PVAD-AlAS 431 ILLINOIS5 UNI v AT URBANA DEPT OF METALLURGY AND MINING--ETC F/A 7/4 9I CHARACTERIZATrON OF AS-GROWN DISLOCATION STRUCTURE IN NIORIUM B--ETCIU)I SEP A1 S R STOCK ,H CHEN, H K BRNBAUM NOi 5C11
UNCLASSIFIED 0047-C1 2N
Emoorh.a.
CHARACTERIZATION OF AS-GROWN DISLOCATION STRUCTUR~E
IN JIOBIUM BY X-RAY DIFFRACTION TOPOGRAPHY,
?'R. Stock, Haydn /Chnand H rn ba urn
ONR Contract USNJ0014-75-C-1O12,
University of Illinois at Urbana-Champaign
Department of Metallurgy and Mining Engineering D7Urbana, Illinois 61801 -.. E
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8110 5 060
CHARACTERIZATION OF AS-GROWN DISLOCATION STRUCTURE IN
NIOBIUM BY X-RAY DIFFRACTION
TOPOGRAPHY
by
S. R. Stock, Haydn Chen and H. K. Bi rnba um
Depdrtment Of >',etd I Iurcy and I'i ni ngE Lng inee ri ng
and th. aeral s Resea rch Labo!rdtory
Uni vr.,!s ity o I I! inois att Urb aii a C a pin
Urba n i, I L 6 1801
The behavior of dislocations in b.c.c. metals has been
extensively examined usinq relatively macroscopic methods, such
as deformation studies and etch pitting, as well as with
microscopic TEM methods. While these studies have led to
significant increases in the understanding of dislocation
behavior, the need remains for a method for studying the
microscopic behavior of dislocations in relatively thick
specimens. One such method, x-ray diffraction topography, has
not been extensively applied due to the relatively high x-ray
absorption of many of the metals of interest, such as niobium,
and the consequent need to prepare highly perfect thin
crystals. In the present note we report the preparation of such
niobiuvi crystals and the use of Lang topography to characterize
their dislocation structures.
Single crystal niob)ium specimens of thickness suitable for
transmiission topographic studies ;ere grown by recrystallization
of a heavily deform~ied polycrystalline ribbon of niobium,;. Ine
procedure consisted of resistance heatinqg at 2200K and 2.6 x 10-8
Pa for times on the order of an hour. It should be noted
howeever, that due to outgassing of the n ioh;ul l, protracLed
annealing .'s required to obt in this pressure. DuIinI thi
annea e. ,Ited .a qr'owth ( ec o:i,i.ry riocr'jstall 1 iz~iti o )
occurred which was apparently dri ven by the s:;rfa e energy of ihe
niobium-vapor inLerface. Grains were prou .iced having surfaLe
area of several cm 2 and having <110> approximat;ly normal to the
surface. Rapid cooling, achieved by cessation of the heating
current, was necessary in order to mini.iizf, severe oxidation at
1/
. . . .. . , , ,..2
intermediate teiperatures. In the particular crystal described
here, the recrystallization anneal was folloived by equilibration
with 2.9 x 10 - 3 Pa of pure N2 clas to introduce nitrogen solutes
into the niobiunt. The nitrogen solutes served to reduce the
sensitivity of tile crystal to strains. The thickness of the
crystal reported in the present note is 76 pin, and the crystal '1
has a surface normal about five degrees fro:m [110]. Single
crystals grown by this method have total dislocation densities of
10 cin/c 3 or less.
A cenventional Lang camera using MoK radiation from a
inicrofocus generator operating at 50 kV and 3.8 wiA was used for
the topography. Ilford 1.4 rInclear emulsions with a thickness of
50 pmn recorded the topographs. Because pt - 1, where p is the
linear absorption coefficient and t is the crystal thickness,
kinematical contrast is expected to predo, iinate. The Burgers
vectors of soi~i of the dislocations in a ntvjork were deterviinn
using the follooing criteria. Neylecting elastic anisotropy, a
dislocation will be coripletely invisible if q • b = 0
dcd q • b x v :; 0, ihcre g is the diffraction vector, b is the
B'.,rqers vector ari , is the dislocition line vector. Residual
cor!trdst nay occur if i . b-- 0, h::t y b x ut- 0; if both
condi ions hold h,, ('t;riel is . isLiC lly anisotropicc as
niob i u; ; or i f t he rc o sPreq a io,i of i pur i ty ato, IS to t i.
dislocations. An a IditiOudl requiremen t for networks is that the
su-m of Burgers vectors of dislocations inretir.y at a node 'must he
zero.
Projue tion tu',ojrp;)h:; w,er taken with the following
(A2
diffraction vectors: 200, 020, 002, 110, 011, 121 and 211.
Figure I shows tWo of these, the 110 and 011 topographs, wii ch
exhibit a variety of features including several different
subgrains and a network of dislocations labeled "N." The thick
dark line extending through the dislocation array is a scratch
placed on the surface. Topographs taken before and after the
scratch was introduced show that the network was only slightly
disturbed and that no dislocations propagated from the immediate
vicinity of the scratch. Feature "0" is a "dent" which was
present prior to crystal growth as a result of local plastic
deformation. The dislocations w'hich formed this "dent" annealed
out, and the remaininq deformation is elastically accomrIodated.
The gradient of elastic strains about the "dent" leads to unusual
contrast of the dislocation network in the 011 topograph: rdther
than the more usual enhanced diffraction at defects, the
radiation is scattered away froi the defect leading to a decrease
in diffracted intensity relative to the backgrOund. This
contrast is similar to that observed in elastically bent silic n
crysta Is (e eran and BI ech , 192) and can be expected fro:m
dynamical scattering thLuo ry (Iart, 1931).
