Intergranular Boron Segregation and Grain Boundary Character in
Alloy 304 Austenitic Stainless Steel
Department of Materials Science and Engineering University of Toronto
A thesis submitted in confonnity with the re~uirements for the Degree of Master of Appüed Science
in the University of Toronto.
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Canada
lntergranulacBoron Segregation and Grain Boundary Character in
Alloy 304 Austenitic Stainless Steel
Masters of Applied Science
2001
Michael Kurban
Department of Materials Science and Engineering
University of Toronto
Abstract
Secondary ion mass spectrometry (SIMS) and orientation imaging microscopy
(OIM) were used to study the relationship between boron segregation susceptibility at
grain boundaries and grain boundary structure in Alloy 304 austenitic stainless steel
containing boron as a trace solute impurity.
Boron segregation was detected at grain boundaries in materials heat treated at
1ûûû and 1100°C and both boron segregation and carbide precipitation were detected at
grain boundaries in materials heat treated at 800°C. Coherent 23 twin boundaries
displayed high resistance to both boron segregation and carbide precipitation. 2 1
(low-angle) boundaries displayed some resistance to boron segregation and high
resistance to carbide precipitation. 29 boundaries satisfj4ng the Palumbo-Aust criterion
displayed some resistance to both boron segregation and carbide precipitation. Other
low-x boundaries and general boundaries (Ea29) displayed low resistance to both boron
segregation and carbide precipitation.
----- . Acknowledgements 1 would like to sincerely thank my supervisors, Prof. U. Erb and Prof. K.T. Aust
at the University of Toronto, for their guidance and support. 1 would also like to thank
Dr. O. Palumbo and Dr. P. Lin at Integran Technologies inc. for providing the materials for
this study. 1 would like to extend my thanks to Dr. G. McMahon at CANMET in ûttawa for
bis assistance with the secondary ion mass spectrometry (SIMS) analysis and Mr. S. Boccia
and Mr. F. Neub at the University of Toronto for their assistance with the orientation
imaging microscopy (OIM) analysis. I would also like to give special thanks to Dr. M.
McNeil at the Nuclear Regulatory Commission in the United States of Arnerica for helping
initiate this research.
The hancial support nom the Natural Sciences and Engineering Research Council
of Canada, Ontario Power Generation and Integran Technologies Inc. was much
eppreciated.
Table of Contents
1. Introduction
2. Literature Review
Geomtric Mode1 of Grain Boundary Structure
Coincidence Site Lattice (CSL)
Displacement Shift Complete @SC) Lattice
Geornetric Criterion for Allowable Angular Deviation
Grain Bouadary Engineering
Intergranular Solute Segregation
Equilibriurn Segregat ion
Structural Effects
Non-Equilibrium Segregation
Structural Effects
Intergranular Precipitation in Steels
Structural Eff'ects
Materials
Heat Treatrnent s
Secondary Ion Mass Spectrometry (SIMS) Analysis
Sample Preparation for SIMS Analysis
Borna Imaging
Orientation Ixnaging Micmscopy (OIM) Analysis
Sample Preparation for OIM Aiialysis
Microstructure Mapping and Grain Bouadary Characterinition
Oxalic Acid Test
Carbide Precipitat ion Detection
Grain Boundary Classifications
Boron Segregation at Oraia Boudaries
Grain Bouiidary St~ctures
Carbide Precipitation at Grain Boundaries
Results and Discussion 1:
Gnin Bounda y Character Distributions (GBCDs)
OIM Maps and Pole Figures
Grain Boundary Frequencies and Length Fractions
Brandon's Criterion
Palumbo-Aust Criterion
Results and Discussion II:
Boron Segregation at Grain Boundaries
SIMS Boron Images
Grain Boundary Distributions and Frequencies
Brandon's Criterion
Palumbo-Aust Criterion
Boundaries
z3 Boundanes
x9 Bowidaries
Other LOW-x Boundaries (m9 Excludhg z1, D and z9)
General Boundaries (m) CP Alloy 304 vs. G B E * ~ Alloy 304
Conclusions
Grain BoundPry Character Distributions (GBCDs)
Boron Segregation at Grain Boudaries
Recommendations F'or Future Work
Rete ren ces
iii
- List o f Figures Fig. 2.1. Schematic representation of the CSL description of a interfice
fonned by a 8-36-87' [lûû] misorientation of two adjohhg lanices (Aust, 1994).
Fig. 2.2. Schematic representation of the CSL-DSC description of a x5 interface that deviates h m exact x5 orientation by 5' (Aust, 1994).
Fig. 2.3. Maximum angular deviation angles (Ag) fiom any low-z boundary in 99.999 wt.% Ni displayhg select ive irnmunit y to intergranular corrosion in 2N H2S04 (Palwnbo and Aust, 1990).
Fig. 2.4. The solute dependence of the primary influences governing the kquency of low-z boundaries in polycrystalline materials (Palurnbo and Aust, 1990).
Fig. 2.5. Grain boundary energy (y) versus misorientation angle (8) for grain boundaries wit h and wit hout solute segregation (Sautter, Gleiter and Bh, 1977).
Fig. 2.6. Hardness-distance profiks near a grain boundary in a dilute lead alloy containing 1 ppm gold afier water quenching h m 300°C and d e r air cooling fiom 300°C . . . (Aust and Westbmk, 1965).
Fig. 2.7. Influence of quenching temperature (1 12 hr anneal at each quenching temperature) on the room temperature hardness of a grain and two grain boundanes in lead containing 0.65 ppm gold ... (Aust and West brook, 1965).
Fig. 2.8. Time-temperature-sensitization c w e s for various carbon contents (W.%) in Alloy 304 austenitic staidess steel. M23C6 carbide precipitation occurs . . . Baseà on data by Brucmmct ( t 986).
Fig .2.9. Time-tempemture-precip9at ion c w e s for various grain boundar ies (having specific surface âee energies show in parentheses in ergdcm at 1O6O0C) in AUoy 304 austenitic stainless steel containing 0.038 wt.% C. &>Cs carbide precipitatbn occm ... (Trillo et al., 1 995) Based on data by Stickler and Vinckier (1 96 1).
Fig. 3.1. Schematic diagram illustrating the setup of a SIMS for direct-ion imaging .
Fig. 3.2. (a) SIMS boron image and (b) SIMS oxygen itnage h m an Alloy 304 sampk. (c) SEM micrograph of the area imaged in (a) ancl (b) &er SIMS analysis.
Fig. 3.3. Schematic diagram illustating the setup of a SEM for OIM analysis.
Fig. 3.4. (a) OIM map nom an Alloy 304 sample (with twin boudaries indicated in thick lines). (b) SEM rnicrograph of the area imaged in (a)*
Fig. 3.5. SEM rnicrograph from an Alloy 304 sarnple k t treated for 20 min at 8ûû°C d e r (a) SIMS aaalysis and (b) SIMS analysis followed by electroetching in oxalic acid for 30 S.
Fig. 4.1. OLM map fiom as-received CP Alloy 304.
Fig. 4.2. OIM map fiom as-received G B E ~ ~ Moy 304.
Fig. 4.3. Intensity pole figures from as-received CP Alloy 304.
Fig. 4.4. Intensity pole figures fiom as-received G B E ~ ~ Allo y 304.
Fig. 4.5. Frequencies of low-X and genera1 boundaries as per Brandon's criterion in (a) as-received CP Alloy 304 and (b) as-received G B E ~ ~ Alloy 304.
Fig. 4.6. Frequencies of low-x and general boundaries as per Brandon's criterion expected in a random distribution (see Appendix).
Fig. 4.7. Length hctions of low-z and general boundaries as per Brandon's criterion in (a) as-received CP Alloy 304 and (b) as-received G B E ~ ~ Alloy 304.
Fig. 4.8. Freqwncies of low-x and general boundaries as per the Palumbo-Aust criterion in (a) as-received CP Alby 304 and (b) as-received GBE.'~ Alloy 304.
Fig. 49. Frequencies of low-x and general boundaries as pet the Palumbo-Aust criterion expected in a random distribution (see Appendix).
Fig. 4.10. ~engih hctions of low-x and generai boundaries as per the Palurnbo-Aust criterion in (a) as-received CP Alloy 304 and (b) as-received GBE'~ Alloy 304.
Fig. 5.1. SIMS boron images h m (a) CP Alloy 304 and (b) G B E ~ ~ Alloy 304 heat treated for 2.5 min at 100O0C.
Fig. 5.2. SIMS boron images fiom (a) CP Alloy 304 and (b) G B E ~ ~ Alloy 304 heat treated for 2 min at 1 100°C.
Fig. 5.3. SIMS bomn images h m (a) CP Alloy 304 and (b) G B E ~ ~ Alloy 304 heat treated for 3 min at 800°C.
-- .
Fig. 5.4. - - . - _ _ _ = _ - _ - _
Fig. 5.5.
Fig. 5.6.
Fig. 5.7.
Fig. 5.8.
Fig. 5.9.
SIMS bomn images h m (a) CP A h y 304 and (b) G B E ~ AUOy 304 hcattritatd fbf IOmMat 8W0€. 59
SIMS bomn images h m (a) CP Alloy 304 and (b) GBE" Alloy 304 heat treated for 20 min at 800°C. 60
(a) Carbide precipitation susceptibüity, (b) detection of boron within carbides and (c) bomn segregation susceptibility at grain boundsries in CP and G B E ~ ~ Ailoy 304 heat heated for 3'10 and 20 min at 8ûû°C. 61
Frequencies of low-x and general boundaries as per Brandon's criterion in (a) CP Alloy 304 and (b) G B E ~ ~ Alloy 304 ka t treated for 2.5 min at 1000°C. 67
Frequencies of low-z and general boundaries as pet Brandon's criterion in (a) CP Alloy 304 and (b) G B E ~ ~ Alloy 304 heat treated for 2 min at 1 100°C. 67
Freqwncies of low-x and general boundaries as per Brandon's criterion in CP and G B E ~ ~ Alloy 304 heat treated for (a) 3 min, (b) 1 O min and (c) 20 min at 8ûû°C. 68
Fig. 5.10. Frequencies of low-z and general boumlaries as per the Palumbo-Aust criterion in (a) CP Alloy 304 and (b) G B E ~ ~ Alloy 304 heat treated for 2.5 min at 1 OOO°C. 72
Fig . S . 1 1. Frequencies of low-X and general boundaries as per the Palumbo- Aust criterion in (a) CP Alloy 304 and (b) G B E ~ Alloy 304 heat treated for 2 min at 1 100°C. 72
Fig. 5.12. Frequencies of low-x and eneral boundaries as per the Paiumbo-Aust 4 criterion in CP and GBE Ailoy 3W heat treated for (a) 3 min, (b) 10 min and (c) 20 cnin at 8ûû°C. 73
Fig. 5.13. Boron segregation susceptibility at x 1 boundaries (5.O03S1 5 .O0) and general boundaries ( s 2 9 and 15.O0cûS20.O0) as per Brandon's criterion in CP and G B E ~ ~ Alloy 304 heat treated at 1000 and 1 1oO0C. 75
Fig. 5.14. Carbide precipitation susceptibility at z 1 boundaries (5 .OOGS1 5 .O0) and general boundanes (>29 and 15.O0~$20.O0) as per Brandon's criterion in CP a d G B E ~ ~ AUoy 304 kat treated for 3'10 and 20 min at 8û0°C. 76
Fig. 5.15. Detcction of boron within carbides and bomn segregation susceptibiîity at xl bundaries (5.0°~~15.00) and general boudaries ( s 2 9 and 15.0°<8a0.00) as per B d o n ' s criterion in CP and G B E ~ AUoy 304 heat treated for 3.10 and 20 min at 800°C. 77
Fig. S. 16. SIMS boron image showing ao b m n segregation at two x1 2 - --
- - - ûotmdmiesfM00 a d = @=+?.ka) 0) CP-Mloy 304 herit treated for 2.5 min at 1oOo0C.
Fig. 5.17. SIMS borm image showing boron segregation at a El boundary (û=9S0) and no boron segregation at a x1 boundary (8=1 2.!J0) in G B E ~ ~ AUoy 304 heat treated for 2.5 min at 1 ûûû°C.
Fig. 5.18. SiMS bom image showing w bomn segregation at a Z1 boundary (0=6S0) in CP AUoy 3 W heat treated for 2 min at 1 1 OO°C.
Fig. 5.19. SEM micrograph showing no l o c a W corrosion (Le., no carbide precipitation) at three z1 boundaries (0=5.g0, 8=6.4O and û=lO.4") in CP Alloy 304 heat treated for 3 min at 800°C. SIMS boron image showing . . .
Fig. 5.20. SEM micrograph showing no localized corrosion (i.e., no carbide precipitation) at a El boundary (û==6.70) in G B E ~ Ailoy 304 heat treated for 10 min at 8W°C. SIMS boron image showing . . .
Fig. 5.21. SEM microgmph showing localized comsion i . , carbide precipitation) at a general boundary (D29 and 8 4 7.2') and no iocalized corrosion (Le., no carbide precipitation) at two x1 boundanes (û=10.2* and 8=13.8O) in CP Alloy 304 heat treated for 20 min at 800°C. SIMS boron image showing . . .
Fig. 5.22. SIMS bron image showing no bomn segregation at several coherent twin boudaries (CT) (A06.0°) and incoherent twin
bouadaries (IT) (A&2.0°) satisfying the Palumbo-Aust critenon in G B E ~ ~ Alloy 304 heat treated for 2.5 min at lûûû°C.
Fig. 5.23. SIMS bomn Unege showing no boron segregation at several coherent twin bouadaries (CT) (AOP.OO) snd inroherent W twin
boundanes (IT) (AOd.OO) satisQing the Palumbo-Aust criterion and no boron segngation at a coherent twia boundary satisfying Brandon's critetion but not the Palumbo-Aust criterion (HT) (Ae8.3O) in CP Alloy 304 k a t treated for 2 min at 1 100°C.
Fig. 5.24. SEM microgreph showing w locaiized comsion (i.e., no carbide precipitation) at several cobrrent D twin bouadarKs (CT) (A8~2.0~) and incoberrnt W twin ôoundaries (IT) (A&2.0°) satisfying the Pdumbo-Aust criterion in G B E ~ ~ Alloy 304 heat treated for 3 min at 8ûû°C. SIMS boron image showing . . .
vii
Fig. 5.25. SEM micrograph showing localized comsion (ie., carbide A-L2 -.. -- - - precipitatioa) an Urohererit twin bouDdary (ZT) (Aû52.0°)
satisfying the Palumbo-Aust criterion and no locaiized comsion (i.e., no carbide precipitation) at several coherent Q twin boundanes (CT) (A&2.0°) satisfying the Palumbo-Aust criterion in CP Alloy 304 k a t treated for 20 min at 800°C. SIMS boron image showing . . .
Fig. 5.26. Boron segregation susceptibüity at x9 boundaries satisfjhg Brandon's criterion but not the Palwnbo-Aust criterion (i.e., 2.40°cAW5.000) in CP and G B E ~ Alby 304 k a t treated at Io00 and 11oO0C.
Fig. 5.27. Boron segregation susceptibility at x9 boundaries satismg the Palumbo-Aust criterion (Le., A012.40°) in CP and G B E ~ ~ Alloy 304 k a t treated at Io00 and 1 1OO0C.
Fig. 5.28. Carbide precipitation susceptibility at x9 bouadaties satisfying Brandon's criterion but not the Palumbo-Aust criterion (i.e., 2.4O040S5.0O0) in CP and G B E ~ ~ Alloy 304 kat treated for 3, 10 and 20 min at 8OO0C.
Fig. 5.29. Carbide precipitation susceptibility at z9 boundaries satiswng the Pahunbo-Aust criterion (Le., A0S2.40°) in CP and G B E ~ ~ Alloy 304 k a t treated for 3,10 and 20 min at 800°C.
Fig. 5.30. Detection of bomn within carbides and boron segregation susceptibility at z9 boundaries satisfying Brandon's criterion but not the Palumb-Aust criterion (Le., 2.4O049S0O0) in CP and GBE~* Alloy 304 heat treated for 3, 10 and 20 min at 800°C.
Fig. 5.3 1. Detection of boron within carbides and boron segregation susceptibility at Z9 boundaries satisfying the Palumbo-Aust criterion (Le., A&2.40°) in CP and G B E ~ Alloy 304 heat treated for 3, 10 and 20 min at 80O0C.
Fig. 5.32. SIMS boron image showing boron segregation at two bw-z boundanes (x 1 1 ( A e 2 Bo) and D3 (AB=2.3O)) satisfyhg Brandon's criterion but not the Palurnbo-Aust criterion and no boron segregation at a D boundary (A8=0.8°) satisfying the Palumbo-Aust criterion (P-A) in CP Alloy 304 k a t treated for 2.5 min at 1oOO0C.
viii
Fig. 5.33. SIMS boron image showing bomn segregation at two low-I: boundiilies 6x1 1- f W2.40F d C17 (A@=LQ0)) satisfying Brandon's criterion but not the Palumbo-Aust criterion, bomn segregation at three D boundaries (clockwise h m bottom-lefi, Aû=O.SO, A8= 1 .O0 a d A0=0.4O) satiseing the Palumbo-Aust criterion (P-A) and no boron segregation at a z 9 bouadas, (center-right, AH.7') satisfying the Paiumbo-Aust cnterion @-A) in G B E ~ Alloy 304 heat treated for 2.5 min at 1000°C. 104
Fig. 5.34. SIMS boron image showing boron segregation at two low-x boundaries (XI5 (A8=2.1°) and n 3 (A0=2S0)) satisfying Brandon's criterion but not the Palumbo-Aust cnterion and no boron segregation at a D ôoundary (A9-0.4O) satisQing the Palumbo-Aust criterion (PA) in CP Alloy 304 heat ûeated for 2 min at 1 100°C. 105
Fig. 5.35. SIMS boron image showing boron segregation at two low-x boundaries ( D l (A8=1.3O) and n 7 (A84 .O0)) satisming Brandon's criterion but not the Palumbo-Aust criterion, boron segregation at two D boundaries (left to right, Aû=l.1° and A9=1.9O) satisfying the Palurnbo-Aust criterion and no boron segregation at a Z9 boundary (A0=0.8O) satisfying the Palumbo-Aust criterion in G B E ~ Alloy 304 heat treated for 2 min at 1 100°C, 1 06
Fig. 5.36
Fig. 5.37.
Fig. 5.38.
SEM micrograph showing localized comsion ( i e . carbide prec ipitat ion) at two a7 boundaries (lefi-to-right, A0=0. 8" and AH.7') satisfjing the Palumbo-Aust criterion (P-A), no localized corrosion (Le., no carbide precipitation) at two Z9 boundaries (lefi-to- right, A0=0.7O and Aû=0.9O) satisfy h g the Palumbo- Aust criterion (P-A) and no localizecî corrosion at a D3 boundary (A8=le8O) satiswg Brandon's criterion but not the Palumbo-Aust cnterion in G B E ~ ALlOy 304 heat treated for 10 min at 800°C. SIMS boron image showing .. . 1 07
SEM rnicmgraph showing localized comsion (i.., carbide precipitation) at two u7 boundaries (lefi-to-right, AO=O.SO and A8=0.6*) satisfjhg the Palumbo-Aust criterion (P-A) and localued comsion at a D boundary (A8=2.ti0) and a I327 boundary (A0=2.4') satisfjhg Brandon's criterion but not the Pahunbo-Aust cnterion in GBE'~ ALlOy 304 heat treated for 10 min at 800°C. SIMS boron image showhg . . . 108
SEM micrograph showing localized comsion ( i . , carbide precipitation) at a x9 boundary (A0+.7O) satisfyhg the Palumbo-Aust criterion (P-A) and m locaiized corrosion (i.e., no carbide precipitation) at a a 7 boimdary (Ae1.4') satiming Brandon's criterion but not the Palumbo-Aust criterion in CP Alloy 304 ka t treated for 20 min at 800°C. SIMS boron image showina ... 109
Fig. 5.39. Bomn segregation susceptiiility at other low-z boundanes (&29 cxchtdhg Ek, and ~9)+atisfyiagBramlon'scnt&n but ~t the Palumbo-Aust criterion (i.e., 1 ~ ~ ~ ~ ~ ~ d & l ~ ~ ~ ' ~ ) in CP and G B E ~ Alloy 304 k a t treated at 1 0 and 1 100°C.
Fig. 5.40. Bomn segregation susceptibility at other low-L bounckies (m9 excluding El, Q and x9) satisfying the Palumbo-Aust criterion (Le., 604 S ~ E " ~ ) in CP and G B E ~ ~ Alloy 304 k a t treated at 1 O00 and 1 1 ûû°C.
Fig. 5.4 1. Carbide precipitation susceptibility at other low-Z boundaries (m9 excluding El , 5 and x9) satisfying Brandon's criterion but not the Palumbo-Aust cnterion (Le., 1 ~ ~ z ~ % A e ~ l S~E-'") in CP and G B E ~ ~ Alloy 304 heat treated for 3, 10 10 20 min at 8OO0C.
Fig . 5.42. Carbide precipitation susceptibility at 0 t h low-z boundaries (mg excluding x1, and W) satisfjhg the Pahunbo-Aust criterion (Le., ~ 0 ~ 1 5 ~ r ~ ' ~ ) in CP and O B E ~ Alioy 304 heat treated for 3, 10 and 20 min at 8OO0C.
Fig. 5.43. Detection of boron within carbides and boron segregation susceptibility at other low-Z bounâaries (w9 excluding x1, D and x9) satisfying Brandon's criterion but not the Palumbo-Aust criterion (Le., 1 5 ° ~ s ' 6 4 0 ~ 1 5 0 ~ ' n ) in CP and G B E ~ Alloy 304 heat treated for 3,lO and 20 min at 800°C.
Fig. 5.44. Detection of bomn wahlli carbides and boron segregation susceptibility at other low-Z boundaties (m9 excluding x1, D and D) satisfying the Palumbo-Aust criterion (Le., ~ 0 ~ 1 5 ~ ~ ~ ' ~ ) in CP and G B E ~ Alloy 304 heat treated for 3, 10 and 20 min at 800°C.
Fig. 5.45. Bomn segregation susceptibility at general boimdaries ( s 2 9 ) in CP and G B E ~ ~ Mloy 304 heat treated st 1000 and 1 lûû°C.
Fig . 5 A6. (a) Carbide precipitation susceptibility, (b) detection of boron within carbides a d (c) bomn segregation susceptibility at general boundaries ( 5 2 9 ) in CP and G B E ~ ~ Aiioy 304 ka t treated for 3,10 and 20 min at 8ûû°C.
Fig. 5.47. SIMS bomn image showhg bomn segregation at several general buadanes (x>29) in G B E ~ ~ M o y 304 heat treated for 2.5 min at 1ooO0c.
Fig. 5.48. SIMS bomn image showing boron segregation at several general bUI1Ci8ties (D29) in CP Alloy 304 heat treated for 2 min at 1 100°C.
Fig. 5.49. SEM micrograph showing localked comsion (i.., carbide - - - pecipiiatioe) at fiwP g& bo- &W) .and no hcalized
corrosion (i.e., no carbide precipitation) at a general boundary (*) in G B E ~ A b y 304 heat tnated for 10 min at 800°C. SIMS boron image showing . . .
