Research ArticleRelationships among the Microstructure Mechanical Propertiesand Fatigue Behavior in Thin Ti6Al4V
Y Fan12 W Tian1 Y Guo1 Z Sun1 and J Xu1
1School of Material Science and Engineering China University of Mining and Technology Xuzhou Jiangsu 221116 China2Faculty of Engineering University of Nottingham University Park Nottingham NG7 2RD UK
Correspondence should be addressed to Y Fan fanyucumteducn
Received 14 September 2015 Revised 9 December 2015 Accepted 10 January 2016
Academic Editor Michele Iafisco
Copyright copy 2016 Y Fan et al This is an open access article distributed under the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
The microstructures of Ti6Al4V are complex and strongly affect its mechanical properties and fatigue behavior This paperinvestigates the role of microstructure on mechanical and fatigue properties of thin-section Ti6Al4V sheets with the aim ofreviewing the effects of microstructure on fatigue properties where suboptimal microstructures might result following heattreatment of assemblies that may not be suited to further annealing for example following laser welding Samples of Ti6Al4Vsheet were subjected to a range of heat treatments including annealing and water quenching from temperatures ranging from650∘C to 1050∘C Micrographs of these samples were inspected for microstructure and hardness 02 proof stress elongationand fracture strength were measured and attributed back to microstructure Fractography was used to support the findings frommicrostructure and mechanical analyses The strength ranking from high to low for the microstructures of thin Ti6Al4V sheetsobserved in this study is as follows acicular 1205721015840 martensite Widmanstatten bimodal and equiaxed microstructure The fatiguestrength ranking from high to low is as follows equiaxed bimodal Widmanstatten and acicular 1205721015840 martensite microstructure
1 Introduction
Ti6Al4V alloy is widely used in the medical device indus-try [1ndash3] for its many desirable properties including itsstrength to weight ratio corrosion resistance biocompat-ibility and processability [4ndash8] Titanium components inmedical devices are usually manufactured from very thinsections (lt1mm) and it is common for such products to behermetically sealed by laser-beam welding [3] Some weldedcomponents made of thin Ti6Al4V sheets are subjectedto static and cyclic loading from which fatigue fractureand failure may eventually occur The mechanical proper-ties (strength and toughness) and fatigue behavior of thinTi6Al4V sheets are evaluated from the viewpoint of findingoptimal properties for use in medical devices Previousresearch has indicated that mechanical properties and fatiguebehavior are quite sensitive to microstructure [5 9 10] Themicrostructures are controlled by heat treatment generally attemperatures in the dual 120572-120573 phase region [11 12]
The microstructures of titanium alloys are generallydescribed by the size and arrangement of their120572 and120573phases
The two extreme cases of phase arrangements are lamellarmicrostructure (with a greater 120572120573 surface area and moreoriented colonies) which is generated upon cooling from the120573 phase field and equiaxed microstructure (a uniform struc-ture composed of 120572 grains and grain boundaries of 120573 [7])which results from a recrystallization and globularizationprocess [13] Previous research has indicated that lamellarmicrostructure exhibits lower strength lower ductility andbetter fatigue propagation resistance comparedwith equiaxedmicrostructure [14] Equiaxedmicrostructure provides betterfatigue initiation resistance but poorer propagation resistance[13] than lamellar microstructure Another kind of structurecalled bimodal microstructure is considered to be a com-bination of lamellar and equiaxed microstructures Bimodalmicrostructures exhibit a well-balanced fatigue propertiesprofile [13] since they combine the advantages of bothlamellar microstructure (ie higher fatigue crack propaga-tion resistance) and equiaxed microstructure (ie higherfatigue crack initiation resistance) During heat treatmentWidmanstatten microstructure may be found in Ti6Al4V[15] this is observed when materials are cooled at a critical
Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2016 Article ID 7278267 9 pageshttpdxdoiorg10115520167278267
2 Advances in Materials Science and Engineering
rate from extremely high temperature For example Ahmedand Rack [16] showed that for heat treatment at 1050∘Cfor 30min and at a low cooling rate (lt20∘C sminus1) 120572 colonieschange into a basket-weave structure (Widmanstatten) Theslow cooling rate results in nucleation and growth of Wid-manstatten plates There is no full definition for the Wid-manstatten microstructure of titanium alloy much beyondthe appearance of the characteristic basket-weave structureThe Widmanstatten microstructure may be considered tobe a special type of lamellar microstructure (with thickerlamellar width and greater orientation with long coarse 120572grain boundaries) Rapid quenching (or laser-beam welding)leads to amartensitic transformation of120573 which causes a veryfine needle-like microstructure Unlike ferrous martensitetitaniummartensite is neither significantly stronger normorebrittle [17] than its parent phase and the hardening effect oftitanium alloy martensite is only moderate
The weld zone in laser-beam welding consists of meltedand resolidified metal resulting from a process lasting onlya short period of time Hence the fusion zone (FZ) hasproperties similar to those resulting from a water-quenchingprocess from a high temperature The fatigue behavior of 1205721015840martensite is generally seen as poor because the dislocationsconcentrate on the martensite interfaces on the tip of thefatigue crack in the FZ forming a high-density dislocationnetwork Microplastic deformation occurs at and near the tipof the crack forming a large deformation zone Fine marten-site laths obstruct the motion and emission consequentlythe stress concentration induces shear fracture of the laths[18 19] In thin Ti6Al4V laser welding the martensite lathstend to be even finer owing to the greater cooling rate whichaggravates fatigue crack [18]
Hence it is necessary to improve the fatigue behavior oflaser-welded Ti6Al4V by transformingmartensite to anothermicrostructure The purpose of this paper is to study theeffect of microstructure produced by different cooling rateson the mechanical properties of as-received (AR) parentmaterial (PM) 07mm Ti6Al4V sheets The fatigue behaviorof bimodal Widmanstatten and martensite microstructurestransformed from the AR materials is also studied
2 Experimental Procedures
The material used in this study was 07mm sheet Ti6Al4Valloy (grade 5) Following rolling the sheet was heat-treatedat 850∘C for 8 h followed by heat treatment at 750∘C for30min and then furnace-cooled to room temperature thisdefines its AR state
Samples for microstructural analysis were mounted inconductive hot-mounting resin The samples were groundwith 240-mesh silicon carbide papers (to remove any oxidelayer) followed by 400- 800- and 1200-mesh silicon carbidepapers and finally polished using a porous neoprene polish-ing disk to a mirror finish The samples were etched with amixture of 2mL HF 5mL HNO
3 and 93mL H
2O A Nikon