The netwo rk of di sloc a tions is labeled in Fig. 2 wit tihe
Burgers vectors deteriin ed using the above contrast criteria.
Residual contrast, however, proved to he a ',ajor problemi in
identifying a consistent set of Burgers vectors. Optical
densitometry was requi red to conclusively determine whether the
images were in contrast or in residual contrast. The difference
in transriitted intensity between the dislocation image and the
background, normalized relative to the intensity transmitted
through the unexposed portiorns of the emulsion, was used as a
parar,,eter to deterni ne whother the image was in contrast or was
exhibiting residual contrast. As can be seen in Fig. 2, the
network's dislocation Burgers vectors consist of <111>, <100>,
and <110> types. While it is energetically favorable for two
a/2<111> slip dislocations to combine to form a single a<100>
dislocation, the reaction of two a/2<111> dislocations to form a
single a<ll> is energetically unfavorable. It appears, however,
that during the high te;perature anneal both a<100> and
a<110> dislocation segmients form in the network. The fractions
of Burgers vectors of each type are 60% a/2<111>, 25%" a<100> and
150, a< 110>. Networks conmpri sed of th? sarie types of dislocations
were observed by Dingley and Hale (1966) using TEM in Fe, Fe -.
0.7 j.I;n and 2 1/4%Cr - IIo steel. They rep)orted si m ilar
distributions of 6urgers vector types with 60 a/2<1II>,
20 a I10 > and 20%, a<10)> ty)es.
Nearly per fect n iob i un crys tIs have been reported before
but apparently the perfocLion of strain-dnnealed and
recrystall izel ribbon speci:7ens hcis not been previously
Oxa:il ied. Re e , ' ::an '!~ I IJR 1 i .11 (1961) have prepared
rl io*,.n1,, crys al; frui, th,. , .'hlc contained vi despreid
net;urks of disloc.atIo:is an :1 dislocation densities approaching
l[)?C./ci 3 . Using 1- crijci'ole-less pulling iiethod, Naramoto (l / 3)
has also grown excellent niobi,:i crystals, continimy long
dipoles of a/2<1l1> eddie dislocations, long segmients of a/2<1lI>
screw type, :hart a<iO > sc;:1ents and small 1/2<111> pris:ttic
Sloops and helices. The presence of the prisviatic loops and
helices was ascribed to vacancy precipitation during growth. The
absence of loops and hel ices in the present study could be due to
a variety of factors. The small specimen thickness and tihe
availability of the surface as a defect sink would tend to limit
vacancy loop formation. The relatively good vacuum used would
also minimize oxide formation during cooling and would limit the
subsequent vacancy injection at moderate temperatures. Studies
in which loops and helices were observed in molybdenum (Becker
and Pegel, 1969) and in niobium ( Naramoto, 1978 and Zedler, 1967)
were characterized by slow:er :ooling rates dnd poorer
vacuu,%I (10 - 5 - 1 0 - 6 torr).
AC K 0!L E 0 G E ,N I-S
The authors would ike tn thdnr" Dr. P. E. Zapp for pruvi-/ y
the particular crystal described here and flr. I). L. Zi erath of
the Illi nois State Geologi cal Survey for his help with the
optical densitoiietry. We gratefully ackre..il ,dkie the us of t.h.e
x-ray facilities a tha ,o t qria is .R oe ;c!r Lah.)ruLory aI th0
s, upport of the ff 0 f f ic ff ,if R esar 1h Lhro ,1! COntract ,",'"ri 1'
I
REFER FNUCE S
1. Becker, C., and Pegel, B., 1969, Phys. Stat. Sol., 32, 443.
2. Dingley, D. J., and Hale, K. F., 1966, Proc. Roy. Soc., A295,
55.
3. Hart, Ml., 1981, Chrceiaino Crystal Growith Defects by
X-ray__Methods, p. 216.
4. Mperan, E. S. , and BIlech, I. A., 1972, J. Appi. Phys ., 43,
265.
5. Narai~ioto, H. , 197211, Crystal Growth, -44, 475.
6. Reed, R. E. , Gwber;,an, H. D. , and Baldwhin, T. 0. , 1967, J.
Phys. Chem. Sol ids Suppi . ,1, 829.
7. Zedler, E. , 1967, J. Appi. Pkys. , 38, 2046.
FIGURE CAPTIONS
Fig. 1. Lang topographs of a niobiu:n crystal with surface normal
approximately [110]. a) Subgrains "A" and "B,"
dislocation network "N," scratch "S" and dent "D" are
seen with g = [110]. b & c) Contrast of the
dislocations in the network differs for g = [110] and
g = [011], respectively, due to the influence of bending
from the nearby dent "D.
Fig. 2. Dislocation Burgers vectors for a network "N" in a
niobiur crystal with surface normal approximately
[110]. Different line segments, single, dcuble, dashed
and dotted, represent dislocation lines with a/2<111>,
a<110>, a<100> and undetermined Burgers vectors,
respectively. The direction of Burgers vectors which
lie in the plane of the foil are indicated by arrows.
- 0
0.
1-6-
CC7
oil a
* -/
O-2A4A
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3 REPORT TITLE
Characterization of As-Grown Dislocation Structure in Niobium by X-rayDiffraction Topography
4 DESCRIPTIVE NOTES (Ty.pe of report and inclusive date*)
Technical Report September 1981S AUTHOR(S) (Last name, first name. initial)
Stock, Stuart S., Chen, Haydn and Birnbaum, Howard K.
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13 P BSTRACT
The use of X-ray topography to study dislocation structures inniobium is described.
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NiobiumX-ray topography
Dislocation structures
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