Fig. 5.50. SEM micmgraph showhg localized comsion (i.., carbide precipitation) at three general boundaries ( s 2 9 ) and no localùed corrosion (Le., no carbide precipitation) at two general boundaries (*) in CP Alloy 304 heat treated for 20 min at 800°C. SIMS boron image showing . . .
Fig. 5.51. Frequencies of grain boundaries displayin resistance to boron ,a segregation in (a) CP Alloy 304 and (b) GBE AUoy 304 heat treated for 2.5 min at 1000°C.
Fig. 5.52. Frequencies of grain boundaries displayin resistance to boron TRI segregation in (a) CP Alloy 304 and (b) GBE Ailoy 304 heat treated
for 2 min at 1 10°C.
Fig. 5.53. Length fhctions of grain boundaties displa ing resistance to bomn X, segregation in (a) CP Alloy 304 and (b) GBE AUoy 304 heat treated for 2.5 min at 1000°C.
Fig. 5.54. Length fiactions of grain boundaries displa ing resistance to bomn X, segregation in (a) CP Alloy 304 and (b) GBE Alloy 304 heat treated for 2 min at 1 10°C.
Fig. 5.55. Fnquencies of grain boudaries displaying resistance to carbide precipitation in CP and G B E ~ Alloy 304 ka t treated for (a) 3 min, (b) 10 min and (c) 20 min at 8ûû°C.
Fig. 5.56. Length hctions of grain boudaries displaying resistance to carbide prccipitation in CP end G B E ~ Nloy 304 heat treated for (a) 3 min, (b) 10 min and (c) 20 min at 800°C.
Fig. 5.57. Frequencies of grain boumlaries displaying resistance to bomn segregation in (a) CP AUoy 304 d (b) G B E ~ ~ Alloy 304 heat treated for 3 min at 80û0C.
Fig. 5.58. Length fhctions of grain boundaries displa g tesistance to boron Xf: segregation in (a) CP Alloy 304 and (b) GBE AUoy 304 ka t treated for 3 min at 800°C.
Table 3.1. Heat Treatments
Table 3.2. Angular deviation limits (A&) for low-x boundaries as per Brandon's criterion and the Palumbo-Aust criterion.
Table 5.1. Distributions of low-x and general bounâaries as pet Brandon's criterion in CP and G B E ~ Alloy 304 ka t treated for 2.5 min at 1 oOo0c.
Table 5.2. Distributions of low-z and general boundaries as pet Brandon's criterion in CP and G B E ~ ~ Alloy 304 ka t treated for 2 min at 1 1oo0c.
Table 5.3. Distributions of low-x and general boundaries es pet Brandon's criterion in CF and G B E ~ ~ Alloy 304 heat treated for (a) 3 min, (b) 10 min and (c) 20 min at 80O0C.
Table 5.4. Distributions of low-Z and general boundanes as per the Paiumbo-Aust criterion in CP and GBE" Alloy 304 heat treated for 2.5 min at 100O0C.
Table 5.5. Distributions of low-z and general botdaries as per the Pdurnbo-Aust criterion in CP a d G B E ~ Alloy 304 heat treated for 2 min at 1 10o0C.
Table 5.6. Distributions of low-x and general boundaries as per the Pahunbo-Aust criterion in CP and G B E ~ Alloy 304 heat treated for (a) 3 min, (b) 1 O min and (c) 20 min at 8ûû°C.
Table 5.7. Boron segregation susceptibility at coherent D twin boundaries satisfjing Brandon's criterion but aot the PaIumbo-Aust criterion (Le., 6.0°490.70) in CP and G B E ~ ~ Alloy 304 heat treated at 1000 and 1 1 ûû°C.
Table 5.8. Boron segregation susceptibility at ooherent anà incoherent W twh bouadaries satisming the Pdumbo-Aust criterion (i.e., AOa.OO) in CP a d G B E ~ ~ Alby 304 heat treated at 1000 and 1 100°C.
Table 5.9. Carbide precipitation susceptibility at c o b t 5 twin boundaries satisfjbg Brandon's criterion but not the Palumbo-Aust criterion (i.e., 6.0°<b0i8.70) in CP and G B E ~ ~ AlEDy 304 heat heaîed for 3, 10 and 20 min at 8ûû°C.
-. - xii
Table 5.10. Carbide precipitation susceptibility at coherent and incoherent 5 A > - " - - - - - ~ ~ e R e s s a t i s f y i a g thePahrmtvl-Aust-(i.e.* AOS.OO)
in CP and G B E ~ Alloy 304 k a t treated for 3, 10 and 20 min at 8OO0C. 88
Table 5.1 1. Detection of bron within carbides at coherent twin boundaries sat ismg Brandon's criterion but not tbe Pdumbo-Aust criterion (Le., 6.0°d0S8.7*) in CP and G B E ~ ~ Alloy 304 k a t treated for 3, 10 and 20 min at 80û°C. 89
Table 5.12. Detection of boron within carbides at coherent and incoherent twin boundaries satisfying the Palurnbo-Aust criterion (i.e., A&6.0°) in CP and G B E ~ AlIoy 304 kat treated for 3, 10 and 20 min at 80O0C. 89
Table 5.1 3. Boron segregation susceptibility at coherent Q twin boundaries satis-g Brandon's criterion but not the Palurnbo-Aust criterion (i.e., 6.0°<609.7*) in CP and G B E ~ AUoy 304 heat treated for 3, 10 and 20 min at 8ûû°C. 91
Table 5.14. Boron segregation susceptibility at coherent and incoherent 5 twin boundaries satisQing the Palumbo-Aust criterion (i.e., A8G.0°) in CP and G B E ~ ~ Alloy 304 heat treated for 3, 10 and 20 min at 8ûû°C. 91
1. Introduction
The cote region of a grain bouadary possesses a number of very general geometric
properties since it is the transition zone between two grains, each with a highly periodic
crystal structure. The coincidence site lattice (CSL) is a three-dimensional lattice that can be
consbvcted with lattice points common to two adjohhg grallis at certain crystallographic
misorientations (Kronberg and Wilson, 1949). The CSL is of importance to the
crystallography of grain boundaries because it determines the relative periodicity of the
atomic structure of a grain boundery based upon the relative misorientation of the adjoining
grains. The degree of periodicity at a grain boundary is represented by the parameter
(Grirnmer et al., 1974), which is the inverse k t i o n of coincident sites (Kronberg and
Wilson, 1949). Ail grain boundaries can be described by an appropriate CSL description if
is allowed to approach inhite values (Warrington, 1979).
Nwnerous d e s using high-purity msterials have clearly show that many grain
boundary properties, Uicludhg solute segregation susceptibility, are dependent upon grain
boundary structure as characterkd by the CSL mode1 (see reviews by Shvindlerman and
Straumal, 1985 and Palumbo and Aust, 1W2). Grain boudaries characterized as iow-;r
(high-periodicity) bowlaries (gewrally w 9 ) were found to display improved physical and
chernical properties as compared to general or high-x (low-periodicity) bouadaries (generally
5 2 9 ) , such as incread resistance to solute segrqation. For exampie, as shown by Gleiter
(1970) in a study using zone-ref'iaed lead containllig controiied amounts of solute, less solute
segregation was detected at certain low-x boundaries due to theu higher degree of structural
order ami lower capacity to adsorb or accommodate white atoms.
Based on the fact thst many low-x bouadanes are characterized by bbspecial"
promes, Watanabe (1984) introduced the concept of "grain bouodary design and control".
It was pmposed tbat the interfiace-dependent buik properties of conventional polycrystdine
materiais wuld be improved beyond ptevious limitations by increasing the fiequency of -A-& -
"speciai" boundaries in the grain boundary character distribution (GBCD). Today, this
concept has evolved into a new field of material processing, commonly referred to ps "grain
boundary engineering". As shown by Lui et al. (1995) in an example of grain bomdary
engineeringy the ikequencies of certain Io w-z boundaries in conventional face-centered cubic
(FCC) meterials with a low stacking tàuh energy (SFE) cm be significantly increased by
appropriate themmechanical treatments (Palumbo, 1996). As a resuk many material
propnt ies, such as resistance to intergranular solute segregation, precipitation aad corrosion,
can be signifcantly improved. As proposeci by Pahunbo et ai. (1992), b d on energetic and
crystaîiographic coiistramts associated with twinnhg, a GBCD consisting entirely of low-x
boundaries (Le., nn with n = 1, 2 ancl possibly x1) is attainable. Such a "iwin-limiteci"
microstructure is of gmt practical importance, particularly with regard to the engineering of
conventional polycrystalline materials for impmved resistance to intergranular degradation,
such as sensitization, embrittlement and comsion, which are al1 associated to a certain degree
with intergranular solute segregation.
Alloy 304 austenitic stainless steel is one of the most fiuniliar and most fiequently
used alloys in the stainless steel farnily. AUoy 304 is commonly used in applications where
properties, such as corrosion resistance, are important. It has long been known that the
physical and chernical properties of conventional polycrystaliine materials, especially
austenitic stainless steels, are heavily dependent upon the local chernical composition at the
grain boundaries. Two well-known examples are the sensitization of austenitic stainless steels
to intergranular conosion due to chromium depletion resulting fimm intergranular chromium
carbide precipitation (Aborn and Baia, 1931) and intergranular comsion of solution-tmated
(carbide-fiee) austenitic stainless steels due to intergranuiar solute impurity segregation
(e.g., Aust, Armijo and Westbmok, 1966).
Boron is an alloying element that is commonly added to certain low-carbon steels
to improve hardenability. In austenitic stainless steels, however, boron is usuaily present only
as a trace solute irnpurity. It is well documented that bomn hm a strong tendency to segregate
at grain boundanes in austenitic stainless steels, such as Ailoy 304. Intergranular boron
segregation in austenitic stainless steels is w t a major problem, however, its study is of
importance to better understand the general segregation behaviour of soiute Unpuritics in
these types of materials and develop new materials tbat are more resistant to solute iqurity
segngation. The scope of this research was to snidy the relationship between boron
segregation susceptibility at grain boundaries and grain boundary structure in
commercial-purity Alby 304 austenitic stainiess steel, a FCC material with a low SFE.
The objectives of this study were to:
Investigate the applicability of the CSL mode1 of grain boundary structure to boton
segregation susceptibility at grain boundaries in commercial-purity Alloy 304
austenitic stainless steel.
Investigate the applicability of graiD boundary engineering as a viable meam of
improvhg the resistance of commercial-purity AUo y 304 austenit ic stainless steel
to intergranular bomn segregation.
Background information on grain boundary structure, grain boundary engineering,
solute segregation mechanisms and second phase precipitation in steels is presented in
Chapter 2. A description of the experimental equipment and procedures used in this study is
presented in Cbapter 3. The results h m tbis shidy are presented and discussed in Chapters 4
and 5. The conclusions h m this study are presented in Chapter 6. The recommenâations for
fiiture work are presented in Chapter 7.
2, Literature Review
2.1. Geometric Mode1 of Grain Bo+ Structure
A grain boundary can be described by five independent crystallographic parameters
(Lange, 1967). Three parameters describe the misorientation between the adjoining grains
and the remaining two describe the grah boundary piane normal with respect to one of the
adjoining grains. In this section the coincidence site lattice (CSL) mode1 of grain boundsry
structure, a a d e l strictly based upon the relative misorientation of the adjoining grains. is
discussed.
2.1.1. Coincidence Site Lattice (CSL)
Kronkg and Wilson (1949) fint introduced the concept of the CSL in describing
grain boundary structures in metals. The CSL is a three-dimensional lattice that can be
constructed with httice points common to two adjoining grains at certain crystallographic
misorientations. The CSL is considered the smallest common sublattice of both grains
(GRmwr et al., 1974). The degree of coincidence or periodicity between adjoining grains is
represented by the parameter which is the volume ratio of the CSL unit ce11 to that of the
crystal lattice unit cell (ûrimmer et al., 1974) or the inverse hction of coincident sites
(Kronberg ami Wilson, 1949). Ail grain bomidaries can be àescribeâ by sn appropriate CSL
description if is allowed to appmach infinite values (Warrington, 1979); however, may
achieve very high values of quest ionable pby sical significance. Figure 2.1 shows a schemat ic
representation of a ES CSL interfhce.
Numemus snidies using high-purity materials have cleerly show that many grain
bouildary properties (e.g., energy, corrosion msceptibility ad solute segregation
swceptibility) are dependent upon grain boundary structure as characterized by the CSL
d e l (see reviews by Shwidlermen and S t r a d , 1985 and Palumbo and Aust, 1992).
Grain b o w e s characterized as low-z @a-periodicity) boundaries (generally m9) were
found to display improved physical and chernical properties relative to general or high-z
(low-periodicity) bouadaries (generally 5 2 9 ) due to their higher degree of structural order.
These properties include lower energy, kreased resistance to localized corrosion and
Uicreased resistance to solute segregation. Based on experimental studies concerning
intergiawlar corrosion and hcture in polycrystalline matcrials, Watanabe ( 1985) proposed
that "special" properties would not be expected of grain boundaries chanicterized by D29.
B a d on this proposal, only grain boundaries characterized by w9 are refened to as
"special" or low-2 boundaries.
Fig. 2.1. Schernatic representation of the CSL description of a x5 interface formed by a 0=36.87O [100] misorientation of two adjoining httices (Aust, 1994).
2.1.2. Disdacement Shifi Comnlete (DSC) Lattice
As propsed by Chalmers and Gleiter (1971), the ability of a grain boundary to
accommodate small deviations fiom an exact low-C orientation and still maintain the
presence of a periodic structure is more important than the exact locations of the coinciding
atoms at the grain boundary. As shown in several transmission electmn microscopy (TEM)
shdies of grain bouadaries characterized as king close to exact bw-z orientations
(e.g., Bgto, Gleiter and Hornbogen, 1968 and Schober and BalIufFii 1970), the existence of
collective atomic relaxations et grain boiuidaries cm lead to the formation of arrays of grain
boundary dislocations (GBDs). In much the same way as lattice dislocations at low-mgle
boundaries act to preserve iattice sûucture, GBDs act to preserve the periodic structures of
high-angle boundaries that deviate slightly from exact low-x orientations (Bo llmam, 1 970).
Such grain boundary structures can be visu al^ as GBDs superimposed on ideally oriented
low-x boundaries (Aust, 198 1).
GBDs m distinct h m iattice dislocations in terms of the& Burgers vectors. The
Burgers vectors of the GBDs represent the translation vectoa of a second lattice called the
displacement shift complete @SC) lattice (Grimmer et al., 1974). The DSC tattice can be
consided as the inverse lattice of the CSL (Grimmer, 1974) with a unit ce11 volume
proportional to r' (GNnmer, Bollmann and Warrington, 1974). Figure 2.2 shows a
schematic representation of the CSL-DSC description of a x5 interface that deviates slightly
fiom exact CS orientation.
Fig. 2.2. Schemtic representation of the CSL-DSC description of a Z5 interface that deviates h m exact Q orientation by S0 (Aust, 1994).
2.1.3. Geometric Criterion for Allowable Annular - Deviation
The presence of GBD structures at a grain bowdary indicates that regions of "good
fit" exist at the grain boundary interface and that such a grain boundary may display special
propexties. For a grain boundary that deviates slightly h m exact low-x orientation there is
an angular deviation limit (ABm) above which it cm be comidered that the cores of the GBDs
begin to overlap, thus resulting in elimination of the periodic structure of the grain boundary
and any special properties it may possess. A geometric criterion for aliowable angular
deviation îiom exact low-x orientation is dependent upon the characteristics of both the CSL
and DSC lanices. The appticability of a geornetric criterion to grain boundary properties is
heavily dependent upon and Ae in defhing GBD structures through the relaxation
associated with the CSL/DSC rnodel.
Several geometric criteria have been proposai to de fine A& for low-x boundaries
(e.g., Brandon, 1966; Ishida and McLean, 1972; Deschamps et al., 1987; and Palumb and
Aust, 1990). Most geornetric criteria are derived h m the Read-Shockley relation (Read and
Shockley, 1950), 0=b/d, where 8 is the misorientation angle between the adjohhg grains, b is
the magnitude of the Burgen vector of the GBDs and d is the dislocation spacing. For
low-angle boundaries (XI), the most commonly used angular deviation limit is 15.0° (Read
and Shockley, 1950). For high-angle boundaries (8>1S0 and 5 1 ) the most commonly used
ctilerion is Brandon's criterion, A ~ ~ S ~ S ~ ~ ' " (Bramlon, 1966). However, as shown by
Palumbo and Aust (1990), a more restrictive criterion, WBS found to be more
consistent with theoretical and experimental observations.
As shown by Palumbo and Aust (1990) in a study of intergranular corrosion in
high-purity nickel containing various sulfur concentrations, locaüzed comsion susceptibility
at grah boundaries was fouad to increase with increashg s u k content and was strongly
dependent upon grah boundary structure* As shown in Figure 2.3, a limited structural field of
immunity defined by m5 and ~ 0 , & 1 5 ~ ~ ~ ' ~ was determined The initiation of localized
CO-msion at grain boundaries was characterized by isolated pitting at the grain bounc&uies. Y=---= - A _ _ _ _ -
Susceptibility to intergranular corrosion was attributed to a dislocation mechanism whereby
corrosion resistance was dependent upon the specific distribution (i.e., density) of the GBDs
d the local cbemistry (i.e., local sohite concentration) at the GBDs. In the absence of
segregated sohite, GBDs were found to act as preferential sites for localized corrosion. With
increasing solute concentration, because GBDs cm act as sinks for solute atoms at grain
boundaries (Gleiter, 1970), the potential required to initiate corrosion at GBDs was reduced.
Fig. 2.3. Maximum angular deviation angles (Ag) h m any low-x boundary in 99.999 wt.% Ni displaying selective i m m d y to intergranuiar corrosion in 2N H2S04 (Palwnbo and Aust, t 990).
2.2. Grain Boundary Engineering
Grain boundary engineering expiesses an approach in which conventional
polycrystalline materials can be designed with improveâ interfice-dependent buk propertîes,
such as improved resistance to intergranular comsion, by the deliberate manipulation of the
grain boundary network using fundamental kmwledge of the structures and pmperties of
grain boundanes. Grain bouadary engineering is besed on the fact that many low-z
boundaries are cbaraçterized by special or beneficial properties, such as increased resistance *
to solute segregation and localhd corrosion (see reviews by Shvindlenaan and Straumal,
1985 and Palumbo and Aust, 1992). Low-x buadenes occur naturaily in ail polycrystalline
materials but at a fkquency that is stiongly dependent upon the processing history of the
material. The objective of grain boundary engineering is to deliberately increase the
fiequency of low-x boundaries in conventional polycrystallhe materials in order to hprove
bulk material properties beyond previous limitations.
The grain boundary character distribution (GBCD) of a d o m crystal orientation,
as defined by Warrington and Boon (1975), is the set of grain boundaries generated fiom a
random polycrystalline aggregate in which every p i n orientation has an equal probability of
occurrence. Based on the calculat ions of Warrington and Boon (1 9751, the total fiequency of
low-x bouDdaries defined by z29 satisQing Brandon's criterion (i.e.. A 0 , , , $ 1 5 0 ~ ' ~ )
(Brandon, 1966) expected in a random distribution is 13.62%. T'his value is, however,
considerably lower than the low-L boundary kquency typicaily found in many metallic
systems, including austenitic stainless steels.
By the assistance of w w technologies, such as electron backscattered dfiaction
(EBSD) and orientation irnag h g rnicroscopy (OIM), various material and processing
parameters (e.g., twinning, grain size and prestrain and annealing) have ken found to
influence the fiequency of low-x boundaries in polycrystalline materiais (e.g., Watanabe,
1985 and 1986; Aust and Palumbo, 1989 and 1991; and Pa1umbo and Aust, 1990). As a
resuit , new thermomechanical treaûnents have been develo ped that can signifcant ly improve
the buik properties of cornmon engineering materials (Paiumbo, 1996). For example, as
shown by Lin et al. (1995), after appropriate thermornechanicai processing, an increase in the
fkquency of low-x boundaries in commercial-purity Ailoy 600 resuhed in a significant
increase in the resistance of the material to sensitization (i.e., intergranular chromium carbide
precipitation) and intergranular corrosion. By increasing the Gequency of low-x boundaries
@sijing - Branàon's criterion fiom 31% in the conventionally - - processed material to 71% in
the grain bouadary engineered material intergranular corrosion resistance for both sensitized
and solution-treated (carbide-fke) Alloy 600 was significantly increased.
As show in several early studies, the introduction of low-Z boundaries in FCC
materials is closely related to the formation of cohemt U twin boundaries (e.g., Aust and
Rutter, 1960; Aust, 1961 ; and Fenan et al., 1963 and 1967). As shown by Aust and Rutter
(1960) in studies of grain growth in high-purity FCC met& the formation of coherent W
twin boundaries tends to result in the repeated replacement of general (high-z) boundaries
with more structuraüy ordered jpin boundaries having lower values. As shown in
Figure 2.4, the fkquency of low-x boundaries in polycrystalline materials is influenced by
energet ic, kinetic and geometric factors and the relative contriit ion of eac h factor is strongly
dependent upon the solute concentration in the material (Palumbo and Aust, 1990).
According to Palumôo and Aust (1990), the maximum contributions fiom energetic and
kinetic factors occur ody within a certain solute concentration range and only the geometric
influence is inclepeadent of sohite concentration.
Energetic infiuences are expected to be significant only at very low solute
concentrations. Eelective solute segregation tends to cause a more rapid decrease in the
energy of general boundaries than low-x boundaries as illustratecl in Figure 2.5, which
minirnizes the energetic preference for Iow-z boundaries (Sautter, Gleiter and B&o, 1 977).
In low-purity FCC materials with low SFEs, such as Ailoy 304 austenitic stainless steel
(Le., ysmm21 ergdcm2 at 2S°C) (Mm, 1975), only coherent 5 twin boudaries are strongly
preferred due to their extremely high degree of structural order and extremely low interfacial
energy as compared to general boundanes (Le., ye20ergslcm2 at 1060°C wrnpared to
y-800 ergslcm2 at 1060°C) (Mm, 1975). The dnving force for the formation of coherent
twin boudaries durllig grain growth is a reduction in the total interhial fke energy
(Fullman and Fisher, 1 95 1).
Fig. 2.4. The solute dependence of the primary influences goveming the frequency of low-x - boundaries in-polycrysta~ine materials (Palumbo and Aust, 1 990).
Fig. 2.5. Grain bodary energy (y) versus misorientation angle (0) for grain boundaries wit h and without solute segrrgation (Sautter, Gleiter aad B h , 1977).
nie maximum contributions fiom kinetic influences on the fiequency of low-X
boundaries occur ody at intermediate solute concentrations where low-x boundaries have
higher mobilities than general boundaries (Aust and Rutter, 1959 and Rutter and Aust, 1965).
With nirther increases in solute concentration, the ciifferences in the mobilities of low-x
boundaries and general bouadaries are mmmmd . . . and thus kinetic influences decrease.
As obsemed in many studies of iow-purity materiais, such as commercial-purity
Alloy 304 austenitic staidess steel thre is a stmng preference for x9 anâ and7 boudaries
.-- be explaineci in ternis of energetic or kinetic fsctors (e.g., Hasson and Goux, -- - -
1971). As proposed by Palumbo and Aust (1990), the fkquencies of these particular low-x
boundaries are attributed primarily to geometnc interactions between strongly preferred
D-related boundaries (i.e., Un where FI, 2 and 3). According to Paiumbo and Aust
(IWO), only g e o d c contributions can be considered to be almost independent of solute
concentration.