Optiphot 200D microscope (Nikon Corporation TokyoJapan) was used for the examination of microstructural fea-tures of the original and heat-treated samples A Philips XL30
20
20
6
40
07
R25
Figure 1 Sample dimensions for the tensile test
tungsten filament scanning electron microscope (FEI NorthAmerica NanoPort 5350NEDawson Creek Drive HillsboroOR 97124 USA) was used to image and characterize thesamples
The tensile test experiments were performed on anInstron 5569 tensile and compression test machine (InstronCoronation Road High Wycombe Bucks HP12 3SY UK)at room temperature with a load cell capacity of 50 kN andcrosshead speed of 1mmminminus1The tensile test samples weredesigned to comply with European Standard EN10002-1 [20]their dimensions are shown in Figure 1 Two strain gaugeswere used for measurement of strain
(i) A strain gaugewas attached to the gauge section of thesample andwas used tomeasure tensile strainslt02
(ii) At higher strains the crosshead movement was usedto calculate the strains in the gauge section of thesample
Three samples of each of the AR materials and heat-treated materials were tested to failure
The microhardness of the samples was measured usingVickers hardness with a Leco M-400 tester (Lecoreg Corpo-ration 3000 Lakeview Avenue St Joseph MI 49085-2396USA) over a 15 s indentation time A 200 gf load was usedfor all hardness measurements
Two types of furnaces were used
(i) A rapid heat furnace (Carbolite RHF 163 ELITEThermal Systems 6 Stuart RoadMarketHarboroughLeicestershire LE16 9PQ UK) from Carbolite wasadopted for the water-quenching process The sam-ples were heat-treated for 8min until the furnacereached the required temperature and then water-quenched
(ii) A Lenton argon furnace (Lenton Furnaces andOvensPO Box 2031 Hope Hope Valley Derbyshire S336BW UK) was used for the annealing process Argonwas used as a shielding gas along with a Cussonsrare gas purifier-4 (Cussons Technology 102 GreatClowes StreetManchesterM7 1RHUK)The sampleswere soaked in the furnace at temperatures of 650∘C750∘C 850∘C 950∘C and 1050∘C for 1 h and thenfurnace-cooled to room temperature Before thesetests the furnace was calibrated and the cooling curvewas recorded The average cooling rate of the argonfurnace was 0075∘C sminus1
A Denison Mayes 250-kN-capacity servohydraulicmachine (Denison Mayes Group Moor Road Leeds West
Advances in Materials Science and Engineering 3
2020
6
10603
15
R50
07
Figure 2 Sample dimensions for tensile fatigue test pieces
Yorkshire LS10 2DE UK) was used for tensile fatigue testingFigure 2 shows a schematic diagram of the tensile fatigue testpieces designed to comply with British Standard 3518-1 [21]
The stress ratio used for all sampleswas 01 and the cyclingfrequencywas 7Hz Five sampleswere tested to at least 107 lifecycles
3 Results
31 Microstructures and X-Ray Diffraction of Heat-TreatedTi6Al4V Themicrostructure of Ti6Al4VARmaterial exhib-ited equiaxed 120572 phase surrounded by a 120573 phase boundarywith fine grain size (15ndash20120583m) The hardness Youngrsquos mod-ulus 02 proof stress and elongation of the AR materialwere respectively 362HV 111 GPa 978MPa and 136Other samples of Ti6Al4V material were heat-treated at650∘C 750∘C 850∘C 950∘C and 1050∘C for 1 h followed byannealing Following heat treatment samples were inspectedfor their microstructural development In the followingresults the nomenclature ldquoArdquo indicates annealed samples thathave been cooled in the furnace and ldquoQrdquo indicates water-quenched samples Optical images of samples at differentheat-treatment temperatures and cooling rates are shown inFigure 3There is no significant difference in samples of 650A(Figure 3(b)) 750A (Figure 3(d)) 850A (Figure 3(f)) 650Q(Figure 3(c)) and 750Q (Figure 3(e)) All of these imagesexhibit equiaxed 120572 phase surrounded by a 120573 phase boundaryThere is a slight difference between 850Q (Figure 3(g)) andothers where the 120573 phase boundary appears thicker In950A (Figure 3(h)) an equiaxed microstructure with pri-mary 120572 phase and a partial 120573 phase boundary still existsbut some transformation from the 120573 phase boundary tolamellar 120572 + 120573 phase has occurred At 950Q (Figure 3(i)) anequiaxed microstructure with primary 120572 phase and a partial120573 phase boundary is observed which is similar to 950AThe difference is that there is no lamellar 120572 + 120573 phase butinstead a region of metastable 120573 phase (dark region) thathas been transformed from the 120573 phase boundary regions isobserved Figure 3(j) (1050A) shows a typical basket-weaveWidmanstatten microstructure with an 120572 grain boundary inthe prior 120573 grains from annealed 1050∘C samples Figure 3(k)(1050Q) shows needle-like 1205721015840martensite microstructure with120573 phase between martensitic laths The grain size tends tobe coarse The results of quantitative microscopy giving thevolume fraction of phases for all different heat treatments aresummarized in Table 1
The SEM backscattered electron images of AR material(equiaxed) 950A (bimodal) 1050A (Widmanstatten) and1050Q (martensite) are shown in Figure 4
Table 1 Volume fraction results of 120572120573 1205731015840 phases
X-ray diffraction patterns of PM 950A 1050A and 1050Qare shown in Figure 5 respectively Reflections of 120572 and120573 phase were detected in diffraction patterns of equiaxedbimodal Widmanstatten and martensite It should be notedthat 120573 reflections are rather weak suggesting a relatively lowvolume fraction of the 120573 phase (Table 1) in PM and 950A
32 Mechanical Properties Hardness testing was conductedon heat-treated Ti6Al4V samples Figure 6 shows the resultsof hardness examination of Ti6Al4V heat-treated at 650∘C750∘C 850∘C 950∘C and 1050∘C for 1 h with annealing (A)and water quenching (Q) respectivelyThere is no significantchange of hardness values among 650A 750A 850A 650Qand 750Q Following the annealing process hardness valuesincreased from 850∘C to 1050∘C the highest hardness valuefor an annealed sample occurred at 1050∘C (741 higher thanfor the AR sample) Following water quenching hardnessvalues increased from between 750∘C and 850∘C to 1050∘Cthe highest hardness value is 1558 higher than that for theAR sample
The trends (Figure 7) of 02 proof stress values ofheat-treated Ti6Al4V following both annealing and waterquenching are very similar to those of the hardness valuesThe highest proof stress value occurred at a 1050∘C water-quenching condition which is 1378 higher than that of theAR sample
The value of elongation to failure of samples deterio-rated following annealing or water quenching following heattreatment in the temperature range 650∘Cndash1050∘C as seen inFigure 7 There is no significant change of elongation valuesamong 650A 750A 850A 650Q and 750Q In annealingelongation values decrease between 850∘C and 1050∘C thelowest elongation value is 2438 less than that of the ARmaterial In the water-quenched condition elongation valuesdropped off more quickly from a point between 750∘C and850∘C and 1050∘C The lowest elongation value was 4273smaller than that of the AR sample
Stress-strain curve of heat-treated Ti6Al4V followingboth annealing and water quenching from different temper-atures are shown in Figure 8