Several studies have shown that by suitable solid state processing the freqwncies
of certain low-x boudaries in commercial-purity FCC materials with low SFEs
(e.g., austenitic stainless steels and nickel-besed alloys) can be increased to levels in the
7040% range (e.g., Lin et al., 1995 and Palumbo, 1996). Such levels are significantly higher
than those commonly found in conventionally processed materials. The high fiequencies of
low-z boundaries in these low-purity materials are generated prllnarily through twinning
events and geometric interactions between strongly preferred D-re!ated boundaries. The
kquencies of coherent twin bolmdaries and 5-related boundaries are heavily dependent
upon material and processing parameters, such as strain, annealing temperature and grain size
(Palumbo, 1 996).
The primary objective of grain boundary engineering, as it pertaim to FCC
materials, is to hcrease the fkquenc y of coherent Q twin boundaries to their highest possible
levels in order to take advantage of both theu intrinsic properties and their ability to generate
othet bw-2 boundaries. As proposed by Palumbo and Aust (IWO), as a consequence of
energetic and crystallographic constraints associated with tsvinning, a GBCD consisthg
entirely of low-x boundaries (ir., un with n=l, 2 and possibly z1) is attainable. Such a
"twin-limiteci" CSL microstructure is expected to occur when the âequency of coherent D
twin boudaries approaches 213. Distributions hawig these characteristics have commonly
been observed as clusters wahiD conventional polycrystaliine materials (e.g., Palumbo and
Aust, 1990).
--- - - - - Geometric models have been proposed to evaiuate the effects of low-z boundary
fkequency on bulk intergranular cracking and corrosion susceptibility (Palumbo et al., 1991
and 1992 and Lehockey et al., 1996 and 1997). Results fiom these models show that only
moderate increases in the frequency of low-Z boundaries may considerably d u c e the degree
of intergranular degradat ion in commo n engineering mateds.
2.3. Interpnular Solute Seprenation
Bomn is a trace solute impurity commniy fomd in austenitic staidess steels, such
as Alloy 304 austenitic stainless steel. It is well documented that boron has a strong tendency
to segregate at grain boundaries in austenitic stainless steels. For example, Karlsson et al.
(1 988) using secondary ion mass spectmmetry (SIMS) showed that bomn segregation at grain
b o d i e s was detectable in AUoy 3 16L austenitic stainless steel containhg cl ppm boron.
Numetous studies on intergranular solute segregation in difTerent materials have shown that
solute enrichment at grain boundaries cm be due to either equilibrium or non-equilibrium
segregation.
Equilibrium segregation is a reversible process whereby certain solute atom
(Le., those with a signifkant binding energy to the crystll lottice) are adsorbeci at lmsely
packed sites, such as grain boudaries, when a materiai is held at a s ~ c i e n t l y high
temperature to permit appreciable diffusion of solute atom (McLean, 1957). For solute
atom with a certain binding energy to the crystal lattice (i.e., lattice strain), at any
temperature, there wül be an increased concentration of that solute at grain boundaries. The
amount of segregated solute at grain bounâaries increases with increasing buk solute
concentration and decreasing temperature (McLean, 1957). The driving force for this
segregation pmcess is a reduction in the binding energy of the solute to the crystal lattice
locate to a strsin-fkee
collsequence of solute
--.- -- -- . - Q!e., pdyction in lattice strain), which results when - the solute atoms
environment at sites of excess volume, such as grain boundaries. As a I
atoms segregating at grain boundaries, grain boundary h e energy is reduced.
As show in several field-ion microscopy (FM) studies of solute segregation at
grain budaries, equüibrium segregation is generally localized to a few atomic layers at the
grain boimdary interface and the total amount of segregated solute is usually in the order of a
monolayer (e.g., Howell et d, 1973 and 1976 and Smith et al., 1973). As shown in numerous
studies, the amount of segregated solute at a grain boundary at m y temperature or solute
concentration is dependent upon the capacity of the grain boundary to adsorb or accommodate
solute atoms. The adsorption capacity of a grain boundary is dependent upon the degree of
grain bounâary coherenL.e or the m u n t of f k e volume associated with the grain boundary.
Aust and Rutter (1 959) and Rutter and Aust (1 965) investigated the variability in
grain boundary rnobility in bicrystals of zone refined lead containhg controlled amounts of
solute (i.e., th). Grain boumlanes chcterized as king close to exact low-z orientations
(Le., B, z7, XI 3 end z17) were found to have higher mobilities and lower activation
energies t h general (high-z) boundaries in the presence of solute atoms. Rutter ad Aust
(1965) proposed that the diffkrences in mobility and activation energy between low-X
bouadaries anâ general boundaries were due to ciifferences in the amount of interaction
between sohite atoms and the diflierent grain boundary structures. In particular, they
attribut4 the lower mbilities a d higher activation energies of general boundaries to the
stiong interaction between solute atom a d the elastic strain fields associated with the high
GBD densities at these grain boundaries on deviation h m exact low-z orientations. The
slower diffusion of solute atoms in the elasticaiîy straiwd regions led to a significant amount
of drag on the mving p h boundaries (Lucke and Detert, 1957 and Cahn, 1962). The
higher ~ b i l i t ies and lower activation energies of ideally oriented low-Z boundaries were
attributed to the lack of significantly large seain fields outside the core regions of these grain
bouidanes due to their higher degree of struchiral order, better atomic fit and lower GBD
density. Solute atoms segregated primarily at the core regions of ideally oriented low-z
boundaries and, as a result, solute atoms were able to migrate with the moving grain
boundaries without slowing them down significantly. As shown by Demiancnik and Aust
(1 975) in a Mher study of boundaries in zone refined aluminum, grain boundary velocity
was hund to decrease and activation energy increase with increasing deviation (up to 5')
h m exact W orientation (i.e., the amount of solute-grain boundary interaction was found to
increase with increasing GBD density).
In the absence of solute or the presence of too much solute, however, no düference
in mobility between low-Z boundaries and general boundaries was observed (Rutter and Aust,
1965). In the former case grain boundary velocity was limited by the rate at which solvent
atoms moved across the grain boundaries. In the latter case grain boundary velocity was not
controlied by the grain boundary structure but by the speed of the solute atoms diffusing in
the vicuiity of the grain boundaries.
Gleiter (1970) studied the effects of solute (Le., mpper) concentration on the
energies of low-x boundanes and general boundaries in zone rehed Iead. It was observed
that with incretising solute concentration selective solute segregation reduced the energies of
general boundaries more rapidly than the energies of low-x boundaries. Gleiter (1970)
attributed this effect to les solute segregation taking place at low-x boundaries. This effect
was illustrateci schematically in Figure 2.5, where it was alyo shown that grain boundary
segregation tended to d u c e the number of low-energy (i.e., low-a boundaries (i.e., cusps).
Tbe higher degree of solute segregation at gewral boundaries was attributed to the lack of
structural order at tbese grth buiidaries and the interaction of solute atoms with tbe elastic
strain fields associated with the high GBD density at these grain boundaries on deviation fiom
exact low-x orientations. Selective solute segregation at grain boundaries was expected to
exist only for a limited white concentration mge (Sautter, Gleiter and B h , 1977). At high
solute concentrations (Le., when grain boundaries are solute satwated) segregation diffierences
between low-C boundaries would be minimal.
Roy, Erb and Gleiter (1982) investigated grain bou11dary embrittlement in copper
induced by solute (i.e., bismuth) segregation using the sintered sphere-on-plate technique
(Sautter, Gleiter and Bbo, 1977). Results showed that grain boundaries that were most
resistant to embrittlement (i.e., sphere detachment durhg ultresonic treatment) were those of
lowest energy (ie., presumably, including some low-z boundaries). It was also found that
grain boundaries became more embrittled with increasing deviation nom low-energy
(Le., low-x) orientations. M e r a~ealing at low temperatures, a significant number of grain
boundaries were found to be resistant to embrittlement, which was unexpected due to the
higher degree of equüibrium segregation at the low temperatures. This effect was attributed
to the presence of a significant number of low-energy (Le., low-Z) grain boundaries that
possessed higher resistance to solute segregation at the low temperatures (Erb and Gleiter,
1979).
Briant (1983) studied the variability in the amount of solute edchment at
high-angle boundafies in phosphorus- and antimony-do@ nickel-chmmium steels using
Auger electmn spectroscopy (AES) on exposed (Le., hctured) grain boundary surfaces.
Results showed that in the d o y s studied the variability that was observed was primarily
within f30°/o of an average value. Variations in solute segregation dong a single grain
bouadary were not as great as the variation aamng grain boudaries. However, for a fkw
grain boudaries much larget deviations were observed both above and below the average.
Mer analyzing a number of sources of this variability, Briant (1983) concluded that one
major source was grain bouadary structure.
Guo et ai. (1999) investigated the relationship between intergmular boron
segregation, intergranular mlting and grain boundary structure in simulated weld heat
affkcted zones in high-purity Inconel 718 using SIMS end OIM. Intergranular melting in
Inconel 7 1 8 was previously attributed to bomn segregation at grain boundaries (e.g., Kelly,
1989 and Huang et al., 1996). As shown by Zhu et al. (1994), bomn segregation at grain
boundaries in Inconel 718 tends to lower the rnelting temperature of the grain bundary
material. Guo et al. (1999) conducteci studies on solution-treated Inconel 71 8 (i.e., heat
treateà at 1050°C and water quencbed at 50O0C/s) thet was thermally cycled (i.e., heated to
1 190-1 220°C at 1 50°C/s, held there for 1-5 s and then air-jet cooled at 250°C/s). Boron
enrichment at grain boundaries was attributed primarily to equilibrium segregation that
resuhed during i s o t h e d annealhg at the solution k a t treatment temperature (Le., 1 050°C)
prior to the& cycling at higher temperatures (Le., grain boundary mehing temperatures).
In the work by Guo et al. (1999), a close relatioaship was observed between
intergranular melt ing, intergranular boron segregation and grain boundary structure. Low-I:
buridaries were defîned by m9 ushg Brandon's criterion (Le., ~ 8 ~ 1 1 5 ~ ~ ' ~ ) (Brandon,
1966). Only 4 out of 48 low-x boudaries were found rnehed after thermal cycling. Nom of
the z 1 (low-angle) boundaties (i.e., 5O&&1S0) or coherent twin boundaries were found
melted. Other non-melted low-X boumlaries included one El 5 and one n3 boundary. One
of the low-z boundaries found melted was a Dl boundary. Al1 207 general boundaries
(539) were found melted.
Guo et al. (1999) attributed the high resistance of low-x boundaries to rnelting
(i.e., bomn segregation) to low-Z bounâaries having a high degree of structural order and thus
a low capacity to adsorb or accommodate borna atoms. Bornn enrichment was detected at
melted grain boundanes and several non-melted low-x boundaries. Guo et al. (1999)
explained that certain low-z bundaries did not melt because the m u n t of boron enrichment
at these grain bouIlclaries was mt high enough to lower the rnelting tempe- of the grain
-bpuodary material (i.e., low-x boundarie exhibited high resistance to boron segregation).
Born enrichment was mt detected at coherent U twin boudaries. For this particular study,
the use of w9 and a more restrictive criterion for low-z boundary characterization, such as
the Palumbo-Aust criterion (Le., ~0&15~)i-"~) (Palumbo and Aust, 1990), might have show
better correlation between grain boundary structure and borna segregation susceptibility.
2.3.2. Non-Eauilibrium Segre~at ion
As shown by Westbrook a d Wood (1 96 1 and 1 %3), microhardness rneasurements
cm be used to study the presence of local concentrations of solute at grain boundaries in a
wide variety of materials. As observed by Westbrook and Aust (1 %3), the equilibrium
segregation process could not, however, fuily explain excess grain bounâary hardening that
was observed in a wnber of quenched dilute lead alloys in which the levels of apparent
solute segregation extended over distances of several micromet ers across grain boundaries.
Figure 2.6 shows an example of excess grain boundary hardering measured in a bicrystai of
zone-refined lead containing 1 ppm gold after water quenching and air cooüng f?om 300°C.
Aust (l%8) oôserved the same phenornenon in solution-treated Alloy 304 austenitic stainless
steel in which signüicant grain boundary hardening was obserwd with no evidence found by
TEM of solute clusters of sizes exceeding 2.5 nm at grain boudaries. In order to explain
these observations, Westbiook and Ausî (1963) proposai that amther segregation process
was involved that develops h m the kinetics of cooling fiom high temperatures. Aust and
Westbrook (1965) and Aust et al. (1968) pmposed a solute c luste~g mode1
(Le., non-equilibrium segregation process) for describing the grah boundary quench
haidenkg phenornenon.
Non-equiliôrium segregation c m be describeci as the resulting effect of attempting
to maintain local equiiibrium between fiee solute atoms, firee vacancies and vacancy-solute
complexes in the vicinity of grain boundaries during temperature variations and thus
- - , v-~atioqs -. in eqyilibrium concentrations. - During annealhg at high temperatures an
equilibrium concentration of vacancies is generated and distributad throughout the material
ami a certain amount of vacancy-solute binding takes place. During coolhg 6om high
temperatures however, the equilibrium concentration of vacancies cannot be maintaineci
except at w w y s u s (e.g., grain b o d i e s , fke surfaces and fkee dislocations), which are
sites where excess vacancies can be adsorbed and easily annihilated. Since a supersaturated
concentration of vacancies forms in the interion of the grains during cooling a concentration
gradient of vacancies develops netu grsin boundaries and, as a result, vacancies within
diffusion distance of the grain boundenes migrate to the grain boundaries.
Fig .2.6. Hardness-distance profiles aear a grain boundary in a dilute kad ab y containhg I ppm gold afker water quenchg from 300°C and &er air cooling fiom 300°C (1 g load, 5 s Ioading time) (Aust a d Westhok, 1965).
At lower temperatutes wcamy-solute interactions are enhanced, therefore, as
vacancies migrate they tend to drag certain solute atorns (Le., those with positive binding
energies to vacancies) toward grain boudaries. The effective uphill difision of solute atoms
produces a solute-rich region at the grain boundaries that is themdynamicaliy dnven by the
decrease in free energy associateci with the annihilation of excess vacancies at the grain
bowjaries. In the vicinity of the grain bunâaries, collisions between vacancy-solute - -- -& - -
complexes increase and such collisions r m l t in the release of a certain number of vacancies
fiom their complexes, which proceed toward grain boundaries, leaviog behind less mobile
clusters of solute atoms. Subsequent collisions between solute clusters and other
vacancy-solute complexes result in the growth of solute cluster complexes. The size and
fkquency of solute cluster complexes increase as the grain boundary intefice is approached.
The resuhing effect of tbis process is the enrichrnent of solute atoms at grain boundaries with
concentration profiles ecrws the grain boundaries resembling the shape to the
hardness-distance profiles shown in Figure 2.6.
The degree of non-equilibrium segregation at grain boundaries (i.e., thc amount of
solute enrichment at the grain boundary internice and the width of the solute-enriched zone
extendhg fiom the grain bounâary interfacr into the grain interior) is dependent upon heat
treatment procedure. As sbown by Aust and Westbrook (1965), the m u n t of excess grain
boundary hatdening (i.e., degree of non-equilibrium segregation) is dependent upon the
quenching temperature. Figure 2.7 shows an example of the influence of quenching
temperature on the room temperature hardness of a grain ad two grain boundarks in lead
containhg 0.65 ppm gold. Karlsson and Norden (1988) observeci this effect in a ~?hdy of
intergranuilar bomn segregation in Alloy 3 16L austenitic stainless steel containhg 40 ppm
boron using a combinat ion of TEM, F M , atom probe (AP) and imaging atorn probe (IAP).
Karlsson and Norden (1988) found that tbe m u n t of boron segregation at grain boundaries
increased with increasing quenching temperature and boron enrichment was detected as fàr
away as -50 nrn fiom the core regions of the grain boundaries. Unlike equilibrium
segregation in which the amount of solute enrichment decreases with increasing temperature,
the degree of wn-equi1ibriu.m segregation increases with increasing temperature due to a
higher concentration of quenched-in vacancies at the higber ternperatwes.
Pb-0.66 ATOM PPM AU
. I . h
O 100 200 300 QUENCHING TEMPERATURE (OC)
Fig. 2.7. Influence ofquenching temperature (1R hr anneal at each quenching temperature) on the rwm temperature hatdness of a grain and two grain boudaries in lead containhg 0.65 ppm gold (5 g load, 5 s loading the) (Aust and Westbrmk, 1965).
As shown in Figure 2.6, the degree of non-equilibrium segregation at grain
boundaries is also dependent upon the coolhg rate (Aust and Westbrook, 1965). As shown
by Karlsson et al. (1 988) in a study of intergranular boron segregation in Alloy 3 16L
autenitic stainiess steel using SIMS, the highest degree of boron segregation at grain
boundaries occurred at an intemediate cooiing rate (i.e., -lO°C/s). The degree of bomn
segregation was highest at this intemdiate cooiing rate because the cooling tirne was
sufficient emugh to aiîow vocancy-boron complexes to diffuse to grain boundaries but not
d o w the deposited boron atoms to difise away. At higher cooling rates (Le., >lO°C/s) less
bomn segregation was founâ at grah bouadsries because vacancies and complexes ended up
king fiozen into solid solution during coohg since not enough tirne was allowed for their
diffusion to grain boumhies. At lower cooiing rates (i.e., <lO°C/s) less boron segregation
was foui at the core regions of grain bouodanes and the width of the boron-enriched zone
was found to be signüicantiy wider. These effects were attributed to the diffusion of boron
bouadaries d e r they were deposited (i.e., boron desegregation),
large concentration gradient of bomn atoms that developed between
-.-. . . at0.e away firom grain
which was driven by the 1
the grain boundaries and grain interiors during the initial stages of cooling.
As show by Westbrook and Aust (1963), additional grain boundary hardening
(Le., solute segregation) followed by grain bouadary so Aening (i.e., so lute desegregation) can
occur during isothecmal amealing at low temperatures after rapid cooling from high
temperatures Karlsson et al. (1 988) observed these effects in a study of intergranular boron
segregation in Alloy 3 1 6L austenitic stainless steel ushg SIMS. Kat lsson et al. ( 1 988) found
that the amount of boron enrichment et grain boundaries iacreased after a very short k a t
treatment at 500°C after water quenching h m 1250°C. Chen et al. (1998) observed similar
effects in a study of intergranular bomn segregation in Inconel 718 using SIMS. Chen et al.
(1998) found that the amount of boron enrichment at grain boundanes increased and then
decreased with increasing annealing tirne at 1 OSO°C after water quenching from 1 200°C.
Solute segregation cm occur during isothermd anwaling after rapid cooling h m
high temperatures if the concentrat ions of vacancies and vacanc y-so lute complexes, created
prior at high temperatures and parîially annihilated d u ~ g cooling (i.e., quenched in), are still
greater than the equilibrium concentrations corresponâing to the new annealing temperature
( W e s t b k and Aust, 1963; Ueno and Inoue, 1973; and He et al., 19898). Sohite segregation
at grain boundaries that occurs during isothed annealing after rapid coohg results firom
the annihilation of excess (i.ee, quenc hed-in) vacanc ies and vacancy-so lute complexes at grain
boundaries during the low-temperature ka t treatment. The diffusion of solute away ftom the
grain bouadaries (i.e., solute desegregation) occurs &er solute segregation if the temperature
and the concentration gradient of free solute atoms between the grain boundaries and grain
interiors are suffïciently high aad is completed once equilibrium concentrations are achieved.
He et al. (1989A and 1989B) studied the segregation aad desegregation of boron at grain
boudaries in a boron-doped low-carbon steel containing 33 ppm bomn annealed at 10ûû°C
J&X. --- quenching h m 1 100°C ying particle tracking autoradiography (PTA). They
found that additional solute segregation occurred very quickly (i.em, cornpleted after several
seconds) but desegregation too k considerably longer (i.e., completed after several minutes).
In aàdition to heat treatment procedure, the degree of non-equilibrium segregation
at grain boundaries is also dependent upon the type of solute and its bulk concentration.
According to Aust et al. (1968), in order for non-equüibrium segregation to occur, solute
atom must interact with vacancies. Solutes with positive binding energies to vacancies will
becorne enrichecl at grain bouadafies whereas solutes with weak or even negative binding
energies to vacancies will not becorne enriched at grain boundaries and may even become
depfeteâ at grain boundaries. In addition to this criterion, solute clusters must also be stable
or metastable (i.e., activity coefficient of the solute in the solvent must be A). The degree of
non-equilibrium segregation increases with increasing solute concentration (Aust et al., 1968);
however, the maximum amunt of solute enrichment that can occur is determined by the
concentration of fiee vacancies available to form complexes, which is detennined by the
quenching temperature.
Non-equilibrium segregat ion cm be different iated h m equilibrium segregat ion by
the size and shape of the solutesmiched zone at the grain boundaries and its t h e and
temperature dependencies. Unlike equlilibriurn segregation, which is dependent upon the
capacity of the grain boundaries to adsorb solute atorns, non-equüibrium segregation is
dependent upon the ability of the grain b o d e s to adsorb and annihilate excess vacancies
(Le., effectiveness or efficiency of the grain boundary to act as a vacancy sink).
Most d e l s pmposed for describing the operation of grain boundaries as vacaacy
sinks (or sources) are based on the existence of GBDs that climb in the graia bounàary plane
by anaüiilating (or creating) vacancies (e.g., Baliuffi, 1980). The operation of a grain
boundary as a vacancy sink is generally expected to be a highly complex process involving a
number of steps that involve the diffusion of vacaacies to the grain boundary, diffusion of
grain bouadary and vacaacy -
pertains to mn-equilibrium
annihilation at
segregation at
effativeness of a grain boundary at d l a t i n g excess
GBDs. The tenn vacancy sink
grain boundaries, refen to the
vacancies during changes in
temgerature whüe maintainhg an equilibrium concentration of vacancies in the close vicinity
of the grain boundary. Highly efficient vacancy sinks lead to a significant amount of solute
enrichment whereas less efficient or inoperative vacancy sinks lead to Little if any solute
enrichment. One major k t o r that aEects the vacancy sink efficiency of a grain bounàary is
the distribution or density of GBDs at the grain boundary intertace (e.g., Balluffi, 1980).
2.3.2.1. Structural Effects
There is expimental evidence that has shown that the efficiencies of grain
boundaries as vacancy sinks or sources is dependent upon grain boudary structure
(e.g., BalluBi, 1980). For example, Jaeger ad Gleiter (1 978) hvestigated the vacancy
sinWsource efficiencies of grain boundaries during diffusional creep in copper. For
difîùsional creep to occur in polycrystaiiine materials grain bondaries must act as both
vacancy sinks ami vacancy sources. Jaeger and Gleiter (1978) found a close relationship
between grain boundary structure and vacancy sinWsource efficiency. General boundaries
( 5 2 9 ) were found to act as very efficient vacancy sinks/somes under very low vacancy
chemical potentials (i.e., wcancy concentration gradients). Grain boundaries characterized as
close to exact low-x orientations (i.e., 5) were found to be inoperative as vacancy
sinks/sources under low vacancy chemical potentials. The lower efficiency of low-I:
boundanes as vacancy sinks/sources was attributed to the signüicantly lower densities of
GBDs at these grain bouadaries due to their higher degree of structural order and better
atomic M. Under high vacancy chemical potentials, however, low-x boundaries were found
to becorne operative as v8cancy sWsources. The efficiency of coherent Q twin
boundaries, however, was f o d to remain siBnificantly lower than that of general boundanes
and other low-z boundaries at high vacancy chernical potentials due to their extremely high
degree of structural order and lack of GBDs.