33 Fatigue Behavior Figure 9 shows the high-cycle fatiguefracture lives of the AR material martensite microstructureWidmanstatten microstructure and bimodal microstructuretransformed by 1050Q 1050A and 950A respectively Fatiguestrength (at 107 cycles) of the AR material is 180MPawhich was the highest for all microstructures 1050Q hadthe highest 02 proof stress but its fatigue strength was
4 Advances in Materials Science and Engineering
(a) PM
(b) 650A (c) 650Q
(d) 750A (e) 750Q
(f) 850A (g) 850Q
(h) 950A (i) 950Q
(j) 1050A (k) 1050Q
Grain orientation 3
Grain orientation 1Grain orientation 2
transformed from prior120573 phase boundary
Start transformation
Typicalbasket-wave
structuregrain
boundary
Needle-like structure
Primary 120572120573 phase boundary
200120583m
Prior 120573
Metastable 120573
120572 grain boundary
200120583m
Primary 120572
120573 phase boundary area
950A
950Q
Figure 3 Optical microstructure of AR material and heat-treated Ti6Al4V The number indicates the heat-treatment temperature ldquoArdquoindicates annealing (furnace-cooled) and ldquoQrdquo indicates quenching
(a) PM (b) 950A
(c) 1050A (d) 1050Q
Figure 4 SEM backscattered electron images of AR-PM 950A 1050A and 1050Q
Advances in Materials Science and Engineering 5
120572 pahse120573 pahse
0
50
100
150
200
250
30 35 40 45 50
2120579 (deg)
1050Q
1050A
950A
PM
Figure 5 X-ray diffraction pattern of PM 950A 1050A and 1050Q
Har
dnes
s (H
V)
Heat treatment temperature (∘C)
QuenchAnneal
440
420
400
380
360
600 650 700 750 800 850 900 950 1000 1050 1100
Figure 6 Effect of annealing temperatures and cooling scheduleson hardness of Ti6Al4V
lowest (135MPa) The fatigue strength of 1050A (158MPa)was slightly higher than that of 950A (153MPa)
34 Ti6Al4V Fractographs Figure 10 shows SEM fracto-graphs of fatigue crack propagation areas of AR mate-rial 1050Q (martensite microstructure) 950A (bimodalmicrostructure) and 1050A (Widmanstatten microstruc-ture)The fractograph of ARmaterial (Figure 10(a)) is flat anduniform Fine dimple patterns were observed However thefracture surface of the martensite microstructure is rougheras shown in Figure 10(b)The appearance of mountain-shapepatterns indicates typical brittle fatigue fracture The fracturesurface of the bimodal microstructure is flat but not as
Heat treatment temperature (∘C)600 700 800 900 1000 1100
02
p
roof
stre
ss (M
Pa)
ElongationAnnealing AnnealingWater quench Water quench
02 proof stress
Elon
gatio
n (
)
15
12
9
6
3
0
1300
1250
1200
1150
1100
1050
1000
950
Figure 7 Effect of different heat-treatment temperatures on 02proof stress and elongation of Ti6Al4V
0
200
400
600
800
1000
1200
1400
0 1 3 5 7 9 11 13 15
Stre
ss (M
Pa)
Strain ()650A650Q750A750Q850A
850Q950A950Q1050A1050Q
Figure 8 Stress-strain curve of heat-treated Ti6Al4V followingboth annealing and water quenching from different temperatures
uniform as exhibited in ARmaterial (Figure 10(c)) River pat-terns are observed in the fractograph of the Widmanstattenmicrostructure (Figure 10(d))
4 Discussion
Thedifferentmicrostructures are generated by heat treatmentof the AR material [10 22 23] Generally most of thecommercially used Ti6Al4V has been heat-treated via acomplex sequence of solution heat treatment deformationaging and annealing [5] for stress relief recrystallization
6 Advances in Materials Science and Engineering
Stre
ss am
plitu
de (M
Pa)
Life (cycle)As-received material950A bimodal
1050A Widmanst atten1050Q martensite
300
250
200
150
100
104 105 106 107 108
Figure 9 Stress amplitude versus life curves of AR material martensite Widmanstatten and bimodal microstructures of Ti6Al4V
(a) (b)
(c) (d)
Figure 10 SEM fractographs of the high-cycle fatigue samples of (a) AR material (b) 1050Q (c) 950A and (d) 1050A
and globularization Temperatures of 650∘Cndash850∘C are nor-mally used for these As shown in Figures 3(b)ndash3(f) themicrostructures were not modified There is no significantdifference in themechanical properties (hardness 02 proofstress and elongation) associated with heat treatment atthese temperatures In industrial application stress reliefrecrystallization and globularization can be realized byannealing at 650∘Cndash850∘C however the associated oxidationlayer thickness increases at the same time [24]
Water quenching from 850∘C to 1050∘C tends to increasethe strength owing to higher 120573 phase content [10] Waterquenching at 1050∘C (Figure 3(k)) transformed the equiaxedmicrostructure into acicular 1205721015840 martensite microstructureThe martensite sample exhibited the highest increases inhardness (419HV) and 02 proof stress (138) comparedto the AR sample but at the cost of a reduction in ductility(427 lower elongation compared to theAR sample) Similar
results for mechanical properties of acicular 1205721015840 martensitemicrostructure of Ti6Al4V were obtained by Jovanovic et al[10] By comparing the results of this paper with Jovanovicet alrsquos data (02 proof stress 1400MPa elongation 15water quenching from 1100∘C) it is obvious that the valuesof 02 proof stress (1114MPa) of acicular 1205721015840 martensitemicrostructure in this study are lower but elongation (78)is rather higher Jovanovic et al suggested two reasons forthese differences (1)Their ARmaterials wereWidmanstattenwith a massive 120573 phase boundary that exhibited higherstrength (1100MPa) but lower ductility (lt15) and (2) thepresence of TiC (formed from carbon diffused into the meltforming carbides with titanium and vanadium during thecasting process) raised the tensile strength but compromisedductility The fatigue strength of the acicular 1205721015840 martensitemicrostructure is the lowest (135MPa) being lower thanthe AR material by 45MPa The term ldquofatiguerdquo relates to
Advances in Materials Science and Engineering 7
Table 2 Summary of the study showing absolute values and variance (Δ) of properties of AR materials
Widmanstatten (1050A)(lamellar width of 10120583m +gt400 120583m120572 grain boundary)
1041 642 103 minus244 153 minus15
Martensite (1050Q)(200 120583m grain size with insideacicular 1205721015840)
1114 138 78 minus427 135 minus25
the initiation and growth of cracks caused by the repeatedapplication of mechanical stresses or strains and any asso-ciated changes in mechanical properties [21] Sun et al [25]reported that low ductility deteriorated fatigue propagationresistance in tensile mode fracture but high strength didlittle to help fatigue initiation resistance [25 26] Hence thepoor ductility in the acicular 1205721015840 martensite microstructureis one reason why its fatigue strength is so low The coarse-grained structure (Figure 3(k)) can be another reason forbrittle fatigue fracture (Figure 10(b)) It is well known thatfatigue cracking in Ti6Al4V alloys starts at prior 120573 grainboundaries or colony boundaries and 120572120573 interfaces [27]A coarse-grained structure will aggravate dislocations at theprior 120573 grain boundaries [28] and eventually lead to failureA third reason for the lower fracture strength in martensitecould be martensite transformation stress generally it iseasier to initiate cracks at a stress-concentration position [29]Martensite transformation stress may aggravate the stressconcentration