Karlsson and Norden (1988) conducted h e scale studies on the segregation of
bomn at grain boudaries in Alloy 316L austenitic stainless steel using a combination of
TEM, FIM, AP and IAP. Two general bounâaties in a sample containing 4 ppm boron heat
treated at 1250°C and cooled at 3 1°C/s (i.e., high vacancy concentration gradient conditions)
were analyzed for bomn segregation The boron concentration at the core regions of the two
gtneral boundaries were both -2.5 at.% and boron enrichment was detected as fat away as
-20 am h m the grain boundary interfiaces. A coherent 5 twin boundary in a sample
containhg 23 ppm boron k a t treated at 1250°C and cooled at 3 1°C/s was also analyzed for
boron segregation. No significant amount of boron enrichment was detected at or near the
coherent twin boundary. The boron concentration at the core region of the coherent W
twin bodary was detemiined to be -0.01 5 atm% and no bomn enrichment was detected in
the region adjacent to the coherent U twh boundary. The higher degree of boron
segregation at the general boundary was attributed to the general boundary operating as a
much more efficient vacancy sink than the coherent W twin boudary during cooling from
high temperatures due to its high GBD density.
Karlsson and Norden (1988) also analyzed a XI (low-angle) boundary in an Ailoy
316L austenitic stainless steel sample containing 40 ppm boron heat treated at 1250°C and
water quenched (i.e., >500°C/s). The concentration of bomn at the core region of the XI
boundary was deterrnined to be -2.5 at.% and boron enrichment was detected as fàr away as
-20 nrn from the grain boundary interfixe. Boron e~chment was attributed primarily to
mn-equilibrium segregation. Boron enrichment at the x1 boundary was not uwxpected since
the structure of a E l bouadary is an array of primary dislocations.
Precipitation of second phase particles at grain boundaries may result when
temperature variations occur and the concentration of segregated solute at grain boundaries
becomes supersaturated. The susceptibiiity of a material to intergranuht precipitation is
dependent upon both the solid solubility and bulk concentration of the solute in the material.
The solid solubility of carbon in austenitic stainless steel (wt.??) Fe-1 8Cr-9Ni is
-0.15 W.% at 1 100°C and -0.05 W.% at 900°C (Rosenberg and Irish, 1952). In austenitic
stainless steels the predominant carbon-containing phase is M2&. The rate et which carbides
nucleate and grow at grain boundaries is dependent upon the carbon content of the material
(Brwmmer, l986), as shown in Figure 2.8.
Fig. 2.8. Tmie-temperature-sensitization curves for various carbon contents (wt.%) in Ai10 y 304 austenitic stables steel. carbide precipitation occurs in the areas to the right of the various carbon content curves (Tdlo et al., 1995). Based on data by Bmemmer (1 986).
The solid solubility of boron in austenitic staialess steel (W.%) Fe-18Cr-1SNi
containing 0.002 W.% carbon is -95 ppm at 1 lûû°C and -30 ppm at 900°C (ûoldschmidt,
1971). The solid solubility of boron in austenitic stainless steels tends to decrease with
increasing carbon content, however, due to the effect of increased occupation of interstitial
sjtes @ the austcnite lanice by an increased number of - - interstitial . carbon atoms (Goldschmidt,
197 1). In low-carbon austenitic stainless steels (i.e., d.03 W.% C) the predominant
boton-containhg phase is M2B (Le., pure brides) (e.g., Goldschmidt, 197 1). In high-carbon
austenit ic stainless steeis (i.e., >0.03 wt.% C) the preQminant bomn-containing phase is
Mu(B,C)6 (Le., borocarbides) (e.g ., Thomas and Henry, 1980).
Yao (1999) investigated intergrandm pmipitation in Alloy 304 austenitic staidess
steel containhg 33 ppm boron and 0.041 wt.% carbon using SIMS and TEM with
energy-dispersive x-ray spectroscopy (EDS). Yao (1 999) found that ai temperatures above
900°C (Cr,Fe)2B borides were the predominant boron-containing phase whereas at
temperatures below 9ûû°C (c~,Fe)~~(c,B)s borocarbides were the predominant
boron-containing phase (Le., M23(C,B)6 borocarbides were thermodynamically more stable
than M2B borides at temperatures below 900°C).
Lundin and Richan (1 995) invest igated the segregation of boron and phosphorus at
carbide/matrix interfaces in a chromium steel (W.%) Fe-9Cr-0.17C containing 80 ppm boron
and 70 ppm phosphorus ushg atom-probe field-ion rnicroscopy (APFIM). Bomn was not
detected at carbidelmatrix interfàces but was detected homogeneously distributed within the
carbides. Phosphoms, on the other hand, was detected only in a monolayer at carbide-matrix
int erfàces.
Liu et al. (1995) studied carbide (i.e., MuCs and M S 3 ) precipitation at grain
botdaries in a low-carbon nickel-based ternary a10 y (wt Ph) Ni- 1 6.2Fe- 1 8.6Cr containhg
0.02 wt.% carbon usiag SEM and OIM. Resuhs showed t b t there was a strong preference for
coherent D twin boundaries and D-related boundaries (i.e., D and n7) in the material.
Liu et al. (1995) fouml tbat the size a d spacing of grain buadary carbides were inûuenced
by grain bouadery structure. Carbides were smaller ami more closely spaced at Z1
Cb~-~tygle) boqadaiies (2°Sû5150) and u-related - buadanes. No carbides were detected at
coherent a twin boundaries but carbides were detected at incoherent D twin boundaries.
Carbides were detected at other low-x boudaries and geaeral (high-n boudaries.
Tri110 and Murr (1998 and 1999) inwstigated the effects of carbon content
(0.0 1 1 -0.07 W.%) and grain bouDdary structure on carbide precipitat ion susceptibilit y at grain
bomdaries in Alloy 304 austenitic stainless steel k a t treated at 670°C for 0.1 to 100 hrs and
water quenched using TEM and OIM. No carbides were detected at coherent twin
hundaries for ali carbon contents but were detected at incoherent C3 twin boundaries for
carbon contents >0.011 W.%. Carbides were detected at other low-x boundaries and general
boundaries for carbon contents 9 . 0 1 1 wt.% and the sizes and densities of the carbides were
found to increase with increasing carbon content, e~eal ing tirne and misorientation angle.
Trillo and Murr (1998) proposed that a critical interfacial free energy was required
for carbide precipitat ion to occur in austenit ic stainless steels, which was between the average
interfacial fiee energy values of coherent twin boundaries and incoherent twin
boundanes. As shown by Trillo et al. (1995), the rate at which carbides precipitate at grain
boundanes in Alloy 304 austenitic stainless steel at any temperature is dependent upon grain
boundas, energy (i.e., gnh boundary structure). As shown in Figure 2.9, grain boundaries
with the highest interfacial energies (i.e., general bouadaries) tend to precipitate carbides &st,
foilowed by incoherent hNin boundaries and then coherent D twin boundaries. The
relative rates at which carbides precipitate at grain boundaries in austenitic stainless steels will
Vary, bwever, dependhg on the carbon content of the material (Trillo and Mm, 1998).
Zhou a al. (2000) investigated the effects of grain bouadary structure on carbide
precipitation in Alioy 3û4L austenitic stainless steel containing 4 0 3 wt.% carbon using
SEM and OIM. Resuhs sbowed that there was a stmng preference for coherent W twin
boundaties and x9 boudaries in the material. Low-x boudaries were characterized using
Brandon's criterion and the Palumbo-Aust criterion. Results showed that carbide
-- - < L pwipitation susceptibility at grain boundaries was dependent upon grain boundary structure.
z1 (low-angle) bouadaries were defined by 5°dlS150. No carbides were detected at XI
boundaries with 0 between S0 and IO0; however, carbides were detected at a fraction of the
zl boundaries with 8 between 10° and Mo. Other low-x boundaries ( 3 ~ 2 9 ) , primarily
coherent U twin budaries and x9 boundaries, displayed significant resistance to carbide
precipitation. Carbides were detected at -20% of the low-x boundaries satisfying Brandon's
criterion and cl 0% of the low-x boundaries satisfjhg the Palumbo-Aust criterion. Carbides
were detected ai dû?% of the gemral boundaries.
Fig . 2 9. The-temperature-precipitat ion curves for various grain boundaries (having specific surface free energies show in parentheses in ergs/cm2 at 1060°C) in Alloy 304 austenitic stainless steel containhg 0.038 wt.% C. &Cs carbide precipitation occurs in the areas to the right of each cwe . The notations G, IT and CT refer to general boundaries, incoherent twin boundaries and coherent Q twin boundaries, respectively (Trillo et al., 1995). Based on data by Stickler and Vinckier (1961).
3. Ex~erimental Procedures 3.1. Materials
The starting matenal for this study was commercial-purity Alloy 304 austenitic
stainless steel with composition (wt.%) Fe-18.7Cr-8.ONi-1.8Mn-0.2M0-0.06C containing
0 0 ppm boron present as a trace solute impurity. Studies were conducted on conventionally
processed (CP) and grain boundary engineered ( G B E ~ ~ ) AUoy 304. n ie G B E ~ ~ material
was prepared by Integran Technologies Inc. (Toronto, Canada). It was themmechanically
processed using a proprietary processing technique (Palumbo, 1996) to contain a higher
fkquency of low-x boundaries (m9) satisQing Brandon's criterion (Le., A0Sl50r")
(Brandon, 1966) ia the grain boundary character distribution (GBCD). The average grain
diameters of the as-received CP and G B E ~ ~ meterials were 15 and 20 Pm, respectively, as
detemined by OIM analysis. The as-received CP and G B E ~ ~ miterials measured 3 and
1 mm in thickness, respectively, and were cut into 5 mm x 5 mm pieces using a low-speed
cutting wheel.
3.2. Heat Treatments
Table 3.1 shows the heat treatments used in this study. The samples were placed
on a pre-heated stainless steel substraîe, inserted into a pre-heatedtube fiunace, heat treated in
a flowing argon atmosphere and then quickly removeci and water quenched. The annealing
times were kept short to minimize grain growth. The samples were water quenched because it
is common practice to rapidly cool austenitic stainkss steel alloys in liquid &et heat
treatment in order to keep al1 the carbon in soüd solution. However, carbide precipitation was
expected to occur in the samples heat treated at 800°C because the solid solubility of carbon
in austenitic stainless steel (wt.%) Fe4 8Cr-9Ni is -0.025 wt.% at 800°C (Rosenberg and
Irish, 1952) and the carbon content of the matenal was -0.06 wt.%.
T-&le 3.1. Heat Treatments P --.= . -.
( Water Quenched)
Temperature
1 100°C
1 000°C
8OO0C
800°C
3.3. Secondan, Ion Mass Spectmmetq ISIMS) Analvsis
Annealing Tirne
2min
2.5 min
3 min
10 min
The segregation of bomn at grain boundaries in each sample was detected using
s e c o ~ ion mass spectrometry (SIMS). SIMS is a surface analysis technique that is
capable of imaging trace elements on samples by bombarding them with high-emrgy prUnary
ions and mass separating and imaging secondary ions that are sputtered fiom their surfàces
(see Secondary Ion Mass Spectrometry - RUiciples and Applications, Vickerman et al. (eds.),
1989). The SIMS technique can detect ail elements in the periodic table including hydrogen,
usually with a detection lirnit in the ppm range, and analyze areas measuring up to -250 pn
in diameter wah s h t d remlution of -1 p m SIMS amlysis was conducted at the Materials
Techaoiogy Laboratory at Natural Resowes Canada (ûttawa, Canada) using a Cameca IMS
4f SIMS equipped with digital imaging.
3.3.1. S q l e Prepamtion for SIMS Aaalvsis
Mer kat treatment, the samples were mechanicaiiy polished using 2400-grit
silicon-carbide paper to remove surtitce contamllration. Mer mechanical polishing, the
samples were electropolished in 10 ml perchîoric acid - 90 mi ethanol at -3S°C using 65 V
@C) for 1 min to remove sudiace scratches and suffixe deformation. Afier electmpolishing, - 4 - -
the samples were uhrasonically cleaned in ethanol.
The SIMS was operated in direct-ion imaging mode or ion microscope mode.
Figure 3.1 shows a schematic diagram illustrating the setup of a SIMS for âirect-ion imaging.
Oz+ ions were used as primary ions. The prirnary ion bearn was focused ont0 the sample
surface to a diameter of -300 pm and maintaineci at a f ied intensity. Bombardment of the
sample with primary Ozf ions causes the sputtering or ejection of sample material h m the
sample surfàce. Monatornic and polyatornic particles of sarnple material and resputtered
primary ions are produced, dong with electrons and photons. The secodary particles, which
consist of atoms, clusters of atoms and molecular hgments, carry negative, positive and
neutral charges and have a range of kinetic energies. The sputtering rate of the sample is
dependent upon primary beam intensity, sample material and grain orientation.
Negatively cbatged scondary ions were extracted h m the sample surface as they
were produced and then analyzed in a double-focushg mass spectrometer system The mass
spectrometer is capable of separating the secondary ions according to theù mass and energy
and transmitting a mas-selected beam of secondary ions to an ion image detector without
affkct ing the lateral distribution of the selected ions. Mass-resolved ion images were acquired
digitally using a resistive anode encoder detector, which counts the arriving ions and records
th& positions. SIMS direct-ion images are usually circuler because ion image detectors are
circular.
SIMS ion images showing the distribution of boron in each sample were acquired
11 16 by mess separating and imaging B 02' m~kcular ions with atomic niass 43. The BOi ion
was the secorxdary ion detennined to pmduce the stmngest boron signal. Figure 3.2 (a) shows
an example of a SIMS boron image (i.e., BW ion image) h m an Alloy 304 sample. SIMS
Mass 1 ' spectrometer '-1
Electrostatic Magne t ic Sector Sector
Sample Exchange II, Chamber Sarnple
Multiplier Used)
--
Mass-Selected Secondary Ion
Beam
Secondary @ Ion Beam
Faraday Cup (Not Used)
Ion [mage Detector (Used)
Fig. 3.1. Schemat ic diagram illustrating the setup of a SIMS for direct-ion imag hg.
Fig. 3.2. (a) SIMS borna image and (b) SIMS oxygen image h m an Alloy 304 sample. (c) SEM micrograph of the erea i m e d in (a) and (b) der SIMS analysis.
boron images show Bw ion intensities as ----a-- -
Bomn signal intensity is depeadeat upon the
35
a function of location on the sample surface.
local boron concentration in the sample. If the
sputtering rate end sputtering tirne of the samples are controlied, the boron signal intensities
cm be used as a relative measure of the local boron concentration in the samples.
SIMS ion images showhg the rnicrostructure of the areas where boron Unaging was
perforrned were acquired by imaging resputtered oxygen ions (i.e., resputtered primary ions).
SIMS oxygen images show resputtered oxygen ion intensities as a function of location on the
ssmple suthce. Figure 3.2 (b) shows a SIMS oxygen image of the same area imageci in
Figure 3.2 (a). The contrast in a SlMS oxygen image results fiom the difbent grain surfaces
sputtering at Werent rates due to ciifferences in crystallographic orientation.
One SIMS boron image and one SIMS oxygen image were acquired fiom each of
the 10 kat-treated samples (Le., 5 CP and 5 G B E ~ samples). All the samples were sputtereà
at the same rate using the same intensity primary 02+ ion beam. Dflerent sputtering times
were wd for the dflerent kat-treated samples in oràer to obtain relatively the sarne number
of BOi ion counts h m each sample. For bomn imaging each sample heat treated at 1Oûû
and 1 100°C was sputtered for 120 s and each sample ka t treated at 800°C was sputtered for
60 S. A shorter sputtering time was used for the samples heat treated at 800°C because the
bomn signal intemity was considerably stronger h m these samples, due to an effect of the
k a t treatment on the distribution of boron at grain boundaries in these sarnples. For oxygen
imaging every sample was sputtered for 120 S.
Mer SIMS d y s i s , the areas that were analyzed by SIMS were fùrther analyzed
using a scanning electmn microscope (SEM). Figure 3.2 (c) shows a SEM micrograph of the
sample surfiice imagecl in Figures 3.2 (a) anâ (b) d e r SIMS analysis. Generally, lattice
impedêctions, eitber already present or introduced by suditce d g , can lead to rougbness
in the sputter Croters thet can take the h m of nbbons, furrows, ridges, cones and
agglomerations of cones. Polycrystalline materiais tenâ to fonn rough sputter craters because
of d@$rential sputter rates that are dependent upon grain orientation, as show in
Figure 3.2 (c).
Comparing al1 the SIMS boron k g e s with SIMS oxygen images and SEM
rnicrographs of the same areas showed that boron was enriched at rnany grain boudaries in
each sample. The SIMS imaging technique cm not be used to differentiate between
equilibriurn boron segregation and non-equilibrium boron segregation because it will detect
boron enrichment at grain boundaries regardless of the mechanism responsible, nor can it be
used to measure the actual concentration of bomn at grain boundaries. However, qualitative
intermation about the relative amount of boron enrichment at grain boundaries can be
detedned by studying the relative intensity of the boron signal fiom grain boundaries in the
SIMS b o m images.
3.4. Orientation hmwin~ Microsco~v IOIh'fl Analysis
ALer SIMS analysis, grain boundary structures in each sample were characterized
in terms of the CSL mode1 using orientation haging microscopy (OIM). OLM is a
microstructurai anal y sis technique that is capable of imeg ing Mcrostmcnires, determinhg
grain orientations and chaiacterizhg grain boundary structures in polycrystalline samples by
irnaging and analyzing electron backscattered ditfiraction pattern (EBSPs) generated fiom
samples in a scanning electron microscope (SEM) (see Adams et al, 1993). The OIM
technique has a spatial resolution of -100 nm and an angular resolution of <1 and can
d y z e areas measuring up to -2cm in diameter. OIM aoalysis was conducted at the
University of Tomnto (Toronto, Canada) using a Hitachi 4500 cold field emission SEM
equipped with a TexSEM OIM system.
3.4.1. Samle Premtion for OIM Anai~sis - - -
M e r SIMS analysis, the perimeter of the sputtered area on each sample where
SIMS anaiysis was conducted was m k e d by hardness indentations in order to help identify
the m a in the SEM. FoUowing this, the samples were electroetched in 10 g oxalic acid -
100 ml water at +25OC using 2 V @C) for 5 s to remove the thin oxide layer that rernained on
the surface of the sputtered areas d e r SIMS anaiysis. Mer electroetching, the samples were
ultrasonically cleaned in ethanol.
3.4.2. Microstructure Ma~ainn and Grain Boundary Characterization
The SEM was operated in low-magnification mode. Figure 3.3 shows a schematic
diagram iilustrating the setup of a SEM for OIM analysis. Each sample was munted on a
sample holder that was tilted at 70° to the incident electron bearn to maximize the nurnber of
hkscattered electrons emitted fiom the sample. The electron beam was focused to a spot on
the sample surface. The position of the electron beam on the sarnple surface was
automatically conhDUed by the OIM cornputer. The electron beam was seqwntially
positioned at preset points covering the sputtered area of the sample. Beam step sizes of 2,
2.5 or 3 pn were used, depending on the average grain size of the area king analyzed.
Difhcted electrons emitted h m the sample at each point were detected using a phosphor
scrcca plaeed c k to the sanipk. The EBSPs were hmged ushg a low-light cpmera,
digitized and analyzed by special crystaiiographic software that is capable of automatically
indexing EBSPs. Data that was calculateci h m each point and recorded hcluded three angles
definhg the crystailographic orientation of the sample with respect to a set of referme
directions and (%y) coordinates iadicating the location on the sample surface where the data
was collected.
The recorded data was processed and displayed in the form of a map of the sarnple
s&e, caiied an OIM map. Figure 3.4 (a) shows an example of an OIM map h m an AUoy
SEM
Mapping
Screen
Indexing 1
C I
-\ Electron Backscattered Backscattered Electrons Difiaction Pattern (EBSP)
Cornputer ( T S L ~ ~ Software)
Low-Light Image Camera Processor
Fig. 3.3. Schematic diagram illustrating the setup of a SEM for OIM analysis.
3 0 4 sample acquired &er SIMS analysis and Figure 3.4 (b) shows a SEM micrograph of the -A--_- - - 2 -- - -- - . - &
same aiea imaged in Figure 3.4 (a). Every pixel in an OIM iaap corresponds to one EBSP
and one crystaliographic orientation meastuement. Lines in the OIM map separate pixels
with misorientations >5.0°. Co-g the OIM rnap and SEM micrograph in Figure 3.4
shows that the microsbuaute of the sarnple was well reproduced in the OIM map.
Fig. 3.4. (a) OIM msp fkom an &y 304 sample (with 5 twin boumhies indicated in thick Lines). (b) SEM micrograph of the area imaged in (a).
In addition to king able to determine the locations of the gniJ boundaries in a
ssmple, the OIM technique is also capable of calculating the vahes of each grain bundary
since the orientations of all the grallis are knom Grain bouadanes with specific values
am be drawn in specfic colors for easy identification a d the values and deviation angles
(A&) for all the grain boundaries can be extracteci h m the recordeci data Length fiactions of
each type of grain bouodary cm also be determined automatically. In this study, however, the
--- --a --@-@ le@ h t i o n of every grain boundaq was determined manualiy by counting and
recordiag the nmber of line segments constituting each grain boundary in each OIM mp.
3.5. Oxalic Acid Test
Carbide precipitation was expected to occur in the samples k a t treated at 8ûû°C
because the solid solubility of carbon in austenitic stainless steel (wt.%) Fe-18Cr-9Ni is
-0.025 wt.% at 800°C (Rosenberg and Irish, 1952) and the carbon content of the material wm
-0.06 wt.%. Carbide precipitation at grain boudaries in the samples heat treated at 800°C
was detected d e r SIMS and OIM analysis using an oxalic acid test. The oxalic acid test used
in this study was based on a standard ASTM test used to detect susceptibility of austenitic
stainless steels to intergranular attack due to carbide precipitation (Le., ASTM A 262-771,
Practice A).
3.5.1. Carbide Recipitation Detection
After SIMS and OIM analysis, the samples were ultrasonically cleaned in ethanol
to remove sutfece contamination. After ultrasonic cleaning, the samples were electroetched
in 10 g oxalic acid - 100 ml water at 2S°C using 2 V (DC) for 30 S. After electroetching, the
samples were ultrasonically cleaned in ethanol.
Figure 3.5 fa) shows an exainpk of a SEM niierograph showkg the sputtered area
of an Alloy 304 sarnple after SIMS analysis. As shown in Figure 3.5 (a), no grain boundary
gmoving was observeci directly after SIMS anaiysis. Figure 3.5 (b) shows a SEM rnicrograph
of the same area in Figure 3.5 (a) &er an oxalic acid test. As shown in Figure 3.5 (b),
locaiized corrosion at nuwmus grain boundaties was observeci &et an oxatic acid test. The
comded regions indicated the locations of the precipitated carbides.
Fig. 3 S. SEM rnicrograph fkom an AUoy 304 sample heat treated for 20 min at 800°C d e r (a) SIMS analysis aod (b) SIMS analysis followed by electroetching in oxalic acid for 30 S.
3.6. Grain Boundary Classifications
3.6.1. Boron S e a ~ a t i o n at Grain Boundaries
In each sample grain boundaries were classified as either susceptible to boron
segregation or resistant to bomn segregation. Grah boudaries were classifd as susceptible
to boron segregation if any boron was observed at the grain boundaries in the SIMS boron
images. ûrain boundaries were classified as resistant to h n segregation if no bomn was
observed at the grain boudaries in the SIMS boron images. In the case that precipitated
carbides were detected at the grain boundary, the grain bouadary segments between the
carbides were analyzed for bomn segregation.
In each sample @ bundaries with m9 were designated as low-z boundaries
and grain ôoundanes with 5 2 9 were designated as general boundaries. Low-z boundaries
were classifiecl using Brandon's criterion (Le., ~ 0 ~ ~ 1 5 ~ ~ ' ~ ) (Brandon, 1966) and the
Palumbo-Aust criterion (i.e., ~e,,,~15~X") (Palumbo and Aust, 1990). Table 3.2 lists the
angular deviation limits (A€),,,) for low-E: boundaries as per Brandon's criterion and the
Palumbo-Aust criterion. Grain boundaries with a misorientation angle (0) between 5.0' and
1 5.0° were designated as XI (low-angle) bounâaries. Grain boundaries with 0c5.0° were not
analyzed. In this study every grain boundary that was characteriad as a U boundary was
detennined to have the twin orientation. Coherent C3 twin boundaries were differentiated
h m incoherent Q twin boundaries by a visuai inspection of the twin boundaries in the SEM
micrographs. Coherent 5 twin boundaries generally appeared long and straight whereas
incoherent Q twin boundaries generally appeared shorter aad less straight.
3.6.3. Carbide Precipitat ion at Grain Boundaries
In the samples heat treated at 800°C grain boundaries were classüied as either
susceptible to carbide precipitation or resistant to carbide precipitation. Grain boundaries
were classified as susceptible to carbide precipitation if any localized corrosion was detected
et the gain boundaries aller an oxalic acid test. Grain b o u n d h were classified as resistant
to carbide precipitation if no localized comsion was detecteâ at the grain boundaries der an
oxalic acid test.
Table 3.2. Aagular deviation limits (Ag,) for low-x boudaries as per Brandon's criterion and the Palumb- Aust criterion.
Angular Deviation Lirnit (A03
Brandon's Criterion
( A ~ & I 5 ' ~ ' ~ )
Palumbo- Aust Criterion
(AO,,,S 1 SOZ-~'~)
4. Results -... and -Discussion 1:
Grain Boundary Character Distributions
4.1. O M Maps and Pole Fimues
OIM analysis was conducted on sarnples of the conventionally processed (CP) and
grain bounàary engineered ( G B E ~ ) materials in as-received condition. Figures 4.1 and 4.2
show the OIM rnaps fiom the as-received CP and G B E ~ ~ materials, respectively. Figures 4.3
and 4.4 show the htensity pole figures ftom the as-received CP and G B E ~ ~ materials,
respectively. The intensity pole figures nom both materials showed that neither material had
a strong texture.
4.2. Grain Boundarv Freauencies and Length Fractions
4.2.1. Brandon's Criterion
Figure 4.5 shows the fkquencies (i.e., nurnber h c t ions) of 10 w-x boundaries
(m9) and general boundaries (x>29) as per Brandon's criterion (i.e., AB< 1 SQX' ')
(Brandon, 1966) h the as-received CP and C5EETM materials. Figure 4.6 shows the calculated
fiequericies of low-z and general botmdanes as per Brandon's criterion expected in a r d o m
distribution (see Appendk). The fkquency of low-x boundanes was considerably higher in
both as-received materials than that expected in a random distribution. The CP and G B E ~ ~
materials contaiaeà 47 and 62% low-Z buadanes, respectively, as compareà to 13.62%
expected in a random distribution.
Al1 the grain boundanes characterized as D bouadaries in both as-received
materiais were deterrnined to have the twin orientation. The hquency of twUi boundaries
satisfying Brandon's criterion was considerably higher in both as-received materials
1- As Per Brandon's Criterion: 1
Fig. 4.1. OIM map h m as-received CP Alloy 304.
I - As Per Brandon's Criterion: 70 pm (step size = 3 pm) -=xi -=W -=W-D9 ==>29
Fig. 4.2. OIM map h m as-received G B E ~ Alloy 304.
Fig. 4.3. Intensitypok figures fiornas-received CP Alloy304.
Fig. 4.4. Intensity pole figures h m 8s-received G B E ~ ~ AUoy 304.
(10% with 5% D7) x9 (1 1% wiîh 2% D7) (8%)
Fig. 4.5. Frequencies of low-z and g e d boundaries as pet Brandon's criterion in (a) as-received CP Alloy 304 and (b) as-received G B E ~ ~ Alloy 304.
Other B 2 9 (8.56% with 0.59% Z27)
Fig. 4.6. Frequencies of low-x and general bouadanes as pet Brandon's criterion expected in a raadom distribution (see Appendix).
Figure A - 4.5) , thm - that expected in a random distribution - (Figure 4.6). The CP and G B E ~ ~
materiais contaimd 30 and 41% twin boundaries, respectively, as compared to 1.76%
expected in a random distribution Don and Majumdar (1986) previously reported a strong
preference for U twin boundaries in Alloy 304. A stn>ng preference for Q twin boundaries
is a characteristic of fiice-centered cubic (FCC) materials with a low stacking fault energy
(SFE), such as Alloy 304.
nie kquency of xl (low-angle) boundaries (5.0°1tK15.00) was slightly higher in
both as-received materials (Figure 4.5) than that expected in a random distribution
(Figure 4.6). The CP and G B E ~ ~ materials containcd 4 and 3% x1 boundaries, respectively,
as compared to 2.28% expected in a random distribution. A lower fiequency of x1 boundaries was expected in the G B E ~ materiai due to its higher twin boundary
hquency. As show by Makita et al. (1988), a pference for z1 boundaries in FCC
materials is generally attributeà to a strong texture and, as show by Gottstein (1984), an
increase in the D twin boundary fiequency tends to lead to a more random texture.
The firequency of D boundaries sat isfying Brandon's criterion was slightly higher
in the as-ieceived CP material (Figure 4.5 (a)) than that expected in a random distribution
(Figure 4.6). On the other hand, the hquency of x 9 boundaries satisfying Brandon's
criterion was considerably higher in the G B E ~ ~ material (Figure 4.5 (b)) than in both the
as-received CP meterid (Figure 4.5 (a)) and that expected in a random distribution
(Figure 4.6). The G B E ~ material contained 8% D boundaries as compared to 2% in the CP
material and 1.02% eexpeted in a random distribution. As proposed by Palumbo and Aust
(IWO), a prefèrence for x9 boboundaries in FCC materials is attributed primarily to geometric
interactions between twin boudaries. The higher kquency of x9 boundaties in the
G B E ~ material was thus likely due to the higher fîequency of 5 twin boundaries in the
G B E ~ material and a higher degree of interaction between them.
-- . - . - - - + - m *uency of low-z
satisfjhg Brandon's criterion was slightly
bouadaries - - - - . - - - (m9 . - excluding Z1, D and Z9)
bigher in both as-received materials (Figure 4.5)
tban that expected in a d o m distribution (Figure 4.6). The CP and G B E ~ ~ materials
contained 1 1 and 1 (r/o other low-x boundaries, respect ively, as compared to 8.56% expected
in a random distribution The a 7 bounâary was determined to be the most strongly
preferred type of other low-z boundary in both materials, uniike in a random distribution in
which boundaries are expected to be the most strongly preferred with 1.22% (see
Appendix). The CP and G B E ~ ~ materials containal 2 and 5% a 7 boundaries, respectively,
as compared to 0.59% expected in a ranàom distribution. As proposed by Palurnbo and Aust
(1990)' a prefèrence for z 9 and D7 boundaries is attribut4 primarily to geomtric
interactions between n-related boundaries (i.e., D", where n=l and 2). The higher
fkquency of D7 bounàaries in the G B E ~ materiai was likely due to the higher fiequemies
of D twin boundaries and x9 boundaries in the G B E ~ ~ material and a higher degree of
interaction between them.
The lkquency of low-x boundaties excluding El and W-related boundaries
(mg excluding XI, Q, t') and a7) satisfying Brandon's criterion was slightly higher in
the as-received CP material (Figure 4.5 (a)) than that expecteâ in a ranâom distribution
(Figure 4.6). On the other hami, the kqwncy of low-x boundaries excluding x1 and
Q-related boundaries satisfjhg Brandon's was slightly lower in the as-received G B E ~ ~
materiai (Figure 4.5 (b)) than in both the as-received CP material (Figure 4.5 (a)) and that
expcteâ in a raadom distribution (Figure 4.6). The CP material contained 90h low-x
bundaries excluding Z1 and ;-3-relateci boudaries as compared to 5% in the G B E ~
material and 7.97% expected in a ranâom distribution. A lower kquency of low-Z
boundaries excluding Z1 and m-related boundaries was expected in the G B E ~ materid due
to its higher D hwin boundary fiequency. As shown by Lin et al. (1995), the frequency of
low~bo~urirlsr+ies excluding XI. and x3-related b o r n e s tends to decrease with increasing
The firquency of general bounâaries as per Brandon's criterion was considerably
lower in both as-received materials (Figure 4.5) than that expected in a random distribution
(Figure 4.6). The CP and G B E ~ ~xlltteriais contained 53 and 38% general boundaries,
respectively, as compared to 86.38% expected in a random distribut ion.
Figure 4.7 shows the length fiactions of low-x and general boundaries as per
Brandon's criterion in the as-received CP and G B E ~ ~ materiels. The length fractions of
low-x boundaries in the CP and G B E ~ materiais were 53 and 67%, respectively.
0 t h e r G 9 (1%) (7% with 2.9% D7) 'z9 (9% with 0.6% a7)
(a)
Fig. 4.7. Length fiactions of low-z and general boundaries as r Brandon's criterion in mpe (a) as-received CP Alloy 304 and (b) as-received GBE Alloy 304.
As shown in Figure 4.7, tk higher length $action of low-z boundaries satisfj4ng
Brandon's criterion in the G B E ~ ~ niaterial was primarily due to the higher length k t i o n s of
twin boundaries and x9 boudaries. The length fiaction of twin b o d i e s in the CP
and G B E ~ ~ materials was 39 and 52N9 respectively, and the length fiaction of x9 boundaries
in the CP and G B E ~ ~ mateds was 1 and 6%, respectively.
The length &action of general taundaries as per Brandon's criterion was
considerably lower in the as-received G B E ~ ~ matenal (Figure 4.7 (b)) than in the as-received
CP material (Figure 4.7 (a)). The length fractions of general boundaries in the CP and ( 3 % ~ ~ ~
materials were 47 and 3 3%, respectively.
4.2.2. Palumbo-Aust Criterion
Figure 4.8 shows the fkquencies (Le., number fractions) of low-Z boundaries
(mg) and general boundaries (x>29) as per the Palumbo-Aust criterion (i.e., ~8115*x~'~)
(Palumbo and Aust, 1990) in the as-received CP and O B E ~ ~ materials. Figure 4.9 shows the
calculated hquencies of low-z and general boundaries as per the Palumbo-Aust criterion
expected in a random distribution (see Appendix). The fkquency of low-z boundaries was
considerably higher in both as-received materials than that expected in a random distribut ion.
The CP and G B E ' ~ materials contained 35 and 53% low-x 'boundaries, respectively, as
cornparrd to 3.73% expected in a random distribution.
The frequency of twin boundaties satisfjing the Pahunbo-Aust criterion was
considerably higher in both as-received materials (Figure 4.8) than that expected in a random
distribution (Figure 4.9). The CP Md G B E ~ ~ IIlsterials contained 29 and 40% twin
boundaries, respectively, as compared to 0.59% expected in a random distribution.
The muencies of x9 boundaries and other low-x boundaries satisfying the
Palumbo-Aust criterion were süghtly higber in the as-received CP material (Figure 4.8 (a))
than that expected in a random distribution (Figure 4.9). On the other hand, the muencies of
D boundaries and other low-x bounâaries satisfjkg the Pahunbo-Aust criterion were
consideraôiy higher in the as-received GBE" material (Figure 4.8 (b)) than in both the
as-received CP material (Figure 4.8 (a)) and that expected in a random distribution
(1% with 0.2% X27)
(a)
Fig. 4.8. Frequencies of low-z and gemal bounâaries as per the Palumbo-Aust criterion in (a) as-received CP Alloy 304 and (b) as-received G B E ~ ~ Alloy 304.
(0.75% with 0.02%
Fig. 4.9. Frequencies of low-I: and general boudaries as per the Palurnbo-Aust cnterion expected in a random distribution (see Appendix).
@igue43]. nie G B E ~ material contained 6% B-boundaries as compared to 1% in the CP
material and 0.1 1% expected in a randorn distribution and the G B E ~ ~ material contained 4%
other low-x boundaries as compared to 1% in the CP material and 0.75% expected in a
random distribution. The D 7 boimdary was determined to be the most strongly preferred
type of other low-z boundary in both rnaterials. The CP and G B E ~ ~ materials contained 0.2
and 3.3% U7 boundaries, respectively, as compared to 0.02% expected in a random
distribution.
The fiequency of general boundaries as per the Paiumbo-Aust criterion was
considerably lower in both as-received meterials (Figure 4.8) than that expected in a random
distribution (Figure 4.9). The CP and G B E ~ ~ materials contained 65 and 47% general
boundaries, respectively, as compared to 96.27% expected in a random distribution.
Figure 4.10 shows the length fhctions of low-x and general boundaries as per the
Palumbo-Aust criterion in the as-received CP and G B E ~ ~ materials. The length fiactions of
low-z boundaries in the CP and G B E ~ ~ materials were 44 and 60%, respectiveiy.
I 2 3 O t h e r B 9 -\ Other =9 (l%) (2% with 1.5% D71 To -
(1% with 0.2% z27) ' LI4
(5%)
Fig. 4.10. Length fiactions of low-z and geaeral boumhies as per the Palumbo-Aust criterion ia (a) as-meived CP Alioy 304 and (b) as-received G B E ~ ~ Alloy 304.
- - - & shown in Figure 4.1 O, the higher - A - -- length k t i o n - of bw-z boundaries satisfjhg
the Pahuabo-Aust criterion in the G B E ~ ~ material was primarily due to higber length
hctions of 5 hNin bundacies and x9 boundaries. The length fiactions of D twin
boudaries in the CP and G B E ~ ~ materials were 38 and SI%, respectively, and the length
hctions of D boundaries in the CP and GBE" materials were 1 and 5%, respectively.
The length &action of general boudaries as per the Palumbo-Aust criterion was
considerably lower in the as-received G B E ~ material (Figure 4.10 (b)) tlmui in the
as-rooeived CP material (Figure 4.10 (a)). The length fiactions of general boundaries in the
CP and G B E ~ materials were 56 and 40%, respectively.
5. Results and Discussion II:
Boron Segregation at Grain Boundaries 5.1. SIMS Borna 1-
Figures 5.1 and 5.2 show the SIMS boron images fiom the conventionally
processed (CP) and grain boundary engineered ( G B E ~ ) sunples heat treated for 2.5 min at
1000°C and 2 min at 1 10O0C, respectively. Comparing the SIMS boron images with SIMS
oxygen images and SEM micrographs of the same areas showed that boron was enriched at
grsin boundaries in each sample.
Boron enrichment at grain boundaries in the CP and G B E ~ ~ simples heat treated at
1000 and 1 100°C was likely due to a combination of equilibriurn xgregation that occurred
during annealhg and non-equilibriwn segregation that occurred during cooling (i.e., water
quenching). As shown in Figures 5.1 and 5.2, the rnajority of the grain boundaries in each
SIMS boron image displayed relatively the same boron signal intensity, which indicated that
the majority of the grain boundaries in each sample were enriched with relatively the same
amount of boron. In the SIMS boron images som grain boundeties displayed considerably
higher intensities than other grain boudaries. As show by Karlsson et al. (1988), the
stronger intensities at some grain boudaries were primarily due to geomtric effects
associated with the orientation of the grain boundary plane with respect to the sample surface
anâ not due to any real variations in the amount of bomn enrichment at the grain boundaries.
As shown by Karlson et al. (1988), the intensity of the bomn sigoal fiom a grain boundary
that is almost parailel to the sample surface tends to be stronger than that h m a grain
boundary that is mre perpendicular because more grain boundary area is exposed at the
former during sputtering.
Fig. 5.1. SIMS boron images h m (a) CP treated for 2.5 min at lûûû°C.
(b)
Moy 304 and (b) GBE" Alby 304 heat
Fig. 5.2. SIMS boron images h m (a) CP treatd for 2 min at 1 100°C.
(b)
Alloy 304 end (b) G B E ~ ~ Aiby 304 heat
-- - - -- As-show in Figure 5.1, the gr^@ ly@ajes in the SIMS boron images fiom the
CP and G B E ~ samples heat treated at 1 0 ° C displayed relatively the same bomn signal
intensity, which indicated that grain bouodanes in both samples were enriched with relatively
the same amount of boron. Similarly, as shown in Figure 5.2, the grain boundaries in the
SIMS boron images fiom the CP and G B E ~ ~ simples heat hpated at 1 100°C also displayed
relatively the same b o m signal intensity, wbich indicated that grain boundaries in both
samples were also enriched with relatively the same amount of boron. Comparing the SIMS
boron images h m the samples k a t treated at l OOO°C (Figure 5.1) with those h m the
samples heat treated at 1 1 W C (Figure 5.2) showed, however, that the intensities were
slightly sboager h m the samples heat veated at lûûû°C. The stronger intensities fiom the
samples ka t treated at lûûû°C indicateà that the grain boundaries in these samples were
enriched with a relatively higher smount of boron; this was likely due to a higher degree of
equilibrium segregation that occurred during anneaiing at the lower temperature.
Figures 5.3 to 5.5 show the SIMS bomn images h m the CP and G B E ~ ~ samples
heat treated for 3, 10 and 20 min at 800°C, respectively. SEM micrographs of the sarne areas
after SiMS analysis showed no grain bowidary grooving on each sample. SEM micrographs
of the sam areas afier SIMS analysis and an oxalic acid test, on the other hand, showed
localized corrosion at grain boundaries on each sample, which indicated the presence of
precipitated carbides at grain boundaries in each sample.
Figure 5.6 (a) shows the carbide precipitation susceptibility at grain boundaries in
the CP and G B E ~ ~ samples heat treated at 800°C. As show in Figure 5.6 (a), carbide
precipitation was detected at grain bouaderies in each sample and carbide precipitation
susceptibility at grtain bormdaries i n c d with increasing annealing thne in both the CP and
G B E ~ sampîes.
Comparing the SIMS borna images h m the CP auci G B E ~ ~ -les ka t treated
et 8ûû°C with SIMS oxygen images of the same mas and SEM micrographs of the same
Fig. 5.3. SIMS boron images fkom (a) C treated for 3 min at 800°C.
(b)
AUoy 304 and (b) GBE" Aüoy 304 heat
(a)
Fig. 5.4. SAM bomn images h m (a) CP treaîed for 10 min at 800°C.
(b)
AUoy 304 and (b) G B E ~ M o y 304 heat
Fig. 5.5. SIMS bron images from (a) CP Alloy 304 and (b) G B E ~ ~ Alloy 304 heat treated for 20 min at 800°C.
3 min 10 min 20 min at 8WC at 800°C at 8 W C
Heat Tteatment
3 min 10 min 20 min at 8ûû°C at 8 W C at 8 W C
Heat Treatment
3 min 10 min 20 min at 8ûû°C at 8 W C at 8û0°C
Heat Treatment
Fig. 5.6. (a) Carbide precipitatioa susceptibility, (b) detection of bomn within carbides ami (c) bron segregation susceptibiîity at grain boundaries in CP and G B E ~ ~ Alby 304 heat treated for 3,10 anà 20 min at 800°C.
areas after anoxalic acid test sbowed @t boron was e ~ c h e d witw grain boundary carbides
in each sample. Figure 5.6 (b) sbws the detection of bomn within carbides at grain
boundanes in the CP and G B E ~ ~ ~arnples heat treated ai 800°C. Cornparhg Figures 5.6 (a)
and (b) showed that boron displayed a strong tendency to become incorporated into
precipitating carbides.
As sbwn in Figures 5.3 to 5.5, the majority of the carbides (i.e., the srnall bright
spots) in the SIMS boron images fiom the CP and G B E ~ ~ simples ka t treated at 800°C
displayed reletively the sarne boron signal intensity, which indicateà that the nnijorîty of the
carbides in each sample were enricheâ with relatively the same amount of boron. In the SIMS
boron images some carbides displayed considerably higher intensities than other carbides.
The stronger intensities at some carbides were iikely due to geometric effects associated with
the orientation of the grain bouadary plane with respect to the sarnple surnice and not due to
any real variations in the amount of bomn within the carbides.
Comparing the SIMS boron images h m the samples k a t treated at 800°C with
SIMS oxygen images of the same areas and SEM micrographs of the same areas afier an
oxalic acid test also showed that bomn was enriched at grain boudaries in some samples.
Figure 5.6 (c) shows the bomn segregation susceptibility at grain boundaries in the CP and
G B E ~ ~ samples k a t treated at 800°C. As show in Figure 5.6 (c), boton was enriched at
grain boundanes in both the CP and G B E ~ ~ -les heat treated for 3 min at 800°C but was
considerably less enriched at grain boudaries in the CP and G B E ~ ~ samples heat treated for
1 O and 20 min at 800°C.
As shown in Figure 5.6, with increasing annealhg time at 800°C d Uicreasing
carbide precipitation at grain bowidaries bomn tendeâ to desegregate b m grah boudaries
and become incorporated into precipitating carbides. These tesults indicated that bomn had a
stronger atnnity for carbides than for grain boundaries. Similar resuhs were obtained by
Karlsson et al. (1988) in a study of intergranular b r o n segregation in Alloy 3 16L austenitic
staipleg steel containhg 23 ppm boron and 0.016 wt.% carbon ka t treateà at 800°C and -. -
water quencheû using SIMS.
Boron enrichment at grain boundaries in the CP and GBE" samples heat treated
for 3 min at 800°C was likely due to a combinat ion of equilibrium segregation that occurred
during anneaiing and non-equiiibrium segregation that occurred during cwling (i.e., water
quenching). As shown in Figure 5.3, the majority ofthe grain boundaries in the SIMS boron
images h m the CP and G B E ~ samples k a t treated for 3 min at 800°C displayed relatively
the same boron signal intensity, which indicated that the majority of the grain boundaries in
both sarnples were enriched with relatively the sarne amount of boron. In the SIMS boron
images some grain boundaries displayed considerably stronger intensities than other grain
boundaries. The stronger intensities at some grain boundaries were ükely due to geometric
effects associated with the orientation of the grain boundary plane with respect to the sample
surke and not due to any na1 variations in the amount of boron enrichment at the grain
boundaries.