Ti6Al4V heat-treated at 950∘C followed by annealing(Figure 3(h)) exhibits slight lamellar microstructure at prior120573 phase boundaries Such bimodal microstructure (con-sisting partly of equiaxed primary 120572 and a lamellar 120572 +120573 matrix transformed from a prior 120573 phase boundary)results from heat treatment just below the 120573 transus tem-perature [13 30] Tensile test results show (Figure 7) thatbimodal microstructures exhibit well-balanced propertiesbetween 02proof strength (1017MPa) and ductility (118)The Widmanstatten microstructure heat-treated at 1050∘Cfollowed by annealing exhibits a slightly higher strength(1041MPa) than the bimodal microstructure but has lowerductility (103) than the bimodal microstructure Thisresearch indicated that Ti6Al4V with bimodal microstruc-ture has a slightly higher fatigue strength than that withWidmanstattenmicrostructure Similar results were obtainedby Zuo et al [30] The fatigue strengths for their bimodaland Widmanstatten microstructures were 493 and 475MParespectively (with both the stress ratio 119877 = minus1 and frequency= 20 kHz increasing the fatigue strength) Both Zuo et al[30] and Gil et al [5] have concluded that crack initiationrather than crack propagation plays the dominant role incontrolling the total life in high-cycle fatigue when there
are not too many interior defects Equiaxed microstruc-ture benefits fatigue initiation resistance Hence bimodalmicrostructure (with the properties of primary 120572 grain ofslightly lower strength and higher ductility) exhibits higherfatigue life compared with Widmanstatten microstructure(which is a special type of lamellarmicrostructure) Howeverbimodal microstructure exhibits lower fatigue life comparedwith equiaxed microstructure owing to its lower ductilitycompared with AR microstructure The fine dimple patternsobserved in fractographs of AR samplesmay indicate a plasticfatigue fracture process The AR material has an equiaxedmicrostructure that exhibited the highest ductility (136)
The correlations among the microstructures mechanicalproperties and fatigue strengths of 07mmTi6Al4V sheet arelisted together in Table 2
5 Conclusions
This study highlights the role of microstructure on fatiguestrength rather than relying on the broader relationshipbetween ductility and fatigue strength Three key observa-tions are apparent
(1) The 02 proof stress ranking from high to lowfor all types of microstructure of thin Ti6Al4Vsheets observed in this study is as follows aci-cular 1205721015840 martensite equiaxed microstructure withmetastable 120573 Widmanstatten bimodal and equiaxedmicrostructure
(2) The ductility ranking from high to low for all types ofthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and martensite microstructure
(3) The fatigue strength ranking from high to low forthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and acicular 1205721015840 martensite microstruc-ture
The fatigue strength of thin Ti6Al4V sheets decreasesinversely proportionally with the proof stress and propor-tionally with ductility this is in response to the types of
8 Advances in Materials Science and Engineering
microstructure and grain sizes set up in the heat-treatmentprocesses
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the materials supportprovided by Timet UK Limited The work is supportedby Fundamental Research Funds for the Central Universi-ties (2014QNA09) Natural Science Foundation of Jiangsu(BK20150205) and Postdoctoral Science Foundation ofJiangsu (1501029A)The authors also wish to express heartfeltthanks to Professor Philip Shipway and Professor Geoff Tans-ley for encouragement supervision and support throughoutthe work
References
[1] G D Tansley M Cook N Zhang M Chung J Woodard andJ Reizes Complete Passive Suspension of the Ventrassist RotaryBlood Pump Univerisy of Technology Sydney Australia 2000
[2] G Tansley S Vidakovic and J Reizes ldquoFluid dynamic char-acteristics of the VentrAssist rotary blood pumprdquo ArtificialOrgans vol 24 no 6 pp 483ndash487 2000
[3] Y Fan Z Chen C H Zhang and A M Liu ldquoA comparisonof microstructure and mechanical properties of welded thinTi6Al4V with three different types of laserrdquoMaterials ResearchInnovations vol 19 no S4 pp S187ndashS192 2015
[4] L W Tsay and C Y Tsay ldquoEffect of microstructures on thefatigue crack growth in Ti6Al4V laser weldsrdquo InternationalJournal of Fatigue vol 19 no 10 pp 713ndash720 1997
[5] F J Gil M P Ginebra J M Manero and J A PlanellldquoFormation of 120572-Widmanstatten structure effects of grain sizeand cooling rate on the Widmanstatten morphologies and onthe mechanical properties in Ti6Al4V alloyrdquo Journal of Alloysand Compounds vol 329 no 1-2 pp 142ndash152 2001
[6] F Caiazzo F Curcio G Daurelio and F M C MinutololdquoTi6Al4V sheets lap and butt joints carried out by CO
2
lasermechanical and morphological characterizationrdquo Journal ofMaterials Processing Technology vol 149 no 1ndash3 pp 546ndash5522004
[7] I J Polmear Light Alloys From Traditional Alloys to Nanocrys-tals Butterworth-Heinemann Oxford UK 2006
[8] N Poondla T S Srivatsan A Patnaik and M Petraroli ldquoAstudy of the microstructure and hardness of two titaniumalloys commercially pure andTindash6Alndash4Vrdquo Journal of Alloys andCompounds vol 486 no 1-2 pp 162ndash167 2009
[9] M Zitnansky and L Caplovic ldquoEffect of the thermomechanicaltreatment on the structure of titanium alloy Ti6Al4Vrdquo Journal ofMaterials Processing Technology vol 157-158 pp 643ndash649 2004
[10] M T Jovanovic S Tadic S Zec Z Miskovic and I BobicldquoThe effect of annealing temperatures and cooling rates onmicrostructure and mechanical properties of investment castTindash6Alndash4V alloyrdquo Materials and Design vol 27 no 3 pp 192ndash199 2006
[11] D Hardie and S Ouyang ldquoEffect of microstructure and heattreatment on fracture behaviour of smooth and precracked ten-sile specimens of Ti6Al4Vrdquo Materials Science and Technologyvol 15 no 9 pp 1049ndash1057 1999
[12] S Shademan V Sinha A B O Soboyejo and W O SoboyejoldquoAn investigation of the effects of microstructure and stressratio on fatigue crack growth in Tindash6Alndash4V with colony 120572120573microstructuresrdquo Mechanics of Materials vol 36 no 1-2 pp161ndash175 2004
[13] C Leyens and M Peter Titanium and Titanium Alloys Fun-damentals and Applications Wiley-VCH Verlag GmbH amp CoKGaA 2003
[14] C Loier G Thauvin A Hazotte and A Simon ldquoInfluence ofdeformation on the 120573 rarr 120572 + 120573 transformation kinetics of Ti-6 wtAl-4wtV alloyrdquo Journal of the Less Common Metalsvol 108 no 2 pp 295ndash312 1985
[15] T Mohandas D Banerjee and V V K Rao ldquoFusion zonemicrostructure and porosity in electron beam welds of an 120572+120573titanium alloyrdquo Metallurgical amp Materials Transactions A vol30 no 3 pp 789ndash798 1999
[16] T Ahmed and H J Rack ldquoPhase transformations during cool-ing in 120572+120573 titanium alloysrdquo Materials Science and Engineeringvol 243 no 1-2 pp 206ndash211 1998
[17] American Welding Society Recommended Practices for LaserBeamWelding Cutting and Drilling AWSANSI 1998
[18] X Li Research on the Microstructure and Fatigue Propertyof Electron Beam Welding Joint in Titanium Alloy HuazhongUniversity of Science and Technology Wuhan China 2012
[19] C Wu X Li X Huang J Ma and C Cao ldquoRelationship offatigue crack propagation and microstructure for TA15 alloyrdquoRare Metal Materials and Engineering vol 36 no 12 pp 2128ndash2131 2007
[20] European Standard EN10002-1 Metal material Tensile test2001