Comparing the SIMS bomn images fiom the CP and G B E ~ ~ samples heat treated
for 3 min at 8ûû°C (Figure 5.3) with those h m the CP and GBE~* samples heat treated at
1000 and 1 1 OO°C (Figures 5.1 and 5.2) showed that boron signal intensities were considerably
stmnger h m the samples heat treated at 800°C. The stmnger intensities h m the sarnples
heat treated at 800°C indicated that the grain boundanes in these samples were enriched with
a considerably higher amount of bmn. The hipher amount of boron enrichment in the
samples k a t treated at 800°C was ükely due to a higher degree of equilibrium segregation
that occurred during annealing at the lower temperature.
5.2. Grain Boundary Distributions and Fm-uencies
Tables 5.1 and 5.2 show the distributions of low-Z grain boundaries (mg) and
general grain boundaries (529) as per Brandon's criterion (i.e., A% 1 S ~ X ' " ) (Brandon,
1966) characterized in the CP and GBE" samples heat treated at Io00 and 11OO0C,
respectively, using SIMS and OIM. Table 5.3 shows the distributions of low-z and general
bounâaries as per Brandon's criterion characterized in the CP and G B E ~ ~ samples heat
treated at 8ûû°C ushg SIMS, OIM and an oxalic acid test.
As shown in Tables 5.1 to 5.3, a larger nurnber of grain boundaries were
chiuacterW in the kat-treated CP samples than in the heat-treated G B E ~ ~ samples. More
grain bouadaries were c b t e r i z e d in the CP samples because the CP samples had a slightly
smaller grain size. The short heat treatments had no large effect on the grain sizes of the CP
and G B E ~ ~ materials. As determined by OIM analysis, the average grain diameters of the
kat-treated CP and G B E * ~ samples were -15 and -20 pn, respectively, which were about
the sarne as those of the as-received CP and G B E ~ ~ materials.
Figures 5.7 anâ 5.8 show the firepuencies (Le., number hctions) of low-x and
general bounâaries as per Brandon's criterion characterized in the CP and G B E ~ ~ samples
k a t treated ai 1000 and 1 100°C, respectively, using SIMS and OIM. Figure 5.9 shows the
hquencies of low-x ancl general boumlaries as per Brandon's criterion characterized in the
CP and GBE" samples heat treated ai 8ûû°C using SIMS, OIM and an oxalic acid test. The
short k t treatments had no large effect on the grain boundary character distributions
(GBCDs) of the CP and G B E ~ mterials. The muencies of Iow-x and general boundaries
as per Braadon's criterion in the heat-treated sarnples (Figures 5.7 to 5.9) were similar to
those in the as-received materials (Figure 4.5).
Table 5.1. Distributions of low-x and general boundaries as per Brandon's criterion in CP and G B E ~ ~ AUoy 304 k a t k t e d for 2.5 min at 1000°C.
General (D29)
Other l i s 9
Total
CP Alloy 304
GBE" Alloy 304
Table 5.2. Distributions of low-x and general boundaties as per Brandon's criterion in CP and G B E ~ Alloy 304 k a t treated for 2 min at 1 100°C.
Total
161
95
Other r<29
General ( 5 2 9 )
CP Alloy 304
Table 5.3. Distributions of low-z and general bouadaries as pet Brandon's criterion in CP and G B E ~ Alloy 304 heat treated for (a) 3 min, (b) 10 min and (c) 20 min at 800°C.
General ( m g )
Total
G B E ~ ~ Alby 304
G B E ~ Alby 304
CP Alby 304
G B E ~ ~ Alby 304
Other z ~ 2 9 Other Es29 ' ~ 9 (6% with 0.8% C27) (7% with 3.5% Z27) (6%)
Fig. 5.7. Frequencies of low-x and general boundaries as per Brandon's criterion in (a) CP Alloy 304 anâ (b) G B E ~ ~ Alloy 304 kat treated for 2.5 min at 1 OOO°C.
(7% with 1.3% E27)
Other ZQ9 - \ (9% with 3.4% Z27) z 9
(5%)
Fig. 5.8. Frequencies of low-x aad general boundaties as per Brandon's criterion in (a) CP Aîloy 304 a d (b) G B E ~ AUoy 304 kat treated for 2 mia at 1 10û°C.
&Y
1 (3%) Other C d 9
(6% with 1.8% C27)
CP Alloy 304
I (3%) Chher C d 9
(5% with 1.5% E27)
CP Alloy 304
Mer ES9 (8% with 2.0% C27)
CP Alloy 304
Othe C d 9 C9 (Ph with 4.4% x27) (5%)
1 O min at 80o0C
ûther C a 9 C9 (7% with 4.5% C27) (S9c)
Fig. 5.9. Frequeacies of low-x and gewral bouadanes as per Brandon's criterion in CP and G B E ~ ~ Alloy 304 heat ûeated for (a) 3 min, (b) 10 min and (c) 20 min at 8ûû°C.
Tables 5.4 and 5.5 show the distributions of low-C grain boundaries (B) and
general grain boundaries (B29) as per the Palumbo-Aust criterion (Le., ~ 0 ~ 1 5 ~ ~ ~ ' ~ )
(Palumbo and Awt, 1990) characterized in the CP and G B E ~ simples heat treated at 1 0
and 1 100°C, respectively, using SIMS and OIM. Table 5.6 shows the distributions of low-x
and general boundaries as per the Palumbo-Aust criterion chsracteriad in the CP and O B E ~
samples k a t treated at 8ûû°C using SIMS, OIM and an oxalic acid test.
Figures 5.10 and 5.1 1 show the kquencies (Le., n w k r hctions) of low-Z and
general boundaries as per the Pahunbo-Aust criterion chsracterized in the CP and GBE"
samples heat treated et 1000 and 1 100°C, respectively, using SIMS mi OIM. Figure 5.12
shows the muencies of low-x and general boundaries as per the Pahunbo-Aust criterion
characterizeâ in the CP and G B E ~ ~ samples k a t treated at 80O0C using SIMS, OIM and an
oxalic acid test. The short ka t treatments haà no large effect on the grain boundary character
distributions (GBCDs) of the CP and O B E ~ ~ materials. The lkquencies of low-E and
general buisdanes as per the Palumbo-Aust criterion in the heat-treated sarnples
(Figures 5.10 to 5.12) were similar to those in the as-received materials (Figure 4.8).
Table 5.4. Distributions of low-x aiad gened boundaries as per the Palumbo-Aust criterion in CP and G B E ~ ~ Alloy 304heat treated for 2.5 An at 1000°C.
Other C129
General (D29)
Total
G B E ~ Alloy 304
Table 5.5. Distributions of low-x and general boundanes as per the Paiumbo- Aust critenon in CP and G B E ~ ~ Alloy 304 k a t treated for 2 min at 1 100aC.
Ot her m9
General (D29)
Total
Table 5.6. Distributions of low-x and general boundaries as per the Palumbo-Aust criterion in CP and G B E ~ ~ Alloy 304 heat treated for (a) 3 min, (b) 10 min and (c) 20 min at 80°C.
Other Cd9
General (D29)
Total
CP Alky 304
G B E ~ ~ Alloy 304
CP Alloy 304
G B E ~ Alloy 304
C B E ~ Alloy 304
(1% with 0.0% Z27)
(4% with 2.7% Z27) (6%)
Fig. 5.10. Frequencies of low-z and general boundaries as per the Palumbo-Aust criterion in (a) CP Alloy 304 and (b) G B E ~ ~ Alloy 304 heat treated for 2.5 min at 1 OOO°C*
(1% with 0.5% Z27) Other Z G 9 z9 (4% with 1 .O% Z27) (4%)
Fig. 5.1 1. Frequencies of low-x and geaeral bolmdanes as per the Palumbo-Aust criterion in (a) CP Alloy 304 and (b) G B E ~ ~ Alloy 304 heat treated for 2 min at 1 lûû°C.
3 min at 80°C
CP Alloy 304
C l .
OmcrLa9 (2% with 1.5% Z27)
CP Alloy 304
Other ES29 Z9 (4% with 3.6% E27) (4%)
G B E ~ ~ Alloy 304
20 min at 8OO0C
(2% with 1 .OY, D7j- - -'
CP Alloy 304
(5% with 3.3% C27)
G B E ~ ~ AUOY 304
Fig. 5.12. Freqwncies of low-z and general boudaries as per the Palumbo-Aust criterion in CP and G B E ~ ~ Alloy 304 kat treated for (a) 3 min, (b) 10 min and (c) 20 min at 8ûû°C.
Figure 5.13 shows the boron segregation susceptibility at z 1 (low-angle) grain
boundaries (5.0°SûS1 5.0') and general grain boundaries (>29 and 1 5.0°<8120.00) as per
Brandon's criterion (i.e., ~ 0 < 1 5 ' ~ " ) (Brandon, 1966) in the CP and G B E ~ ~ samples heat
treated at 1000 and llOO°C. As shown in Figure 5.13, x1 boundaries defined by the
Read-Shockley Iimit of OS1 5.0' (Read and Shockley, 1950) displayed some resistance to
boron segregation whereas general boundaries with 0 between 1 5.0° and 20.0° displayed no
resistance to bomn segregation. A misorientation angle lirnit between 5.0' and 1 5.0' definhg
a structural field of high resistance to boron segregation at I;1 boundaries was not determined.
As shown in Figure 5.13, boron segregation was detected at a x1 boundary with a
misorientation angle as low as 7.5'.
Figure 5.14 shows the carbide precipitation susceptibility at z 1 boundaries
(5.0°48S 1 5.0°) and general boudaries (z>29 and 1 5 .O04%20.O0) as per Brandon's criterion
in the CP and G B E ~ simples heat treated at 800°C. As shown in Figure 5.14, the
Read-Shockley limit of f3s15.0° (Read and Shockley, 1950) was detemrined to define a
structurai field of high resistance to carbide precipitation at x1 boundaries. Z 1 boundaries
with 0S15.0° displayed high resistance to carbide precipitation whereas general boundaries
with 0 between 15.0° and 20.0' displayed low resistance to carbide precipitation. As shown
in Figure 5.14, with increasing annealhg t h e at 800°C E l boundaries continued to display
hig h resistance to carbide precipitat ion whereas general boundaries dispkyed increased
susceptibility to carbide precipitation.
Figure 5.15 shows the detection of boron within carbides and boron segregation
susceptibility at z1 boundanes (5.0°d&15.00) and general boundaries ( 5 2 9 and
1 S.0°<8d0.00) as per Brandon's criterion in the CP and G B E ~ ~ samples heat treated ai
800°C. As shown in Figure 5. L 5, boron was detected within carbides at general boundaries.
Fig. 5.13. Boron s
- -
O =CP A&, 364 O = G B E ~ A U O ~ 304 B = Boron Segregation Detected
2.5 min 2 min at l ûûû°C at 1 1 OO°C
Heat Tmtment
egregation susceptibility at El bounâaries (S.O0&I lies ( 5 2 9 d 1 5 . 0 ° ~ ~ 0 . 0 0 ) as pet Brandon's AUoy 304 heat treated at 1OOO and 1 1 ûû°C.
1S15.0°) a criterion
general CP and
- - TM- - - O = CP A Ü O ~ 304 -O =GBE AUOY 304
C = Carbide Precipitation Detected
3 min 10 min 20 min at 8ûû°C at 800°C at 8OO0C
Heat Treatment
Fig. 5.14. Carbide precipitation susceptibility at Cl bouadaries (5.0°*l 5.0' boundaries (D29 aud 15.O0<8S20.O0) as per Brandon's criterio G B E ~ ~ Aiioy 304 heat treated for 3,10 and 20 min at 800°C.
l gen CP
mil and
Fig.
3 min 10 min 20 min at 800°C at 800°C at 800°C
Hat Treatment
Detection of bomn within carbides and boron segregation susceptibility at boundaries (5.0°SûS15.00) and general boumhies (D29 and 15.O0<840.O0 pet Brandon's criterion in CP and G B E ~ AUoy 304 kat treateâ for 3, 10 i 20 min at 80O0C.
-C~mpariag Figures 5.14 and 5.15 showed that boron had a strong for carbides at
genersl bouadaries.
As shown in Figure 5.15, bomn segregation was detected at some Z1 boundaries
a d al1 general boundaries in the samples heat treated for 3 min at 8ûû°C but was not dctected
at x1 and general boundaries in the samples heat treated for 10 and 20 min at 800°C. As
shown in Figure 5.15, with increasing annealing time at 800°C boron desegregated fiom both
x1 and pneral boudaries and became incorporated into carbides.
As shown in Figure 5.15, Z1 boundaries in the sampks kat treated for 3 min at
800°C displayed some resistance to boron segregation whereas general boundaries in the
same samples displayed no resistance to boron segregation. A misorientation angle limit
between 5.0° and 15.0° definhg a structurai field of high resistaace to bron segregation at
Z1 bouadaries was not determined. As show in Figure 5.15, boron segregation was detected
at a XI boundary with a misorientation angle as low as 6.4O.
Figures 5.16 to 5.18 show SIMS boron images illustrating the resistance and
susceptibility of El boundaries to boron segregation in the samples ka t treated at 1000 and
llOO°C. Figures 5.19 to 5.21 show SIMS boron images illustrating the resistance of x1 boudaries to carbide precipitation and the resistance and suscept ibilit y of 1 boundaries to
boron segregation in the samples k a t treated a - 800°C. The resistance of El boundaries to
both boron segregation and carbide precipitation was likely due to their highly ordered
structures, which are close to that of a single crystal (Le., arrays of primary dislocations).
SIMS Boron Image
OIM Map
Fig. 5.16. SIMS boron image showing m boron segregation at two El boudaries (8=8.0° and H l . 1 O ) in CP Alloy 304 heat treated for 2.5 min at 1 OOO°C.
SIMS Boron Image
OIM Map
Fig. 5.17. SIMS bomn image showhg boron segregation at a z1 boundary (û=9.S0) and no bomn segregation at a XI boundary (8=12.9O) in G B E ~ Alloy 304 heat tteated for 2.5 min at 1 OOO°C.
SIMS Boron Image
OIM Map
Fig .S . 1 8. SLMS boron image showing no boron segregation at a z1 boundary (0=6.S0) in CP Alloy 304 heat treated for 2 min at 1 100°C.
SIMS Bonn image
SEM Micrograph
OIM Map
Fig. 5.19. SEM micmgraph showing no localized corrosion (i.e., no carbide precipitation) at three cl bounâaries (9=5.g0, û=6.4* and 8 = 1 0 . 4 O ) in CP Ailoy 304 heat treated for 3 min at 800°C. SIMS bomn image showiag boron segregation at the 6.4' LI boundary and no boron segregation at the 5.9" and 10.4" Z1 boundaries.
SIMS Boron Image
SEM Micrograph
OIM Map
Fig. 5.20. SEM micmgraph showing no localized corrosion (i.e., no carbide precipitation) at a z1 boundary ( M . 7 O ) in G B E ~ M o y 304 k a t treated for 10 min at 8ûû°C. SIMS bomn Unage showhg no boron segregation at the 6 . 7 O XI bouadaty*
- -
SIMS Boron Image
SEM Micrograph
OIM Map
Fig. 5.21. SEM micrograph sbowing localized corrosion (i.e., carbide precipitation) at a general boumiery (x>29 and e17.2') and no localized corrosion (Le., no carbide precipitation) at two El boundaries (8=10.2* and t3=13.8O) in CP AiIoy 304 bat treated for 20 min at 8ûû°C. SIMS boron image showing bomn within carbides at the 1 7 . 2 O g e d bouadary and no bomn segregation at the 10.2* and 13.8' çl boundaries.
85
5.4.u Boudaries
Ail the grain boundaries chmterized as W bounàaries in the CP and G B E ~ ~
samples k a t treated at 1000, 1 100 and 8ûû°C were determined to have the twin orientation.
The majority (i.e., -95%) of the twin boundanes characterized in each sample were
detennined to have an angular deviation (Ag) 8.0'. A total of 419 D twin boundaries
satisfying Brandon's criterion (i.e., A0G.7O) (Brandon, 1966) were characterized in the 10
heat-treated samples (see Tables 5.1 to 5.3). 399 of the 41 9 W twin boundaries were found
with A012.0° and 20 of the 419 twin boundaries were found with A 0 between 4.6* and
8.7O. No x3 twin boundaries were found with A0 between 2.0" and 4.6'.
In the CP and G B E ~ ~ samples heat treated at 1000 and 1 100°C 175 coherent z3
twin boundaries and 66 incoherent U twin boundaries satisfying Brandon's criterion were
characterized. 164 of the 175 coherent twin boundaries were found with A012.0° and t 1
of the 175 coherent twin boundaries were found with A8 between 4.6" and 8.7". Al1 66
incohetent twin boundaries were found with A812.0°.
Table 5.7 shows the bomn segregation susceptibility at coherent U twin
boundaries satisfying Brandon's criterion but not the Palumbo-Aust criterion
(i.e., 6.0°<60S8.70) (Brandon, 1966 and Palumbo and Aust, 1990) in the CP and G B E ~
samples heat treaîed at 1000 and llûû°C. As s h o w in Table 5.7, coherent U twin
bouadaries with A 0 between 6.0° and 8.7' âisplayed high resistance to boron segregation.
Table 5.8 shows the boron segregation susceptibility at coherent and incoherent D
twin boudaries satisfying the Palwnbo-Aust criterion (i.e., A8S6.0°) (Palumbo and Aust,
1990) in the CP and G B E ~ ~ samples heat treated at 1000 and llOO°C. As show in
Table 5.8, both coherent 5 twin boundaries with A M . O O and incoherent u twin bounâaries displayed high resistance to bomn segregation.
In the CP and G B E ~ ~ samples ka t treated at 8ûû°C 244 coherent D twin
boundaries and 40 iecoberent twin boundaries satismg Brandon's criterion were
---. - -- - Tabic5.7.-Bofon. s!grqetioa swceptibility ot cohereat U tdn boudaries satisEViog Brandon's criterion but not the Pahimbo-Aust criterion (Le., 6.0°d0<8.70) in CP and G B E ~ ~ Alloy 304 heat treated at 1ûûû and 1 1000~.
Co herent Twin
Boundaries (6.0°40S.70)
2.5 min at tOOO°C
2 min at 1 10O0C
Not Detected at U2
Not Detected at 2/2
G B E ~ ~ Alloy 304
Not Detected at 1/1
Not Detected at 1/1
Table 5.8. Boron segregation susceptibility at coheremt and incoherent W twin boundaries satisfying the Palumbo-Aust criterion (Le., A&6.0°) in CP and G B E ~ ~ Alloy 304 kat treated at 1000 and 1 lûû°C.
2.5 min at 1 O0O0C
2 min at 1 100°C
Co herent 5 Twin
Boundaries (6016. 0°)
CP Alloy 304
G B E ~ ~ Alloy 304
Not Detected at 38/38
Not Detected at 36/36
Not Detected at 53/53
Not Detected at 42/42
Not Detected Incoherent 1 Alloy304 Cp 1 N0t57ted 1 at 13/13 z3 Twin
Boundaries (AO*.OO) Not Detected
at 27/27 G B E ~ ~
Alloy 304 Not Detected
at 19/19
charactgizeâ. 235 of the 244 coberent twin boundaries were found with A8s2.0° and 9 of -4. - - the 244 cokent twin boundaries were found with A0 between 4.6" and 8.7O. Al1 40
incoherent C3 twin boundaries were found with A06.0°.
Table 5.9 shows the carbide precipitation susceptibility at coherent twin
boundanes satisfjing Brandon's criterion but w t the Pdumbo-Aust cnterion in the CP and
G B E ~ ~ samples heat treated at 8û0°C. As shown in Table 5.9, carbide precipitation was not
detected at coberent W twin boundaries with A0 between 6.0° and 8.7" in the samples heat
treated for 3 min at 8û0°C but was detected at those in the samples heat treated for 10 and
20 min at 800°C. In the samples heat treated for 10 and 20 min at 800°C coherent W twin
bouadaries with A9 between 6.0° and 8 . 7 O displayed no resistance to carbide precipitation.
Table 5.10 shows the carbide precipitation susceptibility at ccherent and incoherent
twin boundaries satisQing the Palumbo-Aust criterion in the CP and G B E ~ sarnples heat
treated at 8ûû°C. As shown in Table 5.10, carbide precipitation was not detccted at coherent
twin boundaries with A8S6.0° in the samples k a t treated for 3 min at 8OO0C but was
detected at som of those in the samples heat treated for 10 and 20 min at 800°C. In the
sarnples heat treated for 10 and 20 min at 800°C coherent D twin boundaries with A&6.0°
displayed high resistance to carbide precipitation. The two coherent twin boundaries
found susceptible to carbide precipitation were detemrined to have Aû between 5.3O and 6.0°.
Carbide precipitation was not detected at any of the cohcrent Q twin boundaries with
AOQ.OO.
As shown in Table 5.10, carbide precipitation was not detected at incoherent
twin boundaries satisfying the Palumbo-Aust criterion in the sarnples heat treated for 3 and
10 min at 80°C but was detected at those in the samples k a t treated for 20 min at 8OO0C. In
the samples k a t treated for 20 min at 800°C incoherent D twin boundaries displayed no
resistance to carbide precipitation.
Table 5.1 1 shows the detection of boron within carbides at coherent twin
boundaties satisfying Brandon's criterion but aot the Palumbo-Aust criterion in the CP and
-- - - 'Fable 5.9. C~bide precipitation ausceptibiw at cahereat 5 twm bouildaties sat isfying
Brandon's criterion but not the Palumbo- Aust criterion (Le., 6.0°<Aû18.70) in CP and GBE'~ Alloy 304 heat tteated for 3,10 and 20 II& at 800°C.
Not Detected Co herent 1 A l g 3 0 4 1 at l l l 1
3 min at 800°C
None Found
Table 5.10. Carbide precipitation susceptibility at coheient and incoherent twin boudaries satisfying the Palumbo-Aust criterion (Le., AOB.OO) in CP and G B E ~ AUOy 304 heat treated for 3, 10 and 20 min at 800°C.
10 min at 8ûû°C
Co herent C3 Twin
Boundaries (A&6.0°)
20 min at 8OO0C
Inco herent C3 Twin
Boundaries (A0S6.O0)
Alloy 304 at 55/55
3min at 8OO0C
CP Alloy 304
10 niin at 8OO0C
G B E ~ ~ Alby 304
GBP Alloy 304
20 min at 800°C
Not Detected at S/S
Not Detected at 39/39
Not Detected at 13/13
Not Detected at 414
Not Detected at 49/49
Nat Detected at 9/9
Not Detected at 23/23
Table 5.11. Detection of boron within carbides at coherent twin boundaries satisfying B d o n ' s criterion but not the Pslumbo-Aust criterion (Le., 6.0°<b0<8.70) in CP and G B E ~ ~ Alloy 304 kat treated for 3, 10 and 20 min at 800°C.
10 min 20 min at 800°C at 800°C
1 CP INotal;ytedI None AMOY 304 Found
Table 5.12. Detection of boron withh carbides at coherent and incoherent D twin bounderies satisfying the Palumbo-Aust criterion (Le., AO&.OO) in CP and G B E ~ ~ Alloy 304 heat treated for 3,10 and 20 Mn at 8ûû°C.
Coherent C3 Twin
Boundar ie s (A&6.0°)
CP AUoy 304
Not Detected at 55/55
G B E ~ ~ Alloy 304
Not Detected at 39/3 9
Inco herent D Twin
Boundaries (A816.O0)
10 min at 8OO0C
20 min at 80o0C
Not Detected at 49/49
CP Alloy 304
G B E ~ ~ Aihy 304
Not Detected at 23/23
Not Detected at 5 6
Not Detected at 13/13
GBE? e l e s heat treated at 800°C. As shown in Table 5.11, bomn was detected within &
carbides at coherent twin boundaties with A0 between 6.0° and 8.7'. Cornparhg
Tables 5.9 and 5.1 1 showed that bomn had a strong aflhity for carbides at coherent twin
boudaries with A0 between 6.0° and 8.7'.
Table 5.1 2 shows the detection of boron within carbides at coherent and incokrent
twin boundaries satisfjhg the Palumbo-Aust criterion in the CP and G B E ~ ~ samples heat
treated at 800°C. As shown in Table 5.12, boron was detected within carbides at coherent U
twin boundaries with A0S6.0° and incoherent D twin boundaries. Cornparimg Tables 5.10
anâ 5.12 showed that boron had a strong affinity for carbides at coherent Q twin boundaries
with A06.O0 and incoherent C3 twin boundaries.
Table 5.13 shows the bomn segregation susceptibility at coherent U twin
buadanes satisfying Brandon's criterion but not the Palumbo-Aust criterion in the CP and
G B E ~ samples k a t treated ai 800°C. As shown in Table 5.13, boron segregation was
detected at coherent D twin boundaries with A9 between 6.0° and 8.7' in the sampks heat
treated for 3 min at 800°C but was not detected at those in the samples heat treated for 10 and
20 min at 800°C. In the samples heat treated for 3 min at 800°C coherent twin boundaries
with A0 between 6.0° and 8.7' displayed no resistance to boron segregation. Comparing
Tables 5.11 and 5.13 showed that with increasing annealing t h e at 800°C boron
desegregated h m cohmnt twin boudaries with A0 between 6.0' and 8 . 7 O and became
incorporateci into carbides.
Table 5.14 shows the ûoron segregation suxeptibility at wherent and incoherent
D twin bouadaries satisfying the Palumbo-Aust criterion in the CP and G B E ~ ~ SBrnples heat
treated at 8ûû°C. As shown in Table 5.14, boron segregation was detected at some of the
coberent D twin boundaries with A0S6.0° in the samples heat treated for 3 min at 800°C but
was not detected at those in the sampks heat treated for 10 and 20 min at 8000C. In the
samples beat treated for 3 min at 800°C coherent ,twh boumhies with AOS.OO displayed
bigh resistance to bomn segregatioa The one coherent twin bouidary f o u i susceptible
A --.-A - . - - - -
Table 5.13. Bomn segregation susceptibility at coherent twin boundaries satisfjhg Brandon's criterion but not the Palumbo-Aust criterion (Le., 6.0°d0S.70) in - -
CP and G B E ~ ~ AUoy 304 ka t treated for 3,10 and 20 min at 80°C.
Coherent Twin
Boundar ies (6.0°a041.7")
CP Alloy 304
3 min at 8ûû°C
10 min at 800°C
20 min at 800°C
Table 5.14. Boron segregation susceptibility at coherent and hcohereiit twin boundaries satisfy h g the Pahunbo- Aust criterion (Le., A&6.O0) in CP and G B E ~ ~ Alloy 304 heat treated for 3,10 and 20 min at 800°C.
Coherent C3 Twin
10 min at 800°C
Boundarie s (A0<6.O0)
Incoherent Twin
Boundarie s (A0S6.O0)
G B E ~ ~ Alky 304
CP Alloy 304
G B E ~ ~ Alky 304
Not Detected at 39/39
Not Detected at 49/49
Not Detected at 4/4
Not Detected at 919
20 min at 8OO0C
Not Detected at 30/30
Not Detected at 23/23
Not Detected at 515
Not Detected at 414
to boron - - L a & sgpegation was detemiined - to - have A9=5.S0. Bomo - segregation was m t detecteà a! -
any of the coherent U twin boundaries with A052.0°. Comparing Tables 5.12 and 5.14
showed that with increasing annealhg time at 800°C boron desegregated nom coherent U
twin boundaries with AOS.OO and became incorporated into carbides.
As sbwn in Table 5.14, boron segregation was detected at incoherent Q twin
buadaries satis@ing the Palumbo-Aust criterion in the samples k a t treated for 3 min at
800°C but was mt detected at those in the samples ka t treated for 10 and 20 min at 800°C.
In the samples kat tmited for 3 min at 800°C incoherent D twin boundaries displayed no
resistance to bomn segregation. Comparing Tables 5.12 and 5.14 showed that with increasing
annealhg time at 8ûû°C boron desegregated h m incoherent 5 twin boundaries and became
incorporated into carbides.
Figures 5.22 and 5.23 show SIMS bron images illustrating the resistance of
coherent and incoherent W twin boudaries to boron segregation in the samples k a t treated
at 1000 and 1 100°C. Figures 5.24 and 5.25 show SiMS bomn images illustrating the
resistance of coherent W twin boundaries to carbide precipitation and bomn segregation and
the susceptibility of incoherent twin boundenes to carbide precipitation and boron
segregatbn in the samples heat treated at 8ûû°C. The resistance of coherent D twin
boundaries satisfying the Pahunbo-Aust criterion to both bomn segregation and carbide
precipitation was Lürely due to their unique twinned structure, which is highly ordered and
fke of secondary grain boundary dislocations. The susceptibility of incoherent U twin
boundaries to both bomn segregation and carbide precipitation illustrates the effect of
variations h the grain b o u m plaw on grain bouidsry properties. Coherent and incoherent
Z3 twin bounderies share the same misorientation but difEer fkom one another by the
crystallographic orientation of the grain bouadery plane. The susceptibility of coherent
twin boundaries with A0 between 4 . 6 O and 8.6O to both boron segregation end carbide
precipitation was likely due to their high secondary grain boundary dislocation densities
resulting h m th& high aagular deviations h m exact 5 twin orientation.
SIMS Boron Image SEM Micrograph
OIM Map
Fig. 5.22. SIMS boron image showing w bomn segregation at several coherent z 3 twin boudaries (CT) (A&2.0°) and incoherent nvin boundaries UT) (A&2.0°) satisfyiag the Paiumbo-Aust criterion in G B E ~ Alloy 304 heat treated for 2.5 min at 1oOO0C.
SIMS Boron Image r X 1
SEM Micrograph
OIM Map
Fig. 5.23. SIMS bomn image showing no boron segngation at several coherent twin buadanes (CT) (A=.o0) and incoherent twh boudaries UT) (A6a.0°) satisfying the Palumbo-Aust criterion and no boron segregation at a coherent D twin boundary satisfjing Braidon's criterion but wt the Palumbo-Aust criterion 0 (Aû=8.3O) in CP Ailoy 304 heat treated for 2 min at 1 lûû°C.
SIMS Boron Image SEM Micrograph
OIM Map
Fig. 5.24. SEM micmgraph showing no localized corrosion (i.e., no carbide pmipitation) at several coherent twin boundaries (CT) (AOSt.OO) and incoherent twiii boundaries (IT) (A0S2.0°) satisfying the Palumbo-Aust criterion in G B E ~ ~ AUoy 304 hrat treated for 3 min at 800°C. S M S boron image showing boron segregation at the incoherent D hNin bouadanes and no boron segregation at the coherent c3 twin boundaries.
SIMS Boron Image
- -
SEM Micrograph
OIM Map
Fig. 5.25. SEM micmgraph showing localized comsion (Le., carbide precipitation) at an incoheient twin bormdery (IT) (AûG.OO) saîisfLing the P h b o - A u s t criterion and no localiwd comsion (i.e., no carbide precipitation) at several coherent twin boutdaries (CT) (AOâ.O0) satiPfying the Palurnbo-Aust criterion in CP M o y 304 kat treated for 20 min at 800°C. SIMS boron image showing boron within carbides at the incoherent twin boundary and no boron segregation at the coherent Q twin bounàaries.
5.5. X9 Boundaries
In the CP and G B E ~ ~ samples heat treated at 1000, 1100 and 800°C a total of 41
x9 bouadaries satisfying Brandon's criterion (i.e., A0<-5.0°) (Brandon, 1966) were
chsracteri& (see Tables 5.1 to 5.3). 11 of the 41 D boundaries were found satisfjhg
Brandon's criterion but not the Palumbo-Aust criterion (i.e., 2.4°4&5.00) (Brandon, 1966
and Palumbo and Aust, 1990) and 30 of the 41 D boundanes were found satisfjhg the
Pahunbo-Aust criterion (i.e., A08.4O) (Palumbo and Aust, 1990). AU 30 z9 boundaries
satiswg the Palumbo-Aust criterion were found with AOS1.2O. No x9 bundaries were
found with A0 between 1.2O and 2.4O.
Figure 5.26 shows the boron segregation susceptibility at x 9 boboundaries satisfying
Brandon's criterion but not the Palumbo-Aust criterion in the CP and G B E ~ ~ samples heat
treated at 1000 and 1 100°C. As shown in Figure 5.26, x9 boundaries with A 0 between 2.4'
and 5.0° displayed no resistance to boron segregation.
Figure 5.27 shows the bomn segregation susceptibility at x 9 bundaries satisfying
the Palumbo-Aust criterion in the CP and G B E ~ ~ -les k a t tnated ai 1000 and 1 lûû°C.
As shown in Figure 5.27, D boundaries with AOS2.4O displayed some resistance to boton
segregation. Those D boundaries founâ resistant to boron segregation were detemiined to
have AOSO.9O.
Figure 5.28 shows the carbide precipitation sweptibiüty at D bounàaries
satisfying Brandon's criterion but not the Palumbo-Aust criterion in the CP and G B E ~ ~
samples heat treated at 8ûû°C. As shown in Figure 5.28, x9 boundaries with AB between 2.4'
and Seo0 displayed low resistauce to carbide precipitation.
Figure 5.29 shows the carbide precipitation susceptibility at X9 boundaries
satismg the Palumbo-Aust criterion in the CP and G B E ~ ~ samples heat treated at 800°C.
As shown in Figure 5.29, x9 boundaries with A&2.4O displayeâ some resistance to carbide
pneipitation. Wi incmsing anneaihg time at 8W°C Z9 bouedaries with A 8 ~ 2 . 4 ~
2.5 min 2 min at 1000°C at llûû°C
...- - - - . - - . .
Heat Treatment
B = Boron Segregation Detected A
Fig. 5.26. Bomn segregation susceptibility at x9 boundarks satisfying Brandon's criterion but not the Paiumbo-Aust criterion (Le., 2.40°40S5.000) in CP and G B E ~ ~ Alloy 304 heat treated at 1 O00 and 1 1 ûû°C.
--- . -.
1 B = Boron seGgation Detected 1
2.5 min 2 min at Iûûû°C at 1 1 ûû°C
Heat Treatment
Fig. 5.27. Boron segregation susceptibility at w b o a s satisfying the Palumbo-Aust critenon (Le., A0Q.40°) in CP and G B E ~ A b y 304 k a t treated at 1000 and 1 1oO0C.
3 min 1 O min 20 min at 8ûû°C at 8ûû°C at 800°C
Heat Tteatment
Fig. 5-28. Carbide precipitation susceptibility at x9 boundai-ies satisfying Brandon's criterion but not the Palumbo-Aust criterion (Le., 2.4O040d.0û0) in CP and GBE" Alioy 304 heat treated for 3,10 and 20 min at 800°C.
3 mm 10 min 20 mm at 8ûû°C at 8ûû°C at 8W°C
Heat Tnatment
Fig. 5.29. Carbide precipitation susceptibility at bouadanes satismg the Palumbo-Aust criterion (i.e., A0S2.40°) in CP and G B E ~ ~ Alloy 304 heat treated for 3, 10 and 20 min at 8ûû°C.
continueci to display sorne resistaace to carbide precipitation. Those x9 boundaries found -" - -- -%- -
resistant to boron segregation were detennined to have A8S0.9O.
Figure 5.30 shows the detection of bomn within carbides and borna segregation
susceptibility at z9 boundaries satisfying Brandon's criterion but not the Palumbo-Aust
criterion in the CP and G B E ~ ~ amples kat treated at 800°C. As show in Figure 5.34
boron was detected within carbides at z9 boundaries with A0 between 2.4" and 5.0'.
Comparing Figures 5.28 and 5.30 showed that boron haâ a strong affinity for carbides at D
boundaries with A0 between 2.4' and 5.0".
As show in Figure 5.30, boron segregation was detected at z9 boundaries wit h A0
between 2.4' anà 5.0" in the samples heat treated for 3 min at 8ûû°C but was detected at only
one of those in the samples heat treated for 10 and 20 min at 800°C. In the samples heat
treated for 3 min et 800°C z9 boboundaries with A0 between 2.4' and 5.0° displayed no
resistance to boron segregation.
Figure 5.31 shows the detection of bomn within carbides and boron segregation
susceptibility at boudaries satisfying the Palumbo-Aust criterion in the CP and G B E ~ ~
samples heat treated ai 800°C. As shown in Figure 5.3 1, bomn was detected within car bides
at x9 boundaries with A0Q.4°. Comparing Figures 5.29 and 5.3 1 showed that boron had a
strong attinity for carbides at x9 boumlanes with AW.4O.
As shown in Figure 5.31, boron segregation was detected at 2 of the 6 z9
boundaries with AGG.4O in the samples kat treated for 3 min at 8ûû°C but was not detected
at those in the samples heat treated for 10 and 20 min at 800°C. In the samples heat treated
for 3 min at 800°C boundaries with AOS2.4O displayed some resistance to boron
segregation. Those x9 boboundes found resistant to boron segregation were determined to
bave A0S0.g0. As sbown in Figure 5.31, with increasing annealhg tirne at 800°C boron
desegregated h m z9 bouadaries with AOQ.4O anà became incorporated into carbides.
3 min 10 min 20 min at 800°C at 8ûû°C at 800°C
-a--__ -_*- - -
Heat Tnatment
&i = Born Detectcd Within Carbides B = Bomn Segregation Detected At Boundary
Fig. 5.30. Detection of boron within carbides and boron segregation susceptibility at x9 bounùaries satisfying Brandon's criterion but not the Palumbo-Aust criterion (i.e., 2.40°<60S5.000) in CP and GBE" Alioy 304 heat treated for 3, 10 and 20 min at 80O0C.
Bc = Boron C)etected Within Carbides 1 B = B n m Segregation DM«d At Boundary
3 min 10 min 20 min at 8Oo0C at 8ûû°C at 8OO0C
Heat Treaîment
Fig. 5.3 1. Detection of borna within carbides and boron segregation susceptibility at z9 bounderies satisfying the Palumbo-Aust c M o n (Le., A9a.40°) in CP and G* Alloy 304 heat treated tbr 3,10 and 20 min at 80°C.
- Figures 5.32 to 5.35 show SIMS boron images illustrating the tesistance and
susceptibiüty of D boundaries (arnong other low-Z boundaries to be discussed in
Section 5.6.) to bomn segregation in the samples heat treated at 1000 and 1 100°C.
Figures 5.36 to 5.38 show SIMS boron images illustrathg the resistaoce and susceptibility of
D boundaries (among other low-x boundaries to be discussed in Section 5.6.) to carbide
precipitation and boron segregation in the samples heat treated at 800°C. The resistance of
D boundaries satis@ing the Palumbo- Aust criterion to both boron segregation anà carbide
precipitat ion was likely due to theh low secondary grain boundary dislocation densities
resulting fiom theù low deviations nom exact x9 orientation. The susceptibility of some D boundaries satisfying the Palurnbo-Aust criterion to boron segregation and carbide
precipitation was likely due to the effect of variations in the grain boudas, plane on the
distributions of the secoDdary grain boundary dislocations. The susceptibility of D
boundanes satisfying Brandon's criterion but not the Pahunbo-Aust criterion to both bomn
segregation and carbide precipitation was likely due to their high secondary grain boundary
dislocation densities resulting h m th& high deviations h m exact D orientation.
SIMS Bomn Image
SEM Micrograph
SIMS Boron Image
Fig. 5.32. SIMS boron image showhg bomn segregation at two low-x boundaries (zll (Aû=2.8*) aod D3 (A9=2.3°)) satisfying Brandon's criterion but not the Palumbo-Aust criterion ad m, boron segregation at a z9 boundary (AO=O.BO) satisfying the Palumbo-Aust criterion (FA) in CP Alloy 304 heat treated for 2.5 min at lûûû°C.
SIMS Boron Image SEM Micrograph
OIM Map
Fig. 5.33. SIMS boron image showhg borna segregation at two low-x boundaries (XI 1 (Aû=2.4*) and D7 (Ae1 .O0)) satisfying Brandon's criterion but not the Palumb-Aust criterion, boron segregation at tbree z9 boundaries (clockwise fiom bottom-lefi, A9=û.S0, Aû=l.OO and A8i0.4°) satis-g the Paiumbo-Aust criterion (FA) and no boron segregation at a W boundary (center-right, A8-0.7O) satisfjhg the Palumbo-Aust criterion (P-A) in ( 3 I . 3 ~ ~ ~ Alloy 304 heat treated for 2.5 min at 1000°C.
SIMS Boron Image SEM Micrograph
OIM Map
Fig. 5.34. SIMS boron image showing boron segregation at two low-z boundaries (XI5 (A0=2.1°) anci D3 (Aû=2S0)) satisfying Brdoa's criterion but not the Palumbo-Aust criterion and no boron sepgation at a D boundary (A8=0.4*) satisSinp the Pdumbo-Aust criterion (PA) in CP Alloy 304 heat treated for 2 min at 1 1 O°C.
SlMS Born Image
SEM Micrograph
OXM Map
Fig. 5.35. SIMS boron image showing boron segregation at two bw-x boudaries (Dl (Aû=1.3°) and D7 (A9=1.O0)) satisfying Brandon's criterion but not the Pahuabo-Aust criterion, bomn segregation at two z7 boboundaries (lefi to rigbt, AB-1 .1° a d Aû=1 .go) satisfying the Palumbo-Aust criterion and no boron segregation at a w bouadary (AH.8') dsf j ing the Palumbo-Aust criterion in G B E ~ M o y 304 heat tmited for 2 min at 1 lûO°C.
SIMS Bomn Image SEM Micrograph
OM Map
Fig. 5.36. SEM micrograph showing localized corrosion (i.e., carbide precipitation) at two D 7 boudaries (lefi-to-right, A8=0.8* and Aec0.7O) satisfying the Palumbo-Aust criterion (P-A), no locaîîzed comsion (Le., no carbide precipitation) at two z9 boundaries (lefi-to-right, A04.7O and A H . 9 O ) satisfyiog the Palumbo-Aust critenon (P-A) and no localized comsion at a a3 boundary (A0=1.8O) satisfyEg Brandon's criterion but not the Palwnbo-Aust criterion in G B E ~ ~ Alioy 304 kat treated for 10 min at 8ûû°C. SiMS boron image showing boron within carbides at th two a 7 boundaries and no boron segregation at the D3 bomdary and the two W boundaries.
SIMS Boron Image
SEM Micrograph
OIM Map
Fig. 5.37. SEM micrograph showiiig localized corrosion (i.e., carbide precipitation) at two 7 bouadaries ble fi-to-right, A81).S0 and Aû=û.ao) satisfying the Palumbo-Aust criterion (P-A) and bcalized corrosion at a boundary (A0=2.6') and a D 7 bouadary (A8-2.4O) satisfying Brandon's criterion but not the Palumbo-Aust criterion in G B E ~ ALlOy 304 heat treated for 10 min at 80°C. SIMS bomn image showing boron wîthin carbides at the x9 boboundary and the three D 7 boutxiaries.
SIMS Boron Image
SEM Micrograph
OIM Map
Fig. 5.38. SEM rnicrograph showing localized comsion (Le., carbide precipitation) at a D bouadary (A8=0.79 satisfying the Pahunbo-Aust criterion (FA) and no localizeâ corrosion (i.e., m carbide precipitation) at a D7 boundary (A0=1.4O) satiswg Brandon's criterion but not the Pahunbo-Aust criterion in CP Alloy 304 heat treated for 20 min at 8ûû°C. SIMS boron image showing boron within carbides at the x9 bounâary anci no boron segregation at the n7 boundary.
In the CP and G B E ~ ~ samples heat treated a! 1000,1100 and 800°C a total of 80
other low-x boumlaries (m9 excluding El, and Z9) satisfjkg Brandon's criterion
(Le., ~ ~ 1 5 ~ z ' ' ) (Brandon, 1966) were cbaracterized (sa Tables 5.1 to 5.3). 46 of the 80
0 t h low-I: boundaries were foimi satisfjhg Brandon's criterion but not the Palumbo-Aust
criterion (Le., 1 ~ ~ ~ ' ' ~ ~ 8 6 l 5°x") (Brandon, 1966 and Palumbo and Aust, 1990) and 34 of
the 80 other low-x boundaries were found satisfying the Palumbo-Aust criterion
(Le., ~ 8 ~ 1 5 ~ r ' ~ ) (Palwnbo and Aust, 1990). AU of the other low-Z boundaries excluding
627 boudaries satisijhg the Palumbo-Aust criterion were found with AOL1.OO. All a7
boudaries satisfjing tbe Palurnbo-Aust criterion were found with A010.8O.
Figure 5.39 shows the boroa sepgation susceptibility at other low-z bundaries
satisfjhg Brandon's criterion but not the Palumbo-Aust criterion in the CP and G B E ~
samples k a t treated at 1000 and 1 100°C. As shown in Figure 5.39, other low-x boudaries
with A0 between 1 5 and 1 5 displayed no resistance to boron segregation.
Figure 5.40 shows the boron segregat ion susceptibility at other 10 w-x boundaries
satis-g the Palumbo-Aust criterion in the CP and G B E ~ ~ samples heat treated at 1000 and
1 100°C. As shown in Figure 5.40, other low-x boudaries with ABSI S0FM displayed no
resistance to bomn segregation.
Figure 5.41 shows the aubide precipitation susceptibility at other low-z
boundsnes satisfyhg Brandon's criterion but w t the Palumbo-Aust criterion in the CP and
GBE" samples k a t treated at 800°C. As shown in Figure 5.41, other low-x boundaries with
A0 ktween 1 5 0 ~ ~ ' ~ aid 15°r1n displayed low resistance to carbide precipitation. With
increasing annealing t h e at 8ûû°C other low-x bouadsries with A0 between 15°x5'6 and
150x'~ displayed increased susceptibility to carbide precipitation.
Figure 5.42 shows the carbide precipitation susceptibility at other low-x
buadenes satisfyiag the Palumbo-Aust criterion in the CP and G B E ~ ~ samples heat treated
2.5 min 2 min at 1 OOO°C at 1 100°C
Heat Treatment
Fig . 5.39. Bomn segregation susceptibility at other low-x boundaries (m9 exc luding 1 , x3 and D) satiseing Brandon's criterion but not the Palumbo-Aust criterion (Le., 1 5°xs%A€K1 s~x'') in CP and G B E ~ ~ Alloy 3304 heat treated at 1000 and 1 10°C.
1 B = Bomn Segregation Detected 1
2.5 min 2 min at lOOOOC at 1 100°C
Heat Treatment
Fig. 5.40. Bomn segregation susceptibility at other low-z bouidaries (m9 excluding x1, 5 ad x9) satis-g the Palumbo-Aust criterion (i.e., ~ 0 ~ 1 5 ~ ~ ~ ' ~ ) in CP and G B E ~ ~ AllOy 304 heat treated at lûûû and 11 WC.