[21] British Standard 3518-1 Methods of fatigue testing 1993[22] W Sha and Z Guo ldquoPhase evolution of Tindash6A1ndash4V during
continuous heatingrdquo Journal of Alloys and Compounds vol 290no 1-2 pp L3ndashL7 1999
[23] P Sirilar and P Srichandr Grain Refinement of 120572120573 PhaseTi6Al4V Alloy by Thermomechanical Treatment King Mong-kutrsquos University of Technology Thonburi Bangkok Thailand2006
[24] Y Fan G D Tansley P H Shipway and J Niu ldquoThe applicationof laser welding on Left Ventricular Assist Device (LVAD)rdquo inProceedings of the Symposium on Photonics and Optoelectronics(SOPO rsquo11) pp 1ndash3 Wuhan China May 2011
[25] Z Sun L Jiang andZ Ying Failure Analysis Fundamentals andApplications China Machine Press 2005
[26] X L ZhengQuantitativeTheory ofMetal Fatigue NorthwesternPolytechnical University Press 1994
[27] J Oh N J Kim S Lee and E W Lee ldquoCorrelation of fatigueproperties and microstructure in investment cast Tindash6Alndash4Vweldsrdquo Materials Science and Engineering A vol 340 no 1-2pp 232ndash242 2003
[28] F X Gil Mur D Rodrıguez and J A Planell ldquoInfluence of tem-pering temperature and time on the 1205721015840-Ti-6A1-4V martensiterdquoJournal of Alloys and Compounds vol 234 no 2 pp 287ndash2891996
Advances in Materials Science and Engineering 9
[29] S R Lampman and N Dimatteo Fatigue and Fracture ASMInternational Geauga County Ohio USA 1996
[30] J H Zuo Z G Wang and E H Han ldquoEffect of microstructureon ultra-high cycle fatigue behavior of Ti-6Al-4Vrdquo MaterialsScience and Engineering A vol 473 no 1-2 pp 147ndash152 2008
rate from extremely high temperature For example Ahmedand Rack [16] showed that for heat treatment at 1050∘Cfor 30min and at a low cooling rate (lt20∘C sminus1) 120572 colonieschange into a basket-weave structure (Widmanstatten) Theslow cooling rate results in nucleation and growth of Wid-manstatten plates There is no full definition for the Wid-manstatten microstructure of titanium alloy much beyondthe appearance of the characteristic basket-weave structureThe Widmanstatten microstructure may be considered tobe a special type of lamellar microstructure (with thickerlamellar width and greater orientation with long coarse 120572grain boundaries) Rapid quenching (or laser-beam welding)leads to amartensitic transformation of120573 which causes a veryfine needle-like microstructure Unlike ferrous martensitetitaniummartensite is neither significantly stronger normorebrittle [17] than its parent phase and the hardening effect oftitanium alloy martensite is only moderate
The weld zone in laser-beam welding consists of meltedand resolidified metal resulting from a process lasting onlya short period of time Hence the fusion zone (FZ) hasproperties similar to those resulting from a water-quenchingprocess from a high temperature The fatigue behavior of 1205721015840martensite is generally seen as poor because the dislocationsconcentrate on the martensite interfaces on the tip of thefatigue crack in the FZ forming a high-density dislocationnetwork Microplastic deformation occurs at and near the tipof the crack forming a large deformation zone Fine marten-site laths obstruct the motion and emission consequentlythe stress concentration induces shear fracture of the laths[18 19] In thin Ti6Al4V laser welding the martensite lathstend to be even finer owing to the greater cooling rate whichaggravates fatigue crack [18]
Hence it is necessary to improve the fatigue behavior oflaser-welded Ti6Al4V by transformingmartensite to anothermicrostructure The purpose of this paper is to study theeffect of microstructure produced by different cooling rateson the mechanical properties of as-received (AR) parentmaterial (PM) 07mm Ti6Al4V sheets The fatigue behaviorof bimodal Widmanstatten and martensite microstructurestransformed from the AR materials is also studied
2 Experimental Procedures
The material used in this study was 07mm sheet Ti6Al4Valloy (grade 5) Following rolling the sheet was heat-treatedat 850∘C for 8 h followed by heat treatment at 750∘C for30min and then furnace-cooled to room temperature thisdefines its AR state
Samples for microstructural analysis were mounted inconductive hot-mounting resin The samples were groundwith 240-mesh silicon carbide papers (to remove any oxidelayer) followed by 400- 800- and 1200-mesh silicon carbidepapers and finally polished using a porous neoprene polish-ing disk to a mirror finish The samples were etched with amixture of 2mL HF 5mL HNO
3 and 93mL H
2O A Nikon
Optiphot 200D microscope (Nikon Corporation TokyoJapan) was used for the examination of microstructural fea-tures of the original and heat-treated samples A Philips XL30
20
20
6
40
07
R25
Figure 1 Sample dimensions for the tensile test
tungsten filament scanning electron microscope (FEI NorthAmerica NanoPort 5350NEDawson Creek Drive HillsboroOR 97124 USA) was used to image and characterize thesamples
The tensile test experiments were performed on anInstron 5569 tensile and compression test machine (InstronCoronation Road High Wycombe Bucks HP12 3SY UK)at room temperature with a load cell capacity of 50 kN andcrosshead speed of 1mmminminus1The tensile test samples weredesigned to comply with European Standard EN10002-1 [20]their dimensions are shown in Figure 1 Two strain gaugeswere used for measurement of strain
(i) A strain gaugewas attached to the gauge section of thesample andwas used tomeasure tensile strainslt02
(ii) At higher strains the crosshead movement was usedto calculate the strains in the gauge section of thesample
Three samples of each of the AR materials and heat-treated materials were tested to failure
The microhardness of the samples was measured usingVickers hardness with a Leco M-400 tester (Lecoreg Corpo-ration 3000 Lakeview Avenue St Joseph MI 49085-2396USA) over a 15 s indentation time A 200 gf load was usedfor all hardness measurements
Two types of furnaces were used
(i) A rapid heat furnace (Carbolite RHF 163 ELITEThermal Systems 6 Stuart RoadMarketHarboroughLeicestershire LE16 9PQ UK) from Carbolite wasadopted for the water-quenching process The sam-ples were heat-treated for 8min until the furnacereached the required temperature and then water-quenched
(ii) A Lenton argon furnace (Lenton Furnaces andOvensPO Box 2031 Hope Hope Valley Derbyshire S336BW UK) was used for the annealing process Argonwas used as a shielding gas along with a Cussonsrare gas purifier-4 (Cussons Technology 102 GreatClowes StreetManchesterM7 1RHUK)The sampleswere soaked in the furnace at temperatures of 650∘C750∘C 850∘C 950∘C and 1050∘C for 1 h and thenfurnace-cooled to room temperature Before thesetests the furnace was calibrated and the cooling curvewas recorded The average cooling rate of the argonfurnace was 0075∘C sminus1
A Denison Mayes 250-kN-capacity servohydraulicmachine (Denison Mayes Group Moor Road Leeds West
Advances in Materials Science and Engineering 3
2020
6
10603
15
R50
07
Figure 2 Sample dimensions for tensile fatigue test pieces
Yorkshire LS10 2DE UK) was used for tensile fatigue testingFigure 2 shows a schematic diagram of the tensile fatigue testpieces designed to comply with British Standard 3518-1 [21]
The stress ratio used for all sampleswas 01 and the cyclingfrequencywas 7Hz Five sampleswere tested to at least 107 lifecycles
3 Results
31 Microstructures and X-Ray Diffraction of Heat-TreatedTi6Al4V Themicrostructure of Ti6Al4VARmaterial exhib-ited equiaxed 120572 phase surrounded by a 120573 phase boundarywith fine grain size (15ndash20120583m) The hardness Youngrsquos mod-ulus 02 proof stress and elongation of the AR materialwere respectively 362HV 111 GPa 978MPa and 136Other samples of Ti6Al4V material were heat-treated at650∘C 750∘C 850∘C 950∘C and 1050∘C for 1 h followed byannealing Following heat treatment samples were inspectedfor their microstructural development In the followingresults the nomenclature ldquoArdquo indicates annealed samples thathave been cooled in the furnace and ldquoQrdquo indicates water-quenched samples Optical images of samples at differentheat-treatment temperatures and cooling rates are shown inFigure 3There is no significant difference in samples of 650A(Figure 3(b)) 750A (Figure 3(d)) 850A (Figure 3(f)) 650Q(Figure 3(c)) and 750Q (Figure 3(e)) All of these imagesexhibit equiaxed 120572 phase surrounded by a 120573 phase boundaryThere is a slight difference between 850Q (Figure 3(g)) andothers where the 120573 phase boundary appears thicker In950A (Figure 3(h)) an equiaxed microstructure with pri-mary 120572 phase and a partial 120573 phase boundary still existsbut some transformation from the 120573 phase boundary tolamellar 120572 + 120573 phase has occurred At 950Q (Figure 3(i)) anequiaxed microstructure with primary 120572 phase and a partial120573 phase boundary is observed which is similar to 950AThe difference is that there is no lamellar 120572 + 120573 phase butinstead a region of metastable 120573 phase (dark region) thathas been transformed from the 120573 phase boundary regions isobserved Figure 3(j) (1050A) shows a typical basket-weaveWidmanstatten microstructure with an 120572 grain boundary inthe prior 120573 grains from annealed 1050∘C samples Figure 3(k)(1050Q) shows needle-like 1205721015840martensite microstructure with120573 phase between martensitic laths The grain size tends tobe coarse The results of quantitative microscopy giving thevolume fraction of phases for all different heat treatments aresummarized in Table 1
The SEM backscattered electron images of AR material(equiaxed) 950A (bimodal) 1050A (Widmanstatten) and1050Q (martensite) are shown in Figure 4
Table 1 Volume fraction results of 120572120573 1205731015840 phases
X-ray diffraction patterns of PM 950A 1050A and 1050Qare shown in Figure 5 respectively Reflections of 120572 and120573 phase were detected in diffraction patterns of equiaxedbimodal Widmanstatten and martensite It should be notedthat 120573 reflections are rather weak suggesting a relatively lowvolume fraction of the 120573 phase (Table 1) in PM and 950A
32 Mechanical Properties Hardness testing was conductedon heat-treated Ti6Al4V samples Figure 6 shows the resultsof hardness examination of Ti6Al4V heat-treated at 650∘C750∘C 850∘C 950∘C and 1050∘C for 1 h with annealing (A)and water quenching (Q) respectivelyThere is no significantchange of hardness values among 650A 750A 850A 650Qand 750Q Following the annealing process hardness valuesincreased from 850∘C to 1050∘C the highest hardness valuefor an annealed sample occurred at 1050∘C (741 higher thanfor the AR sample) Following water quenching hardnessvalues increased from between 750∘C and 850∘C to 1050∘Cthe highest hardness value is 1558 higher than that for theAR sample
The trends (Figure 7) of 02 proof stress values ofheat-treated Ti6Al4V following both annealing and waterquenching are very similar to those of the hardness valuesThe highest proof stress value occurred at a 1050∘C water-quenching condition which is 1378 higher than that of theAR sample
The value of elongation to failure of samples deterio-rated following annealing or water quenching following heattreatment in the temperature range 650∘Cndash1050∘C as seen inFigure 7 There is no significant change of elongation valuesamong 650A 750A 850A 650Q and 750Q In annealingelongation values decrease between 850∘C and 1050∘C thelowest elongation value is 2438 less than that of the ARmaterial In the water-quenched condition elongation valuesdropped off more quickly from a point between 750∘C and850∘C and 1050∘C The lowest elongation value was 4273smaller than that of the AR sample
Stress-strain curve of heat-treated Ti6Al4V followingboth annealing and water quenching from different temper-atures are shown in Figure 8
33 Fatigue Behavior Figure 9 shows the high-cycle fatiguefracture lives of the AR material martensite microstructureWidmanstatten microstructure and bimodal microstructuretransformed by 1050Q 1050A and 950A respectively Fatiguestrength (at 107 cycles) of the AR material is 180MPawhich was the highest for all microstructures 1050Q hadthe highest 02 proof stress but its fatigue strength was
4 Advances in Materials Science and Engineering
(a) PM
(b) 650A (c) 650Q
(d) 750A (e) 750Q
(f) 850A (g) 850Q
(h) 950A (i) 950Q
(j) 1050A (k) 1050Q
Grain orientation 3
Grain orientation 1Grain orientation 2
transformed from prior120573 phase boundary
Start transformation
Typicalbasket-wave
structuregrain
boundary
Needle-like structure
Primary 120572120573 phase boundary
200120583m
Prior 120573
Metastable 120573
120572 grain boundary
200120583m
Primary 120572
120573 phase boundary area
950A
950Q
Figure 3 Optical microstructure of AR material and heat-treated Ti6Al4V The number indicates the heat-treatment temperature ldquoArdquoindicates annealing (furnace-cooled) and ldquoQrdquo indicates quenching
(a) PM (b) 950A
(c) 1050A (d) 1050Q
Figure 4 SEM backscattered electron images of AR-PM 950A 1050A and 1050Q
Advances in Materials Science and Engineering 5
120572 pahse120573 pahse
0
50
100
150
200
250
30 35 40 45 50
2120579 (deg)
1050Q
1050A
950A
PM
Figure 5 X-ray diffraction pattern of PM 950A 1050A and 1050Q
Har
dnes
s (H
V)
Heat treatment temperature (∘C)
QuenchAnneal
440
420
400
380
360
600 650 700 750 800 850 900 950 1000 1050 1100
Figure 6 Effect of annealing temperatures and cooling scheduleson hardness of Ti6Al4V
lowest (135MPa) The fatigue strength of 1050A (158MPa)was slightly higher than that of 950A (153MPa)
34 Ti6Al4V Fractographs Figure 10 shows SEM fracto-graphs of fatigue crack propagation areas of AR mate-rial 1050Q (martensite microstructure) 950A (bimodalmicrostructure) and 1050A (Widmanstatten microstruc-ture)The fractograph of ARmaterial (Figure 10(a)) is flat anduniform Fine dimple patterns were observed However thefracture surface of the martensite microstructure is rougheras shown in Figure 10(b)The appearance of mountain-shapepatterns indicates typical brittle fatigue fracture The fracturesurface of the bimodal microstructure is flat but not as
Heat treatment temperature (∘C)600 700 800 900 1000 1100
02
p
roof
stre
ss (M
Pa)
ElongationAnnealing AnnealingWater quench Water quench
02 proof stress
Elon
gatio
n (
)
15
12
9
6
3
0
1300
1250
1200
1150
1100
1050
1000
950
Figure 7 Effect of different heat-treatment temperatures on 02proof stress and elongation of Ti6Al4V
0
200
400
600
800
1000
1200
1400
0 1 3 5 7 9 11 13 15
Stre
ss (M
Pa)
Strain ()650A650Q750A750Q850A
850Q950A950Q1050A1050Q
Figure 8 Stress-strain curve of heat-treated Ti6Al4V followingboth annealing and water quenching from different temperatures