3 min 10 min 20 min at 8û0°C at 800°C at 80°C
Heat Treatment
Fig . 5.4 1. Carbide precipitation susceptibility at other low-x boundaries (w9 exc luding Xl, x3 and D) satisfying B d o n ' s criterion but mt the Palumbo-Aust criterion (Le., 15°~s16d8~150~'n) in CP and G B E ~ ~ Alloy 304 k a t treated for 3,10 and 20 min at 800°C.
1 C = Carbiie Precipitation Detected 1
3 min 10 min 20 min at 8ûû°C at 8ûûaC at 80o0C
Fig. 5.42. Carbide precipitation susceptibility at othet low-x boundaries (m9 excluding xi, and D) satisfying the Palumbo-Aust criterion (Le., A Z K ~ SOZ*~~) in CP a d G B E ~ Alloy 304 heat treatcd for 3,10 d 20 min at 8ûû°C.
- . _ -.-- at-8W°C. As shown in Figure 5.42, other - - low-x boundaries with ABS~SOZ~'~ displayed low
resisiaace to carbide precipitation. With increasing annealing tirne at 800°C other low-x
boundanes with ~ 0 ~ 1 5 0 ~ ~ ' ~ displayed increased susceptibility to carbide precipitation.
Figure 5.43 shows the detection of borna within carbides and boron segregation
susceptibility at other low-x boundaries satisfying Brandon's criterion but not the
Palumbo-Aust criterion in the CP and G B E ~ ~ samples heat treated at 800°C. As shown in
Figure 5.43, boron was detecteà within carbides at other low-x boundaries with A0 between
15°~"6 and ~ S ~ Z ' ~ . Camparing Figures 5.41 end 5.43 showed that boron had a strong
afhity for carbides at other low-z boundaries with A0 ktween 1 5°z5'6 and 1 s ~ ~ ' ' . As shown in Figure 5.43, boron segregation was detected at other low-T,
boundanes with A0 between 1 5 ~ x " and 1 5 0 ~ " in the samples heat treated for 3 min at
800°C but was detected at only one of those in the samples heat treated for 10 and 20 min at
800°C. In the samples heat treated for 3 min at 8W°C other low-x boundaries with A 0
between 15°x5'6 and 1 5 0 r ' ~ displayed no resistance to boron segregation. As shown in
Figure 5.43, with increasing annealhg t h e at 800°C boron desegregated nom other low-x
boundaries with A0 between 15°x5'6 and 1~~x'" and becme incorporated into carbides.
Figure 5.44 shows the detection of bomn within carbides and bron segregation
susceptibility at other low-x boundaries satisfying the Palumbo-Aust criterion in the CP and
G B E ~ ~ saaiples heat treated at 800°C. As show in Figure 5.44, boron was detected within
carbides at other low-x boundaries with AM 50r'". CompaMg Figures 5.42 and 5.44
showed that boron had a strong affinity for carbides at other low-Z boundaries with
~ 0 ~ 1 5°x5'6.
As shown h Figure 5.44, boron segregation was detected at other low-x
bouadaries with ~ 0 ~ 1 5 ~ x ~ ' ~ in the samples heat treated for 3 min at 800°C but was detected
at only oae of those in the samples heat treated for 10 and 20 min at 8ûû°C. In the samples
bat treated for 3 min at 800°C other bw-z boundaries with A ~ S ~ S O ~ " displayed no
& = Boron Detectd Within Carôides B = Boron Segregation Detected At Boundary
3 min 10 min 20 min at 800°C at 800°C ai 800°C
Heat Treatment
Fig. 5.43. Detection of bomn withh carbides and boron segregation susceptibility at other low-x boundaries (m9 excluding XI, and D) satisfying Brandon's criterion bit mt the Palumbo-Aust criterion (i.e., 1 ~ ~ 9 ' ~ e W ~ l 5°r'12) in CP and G B E ~ ~ Alloy 304 kat treated for 3,10 and 20 min at 800°C.
&r = Bomn Detected Within Carbides B = Boron Segregation Detected At Boundary 1
3 min 10 min 20 min at 8ûû°C at 8ûû°C at 800°C
Heat Trmtment
Fig. 5.44. Detection of bomn withm carbides and boton segregation susceptibility at other low-x boundaries (m9 excluding 1 U a d x9) satisfjhg the Palumbo-Aust criterion (Le., ~ 0 ~ 1 5 ~ ~ ~ ~ ~ in CP and G B E ~ ~ Alloy 304 k a t treated for 3 ,10 and 20 min at 8000C.
- &stance ta bumnsegregatian, As show in Figure 5.44, with increasing anneaiing tirne at --
80°C bron desegregated fiom other low-x boundaries with ~ 0 ~ 1 5 ~ ~ ~ ' ~ and became
incorporateci into carbides.
Figures 5.32 to 5.35 show SIMS boron images ülustrating the susceptibility of
other low-x boundaries (among x9 bouadaries discussed in Section5.5.) to boron
segregation in the samples heat treated at 1000 and 1 1 OO°C. Figures 5.36 to 5.3 8 show SIMS
boron images illutrat hg the suscept ibilit y of other low-x boundaries (among other z 9
bouadaries discussed in Section 5.5.) to carbide precipitation and boron segregation in the
samples heat treated at 800°C. The susceptibility of other low-z boundaries excluding a 7
boundaries with A84).8* to both boron segregation and carbide precipitation was likely due to
theH high secondary grain boundary dislocation densities resuhing fiom their high deviations
fiom exact low-x orientations. The susceptibility of D7 boundaries with A0S0.8° to b t h
boron segregation and carbide precipitation tends to indicate that a value of 27 is likely too
high for grain boudaries to display signiticant resistance to boron segregation and carbide
precipitat ion.
5.7. General Boundaries c .29 ) - . --
Figure 5.45 shows the boron segregation susceptibility at general boundaries
(>29) in the CP and G B E ~ satnpies k a t treated at 1000 and 1 lûû°C. As shown in
Figure 5.45, generai boundaries displayed no resistance to bomn segregation.
Figure 5.46 shows the carbide precipitation susceptibility, detection of boron within
carbides and boron segregation susceptibility at general boundaries in the CP and GBE"
sampies heat treated ai 800°C. As shown in Figure 5.46 (a), general boundaries displayed low
resistance to carbide precipitation. With increasing annealing t h e at 8W°C general
boundaries displayed increased susceptibility to carbide precipitation.
As shown in Figure 5.46 (b), boron was detected within carbides at general
boundaries. Cornparhg Figures 5.46 (a) and (b) showed that bomn had a strong affinity for
carbides at general ôoundaries.
As show in Figure 5.46 (c), boron segregation was detected at general boundaries
in the samples heat treated for 3 min at 800°C but was detected et considerably less of those
in the ssmples heat treated for 10 and 20 min at 8ûû°C. In the sarnples heat treated for 3 min
at 8ûû°C general boundaries displayed low resistance to bomn segregation.
Figures 5.47 and 5.48 show SIMS bomn images illustnuing the susceptibility of
general boundaries to boron segregation in the simples heat treated at 1000 and 1 100°C.
Figues 5.49 and 5.50 show SlMS boron images illustratiPig the susceptibility and resistance
of general boundaries to carbide precipitation and boron segregation in the sarnples heat
treated at 800°C. The susceptibility of general bounderies to both boron segregation and
carbide precipitation was likely due to th& disodered structures and high secondary grah
boundary dislocation densities resuhing h m th& high deviations kom exact low-z
orientations.
2.5 min 2 min at 1000°C at 1 HIO°C
Heat Treatment
Fig. 5.45. Bomn segregation susceptibility at geaeral bomdaries ( b 2 9 ) in CP and GBE" Alloy 304 heat treated ai 1000 and 1 lûû°C.
3 min 10 min 20 min ai 80 °C a i 8ûû°C a! 8û0°C
3 min 10 min 20 min ai 800°C at 8000C at 8û0°C
3 min 10 min 20 min at W°C at 800°C at 8000C
Fig. 5.46. (a) Carbide precipitation susceptibility, (b) detection of boron witbin carbides and (c) bomn segregation susceptibility at g e d boumlaries (D29) in CP and G B E ~ Moy 304 heat treaîed for 3,10 and 20 min at 800°C.
SMS Boron Image SEM Micrograph
OIM Map
Fig. 5.47. SIMS boron image showing bomn segregation at severai general boundaries (b29) in G B E ~ Alhy 304 beat treated for 2.5 min at 10ûû°C.
SIMS Boron Image SEM Micrograph
OIM Map
Fig. 5.48. SIMS boron image showing bron segregation at several gewral boundaries (b29) in CP Alloy 304 heat treated for 2 min at 1 lûû°C.
SIMS Boron Image SEM Micrograph
OIM Map
Fig. 5.49. SEM micrograph showing localwd corrosion (i.e., carbide precipitation) at four generai boudaries ( s 2 9 ) and no localized corrosion (ie., m carbide precipitation) at a g e n d boundary (*) in G B E ~ ~ Alloy 304 heat treated for 10 min at 8ûû°C. SIMS borm image showing boron within carbides at the four generai bounderies displaying susceptibility to carbide precipitation and no boroa segregation at the g e d boundary displaying resistance to carbide precipitat ion.
SIMS Boron Image SEM Micrograph
Fig . 5.50. SEM rnicmgraph showing IocaLized conosion (Le., carbide precipitation) at t h general boundaries (>29) and no localized corrosion (i.e., no carbide pmcipitation) at two generai boumlaries (*) in CP Alloy 304 kat treated for 20 min at 800°C. SIMS boron image showing boron withh carbides at the three gewraî boundaries displaying susceptibility to carbide precipitation and no boron segrcgation at îhe two general boundenes displaying resisîance to carbide precipitat ion.
5.8. CP AUov 304 vs. G B E ~ Alloy 304 - - -
Figures 5.51 and 5.52 show the muencies (i.e., number fractions) of grain
ôoundanes displaying resistance to boron segregation in the CP and GBE" samples heat
treated at 1000 and llûû°C, respectively. As show in Figures 5.51 aad 5.52, the G B E ~ ~
samples contained a higher hquency of grain bouadaries displaying resistance to boron
segregation than the CP samples. In the samples k a t treated ai 1000°C the G B E ~ ~ sample
contained 48% grain bundaries resistant to bomn segregation as compared to 34% in the CP
sample. Sirnilarly, in the samples k a t treated at 1 lûû°C the G B E ~ ~ Qarnple contaiaed 49%
grain boundaries resistant to boron segregation as compareci to 36% in the CP sarnple.
Figures 5.53 and 5.54 show the length fktions of grain boundaries displaying
resistance to bomn segregation in the CP and G B E ~ ~ samples heat treated at 1000 and
1 100°C, respectively. As shown in Figures 5.53 and 5.54, the G B E ~ samples contained a
considerably higher length âact ion of grain bouadaries display ing resistance to boron
segregation than the CP samples. In the samples ka t treated at 1000°C the GBETM -le
contained a 59% length fiaction of grain boundaries resistant to boron segregation as
compareci to 43% in the CP sample. Similarly, in the samples heat treated at 1 lûû°C the
G B E ~ ~ m p l e contained a 62% length fiaction of grain boundaries resistant to boron
segregation as compareà to 45% in the CP sample.
Figure 5.55 shows the fiequencies of grain boundaries displayhg resistance to
carbide precipitation in the CP and G B E ~ ~ simples heat treateà at 800°C. As shown in
Figure 5.55, with increasing annealhg tirne at 800°C carbide precipitation susceptibility at
grain bouadaries increaseâ in both the CP and G B E ~ samples. The highest amount of
carbide precipitation at grain bouadaries occurred in the samples k a t treated for 20 min at
8ûû°C. In the samples heat treated fbr 20 min at 800°C the G B E ~ ~ -le contained a higher
muency of grain boumlaries displayhg resistance to catbide precipitation than the CP
a= No Boron Segregation Detected / *= aomn Segregation Dctected
Fig. 5.51. Frequencies of grain boundaries displaying resistance to bomn segregation in (a) CP Alloy 304 and (b) G B E ~ Alloy 304 k a t treated for 2.5 min at 1 OOO°C.
= No Boron Segregation Detected = Boron Segtegation Detected
Fig. 5.52. Freqwncies of grain boundaries displaying resisîance to boron segregation in (a) CP Alloy 304 and (b) GBE" AUoy 304 k a t treated for 2 min at 1 1OO0C.
[3 = No Bomn Segregation Detected
Fig . 5.53. Length h c t ions of grain boundaries display ing resistance to boron segregation in (a) CP Alioy 304 and (b) G B E ~ ~ Alloy 304 heat treated for 2.5 min at 1oOo0c.
O = No Bomn Segregation Detected = Bonn Segregation Detected
;
Fig. 5.54. Length fiactions of grain boimdaries displayhg resistance to boron segregation in (a) CP AUoy 304 and (b) G B E ~ Alloy 304 heat treated for 2 min at 1 100°C.
- - *
O = No Carbide Precipitation Detected -- - .. . - -- & - - - -
CP Alloy 304 G B E ~ A U O ~ 304
min at 800
CP Alloy 304 G B E ~ ~ ~ l l o y 304
min at
Fig. 5.55. Frequeecies of grain boundaries displayhg resistance to carbide precipitation in CP and G B E ~ ~ Afloy 304 heat tteated for (a) 3 min, (b) 10 min and (c) 20 min at 8ûû°C.
.sampleC The G B E ~ ~ sample contained 49?! grain boundaries resistant to carbide precipitation
as compared to 38% in the CP sarnple.
Figure 5.56 shows the length fiactions of grain boundaries displaying resistance to
carbide precipitation in the CP and G B E ~ samples heat treated at 800°C. As shown in
Figure 5.56, the highest amount of carbide precipitation at grain boundaries occurred in the
samples k a t treated for 20 min at 800°C. In the samples heat treated for 20 min at 800°C the
G B E ~ sample containeû a 61% length fiaction of grain boudaries resistant to carbide
precipitation as compared to 47% in the CP sample.
Boron segregation was detected at grain boudaries in the samples heat treated for
3 min at 8OO0C but was detected at considerably fewer grain boundaries in the samples heat
treated for 10 and 20 min at 800°C. Figure 5.57 shows the kquencies of grain boundaries
displaying resistame to boron segregation in the CP and G B E ~ ~ samples k a t treated for
3 min at 800°C. As shown in Figure 5.57, the G B E ~ Qgrnple contained a higher fiequency of
grain bouadaries displaying resistance to boron segregation than the CP sarnple. In the
samples k a t treated 3 min at 800°C the G B E ~ ~ sample contained 47% grain boundaries
resistant to bomn sgregation as compared to 3% in the CP sample.
Figure 5.58 shows the length fiactions of grain boundaries displaying resistance to
bomn sepgation in the CP and G B E ~ ~ -les k a t treated for 3 min at 800°C. As shown
in Figure 5.58, the GBE** simple contained a considerably higher length b t i o n of grain
bouadaries displaying resistance to boron segregation than the CP sample. In the samples
k a t treated for 3 min at 800°C tk G B E ~ ~ sample contained a 57% kngth fraction of grain
boudaries resistant to b r u n segregatioa as cornparrd to 46% in the CP sample.
The higher resistance of the G B E ~ samples to both intergranular boron
segregation ami intergraaular carbide precipitation was likely due to the higher muencies
and higher length fractions of coherent twin boundaries anà x9 bouadaries satisSing the
Palumbo-Aust criterion in the grain bounâary cbaracter distribution.
CP Alloy 304 G B E ~ ~ Alloy 304
min at 8OO0C c CP Alloy 304 G B E ~ ~ Al10 y 304
min at 800
Fig. 5.56. Length fractions of grain bouadaries displaying resistance to carbide precipitatioa in CP and GBE" Alloy 304 heat treated fôr (a) 3 min, (b) 10 min aml (c) 20 min at 8ûû°C.
O = No Boron Segregation Detected = Boron Segregation Detected
Fig. 5.57. Frequencies of grain boundaries displayhg resistance to bomn segregation in (a) CP Alloy 304 and (b) G B E ~ ~ AllOy 304 heat treated for 3 min at 800°C.
--
O = No Boron Segregation Detected = Boron Segregation Detected
Fig. 5.58. Length fkactioas of grain boundaries displaying resistance to boron segregation in (a) CP Aiioy 304 and (b) G B E ~ AUoy 304 kat treated for 3 mi. at 800°C.
---- a . 6 . Conclusions
6.1. Grain Boundarv Chanicter Distributions IGBCDs) (Cha~ter 4.)
1. The âequencies (i.e., number hctions) of low-x boundaries (mg) satisfying
Brandon's criterion (i.e., ~ 0 ~ 1 5 ~ ~ ' ~ ) in conventionaly processed (CP) and grain boundary
engineered (GBE") Alloy 304 in as-received condition were determined to be 47 and 62%,
respectively. The as-received CP material contained 4% x1, 3W Q, 2% x9 and 1 1% other
bw-z boundaries satisfjhg Brandon's criterion. The as-received G B E ~ ~ material contained
3% El , 41% u, 8% x9 and 10% other low-z boundaries satisfying Brandon's criterion.
2. The fkequenc ies of low-I: boudaries sat isQing the Palumbo- Aust c riterion
(Le., ~ 0 ~ 1 5 * x * ' ~ ) in as-received CP and G B E ~ ~ Alloy 304 were determineci to be 35 and
53%' respectively. The as-received CP material contained 4% x1, 29% Z3, 1% z9 and
1% other low-x boundaries satisfying the Palumbo-Aust cnterion. The as-received GBE'~
material contained 3% El, 40% u, 6% z9 and 4% other low-x boundaries satisQing the
Palumbo-Aust criterion.
6.2. Boron Seeregation at Grain Boundaries (Chapter 5.1
1. In both the CP and G B E ~ samples heat treated at 1000 and 1 1 OO°C:
(a) Boron segregation was detected at grain boundaries.
(b) 1 (Io w-mg le) boundaries (5 .0°481 1 5 .O0) displayed some resistance to boron
segregat ion.
(c) Coherent twin boundaries satisfying Brandon's criterion with AOi8.3
displayed high resistance to boron segregation. Incoherent 5 twin boundaries
sat ismg the Palumbo-Aust criterion with A8S2.0° displayed high resistance to
boron segregation.
D bouadanes sa t i swg Braadon's criterion but not the
with A0 between 2.4O and 5.0' displayed w resistance to
Palu&-Aust criterion
bomn segregation. x9
boundaries satisfjhg the Pahunbo-Aust criterion with A0~2.4~ displayed some
&stance to b o m segregation. Those x9 ôoundaries found tesistant to boron
segregat ion were de termined t O have AOSO.gO.
Other low-z boundaries ( X e 9 excluding z1, and x9) satisfjkg Brandon's
criterion but not the Palumbo-Aust criterion with A0 between 1 5°cm and 1 5 0 ~ ' ~
and other low-x boundaries satisfying the Palumbo-Aust criterion with A051 5 0 p ' ~
displayed no resistance to boron segregation.
General boundsties (D29) displayed no resistance to bomn segregation.
In both the CP and G B E ~ ~ samples heat treated at 800°C:
Carbide precipitation was detected at grain boundaries.
Wit h increasing annealing tirne at 800°C carbide precipitat ion suscept ibiiit y at
grain boundaries increased and boron tended to desegregate fiom grain boundaries
and becorne incorporated hto carbides.
1 boundaries displayed some resistance to boron segregat ion and high resistance
to carbide precipitation.
Cofierent x3 twin boundarîes satisfying Branûon's criterion but not the
Palumbo-Aust criterion with A0 between 6.0° and 8.6" and coherent C3 twin
boundaries satisQing the Palumbo-Aust criterion with A0 between 4 . 6 O and 6.0°
displayed low resistance to both boroa segregation and carbide precipitation.
Coherent D twin boundaries satisfying the Paiumbo-Aust criterion with A&2.O0
displayed high resistance to both boron segregation and carbide precipitation.
Incoherent twin boundaries satisfying the Palumbo-Aust criterion with A012.O0
displayed no resistance to boron segregation and low resistance to carbide
precipitat ion.
(e) D boundanes satisfying Brandon's criterion but not the Palumbo-Aust criterion
with A0 between 2.4' and 5.0° displayed low resistance to both boron segregation
and carbide precipitation. x9 boundafies satis@ing the Palumbo-Aust criîerion
with A0a.4O displayed some resistance to both boron segregation and carbide
precipitation. Those z9 bounderies found resistant to both bomn segregation and
carbide prec ipitation were detetmitled to have A8'0.g0.
(s) Other low-z boudaries satisfying Brandon's criterion but not the Palumbo-Aust
criterion with A0 between 15°rs'6 and 1 and other low-x boundaries
satisfying the Palumbo-Aust criterion with ~ 9 ~ 1 5°r5'6 displayed low resistance to
both boron segregation and carbide precipitation.
(h) General bouadaries displayed low resistance to both boron segregation and carbide
precipitat ion.
3. G B E ~ AUoy 304 displayed higher resistance to both intergranular boron
segregation and intergranular carbide precipitation t h a ~ CP Alloy 304. The higher resistance
of the G B E ~ ~ material to both intergranular bomn segregation and intergranular carbide
precipitation was likely due to the higher muencies of low-z boundaries in the grain
boundary character distribution, primady cohetent Z3 twin bounâaries and D boundaries
satis-g the Palumbo-Aust criterion.
7, Recammendations For Future Work
Recommendations for fiiture work include:
Investigating the effects of variables such as buik boron concentration and bulk
carbon concentration on the boron segregation susceptibility at grain boundaries in
Alloy 304.
Conducting similar SIMS and OIM studies on other related materials such as
nickel-based alloys.
Warrington and Boon (1975) demonstmied that the random probability of
occurrence (p) for any CSL grain boundary can be detemiiaed through consideration of
(1) the number of equivalent rotations (n) leadhg to the sarne CSL (i.e., multiplicity) anà
(2) the angular deviation limit (A@ imposed on the CSL. The number of equivalent
rotations (n) is given by,
n = 24w, (1)
where w is the number of distinct crystdographic forms for each CSL. Table A shows the
n values for x=l to 29 derived h m the tabulated data of Mykura (1979). As shown in
Table A, the multiplicity of CSL boundaries tends to increase with increasing value.
Table A. Multiplicity of CSLs (n) from z=l to 29 (Mykura, 1979).
Warrington and Boon (1975) showed that the pmbability of a boundary lying
within an angular deviation l h i t of A0 (rad) h m any given CSL cm be deterniined by
considering the radial density distribution d i n g h m spheres of radius AB(CSL range)
-- _ - ég@ 180°(total distribution). The probability (P) of a boundary lying within an angular
radius of A0 is given by
P = (A0 - sin Ae)/n. (2)
The random probability of occurrence @) for any CSL is thus given by,
p=nP. (3)
Using this analysis and an approximation of Brandon's criterion (i.e., ~ & 0 . 2 5 ~ " rad)
Warrington and Boon (1975) calcufated the proùabilities of CSLs in the range of z=l to 25.
In this study the probabilities of CSLs were recalculated using the actual form of Brandon's
criterion (Le., AOSI S O ~ ' ~ ) and the Palumbo- Aust cnterion (Le., A€& 1 and extended
to x=29. Table B shows the probabilities of CSLs in the range of x=l to 29 using
Brandon's criterion and the Palumbo-Aust criter ion.
Table B. Random probability @) for specüic CSLs in the range of Z=l to 29 using
Brandon's criterion and the Palumbo-Aust criterion.
x Value l-44 Brandon PaloimbAust I = Ï
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