uniform as exhibited in ARmaterial (Figure 10(c)) River pat-terns are observed in the fractograph of the Widmanstattenmicrostructure (Figure 10(d))
4 Discussion
Thedifferentmicrostructures are generated by heat treatmentof the AR material [10 22 23] Generally most of thecommercially used Ti6Al4V has been heat-treated via acomplex sequence of solution heat treatment deformationaging and annealing [5] for stress relief recrystallization
6 Advances in Materials Science and Engineering
Stre
ss am
plitu
de (M
Pa)
Life (cycle)As-received material950A bimodal
1050A Widmanst atten1050Q martensite
300
250
200
150
100
104 105 106 107 108
Figure 9 Stress amplitude versus life curves of AR material martensite Widmanstatten and bimodal microstructures of Ti6Al4V
(a) (b)
(c) (d)
Figure 10 SEM fractographs of the high-cycle fatigue samples of (a) AR material (b) 1050Q (c) 950A and (d) 1050A
and globularization Temperatures of 650∘Cndash850∘C are nor-mally used for these As shown in Figures 3(b)ndash3(f) themicrostructures were not modified There is no significantdifference in themechanical properties (hardness 02 proofstress and elongation) associated with heat treatment atthese temperatures In industrial application stress reliefrecrystallization and globularization can be realized byannealing at 650∘Cndash850∘C however the associated oxidationlayer thickness increases at the same time [24]
Water quenching from 850∘C to 1050∘C tends to increasethe strength owing to higher 120573 phase content [10] Waterquenching at 1050∘C (Figure 3(k)) transformed the equiaxedmicrostructure into acicular 1205721015840 martensite microstructureThe martensite sample exhibited the highest increases inhardness (419HV) and 02 proof stress (138) comparedto the AR sample but at the cost of a reduction in ductility(427 lower elongation compared to theAR sample) Similar
results for mechanical properties of acicular 1205721015840 martensitemicrostructure of Ti6Al4V were obtained by Jovanovic et al[10] By comparing the results of this paper with Jovanovicet alrsquos data (02 proof stress 1400MPa elongation 15water quenching from 1100∘C) it is obvious that the valuesof 02 proof stress (1114MPa) of acicular 1205721015840 martensitemicrostructure in this study are lower but elongation (78)is rather higher Jovanovic et al suggested two reasons forthese differences (1)Their ARmaterials wereWidmanstattenwith a massive 120573 phase boundary that exhibited higherstrength (1100MPa) but lower ductility (lt15) and (2) thepresence of TiC (formed from carbon diffused into the meltforming carbides with titanium and vanadium during thecasting process) raised the tensile strength but compromisedductility The fatigue strength of the acicular 1205721015840 martensitemicrostructure is the lowest (135MPa) being lower thanthe AR material by 45MPa The term ldquofatiguerdquo relates to
Advances in Materials Science and Engineering 7
Table 2 Summary of the study showing absolute values and variance (Δ) of properties of AR materials
Widmanstatten (1050A)(lamellar width of 10120583m +gt400 120583m120572 grain boundary)
1041 642 103 minus244 153 minus15
Martensite (1050Q)(200 120583m grain size with insideacicular 1205721015840)
1114 138 78 minus427 135 minus25
the initiation and growth of cracks caused by the repeatedapplication of mechanical stresses or strains and any asso-ciated changes in mechanical properties [21] Sun et al [25]reported that low ductility deteriorated fatigue propagationresistance in tensile mode fracture but high strength didlittle to help fatigue initiation resistance [25 26] Hence thepoor ductility in the acicular 1205721015840 martensite microstructureis one reason why its fatigue strength is so low The coarse-grained structure (Figure 3(k)) can be another reason forbrittle fatigue fracture (Figure 10(b)) It is well known thatfatigue cracking in Ti6Al4V alloys starts at prior 120573 grainboundaries or colony boundaries and 120572120573 interfaces [27]A coarse-grained structure will aggravate dislocations at theprior 120573 grain boundaries [28] and eventually lead to failureA third reason for the lower fracture strength in martensitecould be martensite transformation stress generally it iseasier to initiate cracks at a stress-concentration position [29]Martensite transformation stress may aggravate the stressconcentration
Ti6Al4V heat-treated at 950∘C followed by annealing(Figure 3(h)) exhibits slight lamellar microstructure at prior120573 phase boundaries Such bimodal microstructure (con-sisting partly of equiaxed primary 120572 and a lamellar 120572 +120573 matrix transformed from a prior 120573 phase boundary)results from heat treatment just below the 120573 transus tem-perature [13 30] Tensile test results show (Figure 7) thatbimodal microstructures exhibit well-balanced propertiesbetween 02proof strength (1017MPa) and ductility (118)The Widmanstatten microstructure heat-treated at 1050∘Cfollowed by annealing exhibits a slightly higher strength(1041MPa) than the bimodal microstructure but has lowerductility (103) than the bimodal microstructure Thisresearch indicated that Ti6Al4V with bimodal microstruc-ture has a slightly higher fatigue strength than that withWidmanstattenmicrostructure Similar results were obtainedby Zuo et al [30] The fatigue strengths for their bimodaland Widmanstatten microstructures were 493 and 475MParespectively (with both the stress ratio 119877 = minus1 and frequency= 20 kHz increasing the fatigue strength) Both Zuo et al[30] and Gil et al [5] have concluded that crack initiationrather than crack propagation plays the dominant role incontrolling the total life in high-cycle fatigue when there
are not too many interior defects Equiaxed microstruc-ture benefits fatigue initiation resistance Hence bimodalmicrostructure (with the properties of primary 120572 grain ofslightly lower strength and higher ductility) exhibits higherfatigue life compared with Widmanstatten microstructure(which is a special type of lamellarmicrostructure) Howeverbimodal microstructure exhibits lower fatigue life comparedwith equiaxed microstructure owing to its lower ductilitycompared with AR microstructure The fine dimple patternsobserved in fractographs of AR samplesmay indicate a plasticfatigue fracture process The AR material has an equiaxedmicrostructure that exhibited the highest ductility (136)
The correlations among the microstructures mechanicalproperties and fatigue strengths of 07mmTi6Al4V sheet arelisted together in Table 2
5 Conclusions
This study highlights the role of microstructure on fatiguestrength rather than relying on the broader relationshipbetween ductility and fatigue strength Three key observa-tions are apparent
(1) The 02 proof stress ranking from high to lowfor all types of microstructure of thin Ti6Al4Vsheets observed in this study is as follows aci-cular 1205721015840 martensite equiaxed microstructure withmetastable 120573 Widmanstatten bimodal and equiaxedmicrostructure
(2) The ductility ranking from high to low for all types ofthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and martensite microstructure
(3) The fatigue strength ranking from high to low forthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and acicular 1205721015840 martensite microstruc-ture
The fatigue strength of thin Ti6Al4V sheets decreasesinversely proportionally with the proof stress and propor-tionally with ductility this is in response to the types of
8 Advances in Materials Science and Engineering
microstructure and grain sizes set up in the heat-treatmentprocesses
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the materials supportprovided by Timet UK Limited The work is supportedby Fundamental Research Funds for the Central Universi-ties (2014QNA09) Natural Science Foundation of Jiangsu(BK20150205) and Postdoctoral Science Foundation ofJiangsu (1501029A)The authors also wish to express heartfeltthanks to Professor Philip Shipway and Professor Geoff Tans-ley for encouragement supervision and support throughoutthe work
References
[1] G D Tansley M Cook N Zhang M Chung J Woodard andJ Reizes Complete Passive Suspension of the Ventrassist RotaryBlood Pump Univerisy of Technology Sydney Australia 2000
[2] G Tansley S Vidakovic and J Reizes ldquoFluid dynamic char-acteristics of the VentrAssist rotary blood pumprdquo ArtificialOrgans vol 24 no 6 pp 483ndash487 2000
[3] Y Fan Z Chen C H Zhang and A M Liu ldquoA comparisonof microstructure and mechanical properties of welded thinTi6Al4V with three different types of laserrdquoMaterials ResearchInnovations vol 19 no S4 pp S187ndashS192 2015
[4] L W Tsay and C Y Tsay ldquoEffect of microstructures on thefatigue crack growth in Ti6Al4V laser weldsrdquo InternationalJournal of Fatigue vol 19 no 10 pp 713ndash720 1997
[5] F J Gil M P Ginebra J M Manero and J A PlanellldquoFormation of 120572-Widmanstatten structure effects of grain sizeand cooling rate on the Widmanstatten morphologies and onthe mechanical properties in Ti6Al4V alloyrdquo Journal of Alloysand Compounds vol 329 no 1-2 pp 142ndash152 2001
[6] F Caiazzo F Curcio G Daurelio and F M C MinutololdquoTi6Al4V sheets lap and butt joints carried out by CO
2
lasermechanical and morphological characterizationrdquo Journal ofMaterials Processing Technology vol 149 no 1ndash3 pp 546ndash5522004
[7] I J Polmear Light Alloys From Traditional Alloys to Nanocrys-tals Butterworth-Heinemann Oxford UK 2006
[8] N Poondla T S Srivatsan A Patnaik and M Petraroli ldquoAstudy of the microstructure and hardness of two titaniumalloys commercially pure andTindash6Alndash4Vrdquo Journal of Alloys andCompounds vol 486 no 1-2 pp 162ndash167 2009
[9] M Zitnansky and L Caplovic ldquoEffect of the thermomechanicaltreatment on the structure of titanium alloy Ti6Al4Vrdquo Journal ofMaterials Processing Technology vol 157-158 pp 643ndash649 2004
[10] M T Jovanovic S Tadic S Zec Z Miskovic and I BobicldquoThe effect of annealing temperatures and cooling rates onmicrostructure and mechanical properties of investment castTindash6Alndash4V alloyrdquo Materials and Design vol 27 no 3 pp 192ndash199 2006
[11] D Hardie and S Ouyang ldquoEffect of microstructure and heattreatment on fracture behaviour of smooth and precracked ten-sile specimens of Ti6Al4Vrdquo Materials Science and Technologyvol 15 no 9 pp 1049ndash1057 1999
[12] S Shademan V Sinha A B O Soboyejo and W O SoboyejoldquoAn investigation of the effects of microstructure and stressratio on fatigue crack growth in Tindash6Alndash4V with colony 120572120573microstructuresrdquo Mechanics of Materials vol 36 no 1-2 pp161ndash175 2004
[13] C Leyens and M Peter Titanium and Titanium Alloys Fun-damentals and Applications Wiley-VCH Verlag GmbH amp CoKGaA 2003
[14] C Loier G Thauvin A Hazotte and A Simon ldquoInfluence ofdeformation on the 120573 rarr 120572 + 120573 transformation kinetics of Ti-6 wtAl-4wtV alloyrdquo Journal of the Less Common Metalsvol 108 no 2 pp 295ndash312 1985
[15] T Mohandas D Banerjee and V V K Rao ldquoFusion zonemicrostructure and porosity in electron beam welds of an 120572+120573titanium alloyrdquo Metallurgical amp Materials Transactions A vol30 no 3 pp 789ndash798 1999
[16] T Ahmed and H J Rack ldquoPhase transformations during cool-ing in 120572+120573 titanium alloysrdquo Materials Science and Engineeringvol 243 no 1-2 pp 206ndash211 1998
[17] American Welding Society Recommended Practices for LaserBeamWelding Cutting and Drilling AWSANSI 1998
[18] X Li Research on the Microstructure and Fatigue Propertyof Electron Beam Welding Joint in Titanium Alloy HuazhongUniversity of Science and Technology Wuhan China 2012
[19] C Wu X Li X Huang J Ma and C Cao ldquoRelationship offatigue crack propagation and microstructure for TA15 alloyrdquoRare Metal Materials and Engineering vol 36 no 12 pp 2128ndash2131 2007
[20] European Standard EN10002-1 Metal material Tensile test2001
[21] British Standard 3518-1 Methods of fatigue testing 1993[22] W Sha and Z Guo ldquoPhase evolution of Tindash6A1ndash4V during
continuous heatingrdquo Journal of Alloys and Compounds vol 290no 1-2 pp L3ndashL7 1999
[23] P Sirilar and P Srichandr Grain Refinement of 120572120573 PhaseTi6Al4V Alloy by Thermomechanical Treatment King Mong-kutrsquos University of Technology Thonburi Bangkok Thailand2006
[24] Y Fan G D Tansley P H Shipway and J Niu ldquoThe applicationof laser welding on Left Ventricular Assist Device (LVAD)rdquo inProceedings of the Symposium on Photonics and Optoelectronics(SOPO rsquo11) pp 1ndash3 Wuhan China May 2011
[25] Z Sun L Jiang andZ Ying Failure Analysis Fundamentals andApplications China Machine Press 2005
[26] X L ZhengQuantitativeTheory ofMetal Fatigue NorthwesternPolytechnical University Press 1994
[27] J Oh N J Kim S Lee and E W Lee ldquoCorrelation of fatigueproperties and microstructure in investment cast Tindash6Alndash4Vweldsrdquo Materials Science and Engineering A vol 340 no 1-2pp 232ndash242 2003
[28] F X Gil Mur D Rodrıguez and J A Planell ldquoInfluence of tem-pering temperature and time on the 1205721015840-Ti-6A1-4V martensiterdquoJournal of Alloys and Compounds vol 234 no 2 pp 287ndash2891996
Advances in Materials Science and Engineering 9
[29] S R Lampman and N Dimatteo Fatigue and Fracture ASMInternational Geauga County Ohio USA 1996
[30] J H Zuo Z G Wang and E H Han ldquoEffect of microstructureon ultra-high cycle fatigue behavior of Ti-6Al-4Vrdquo MaterialsScience and Engineering A vol 473 no 1-2 pp 147ndash152 2008
Figure 2 Sample dimensions for tensile fatigue test pieces
Yorkshire LS10 2DE UK) was used for tensile fatigue testingFigure 2 shows a schematic diagram of the tensile fatigue testpieces designed to comply with British Standard 3518-1 [21]
The stress ratio used for all sampleswas 01 and the cyclingfrequencywas 7Hz Five sampleswere tested to at least 107 lifecycles
3 Results
31 Microstructures and X-Ray Diffraction of Heat-TreatedTi6Al4V Themicrostructure of Ti6Al4VARmaterial exhib-ited equiaxed 120572 phase surrounded by a 120573 phase boundarywith fine grain size (15ndash20120583m) The hardness Youngrsquos mod-ulus 02 proof stress and elongation of the AR materialwere respectively 362HV 111 GPa 978MPa and 136Other samples of Ti6Al4V material were heat-treated at650∘C 750∘C 850∘C 950∘C and 1050∘C for 1 h followed byannealing Following heat treatment samples were inspectedfor their microstructural development In the followingresults the nomenclature ldquoArdquo indicates annealed samples thathave been cooled in the furnace and ldquoQrdquo indicates water-quenched samples Optical images of samples at differentheat-treatment temperatures and cooling rates are shown inFigure 3There is no significant difference in samples of 650A(Figure 3(b)) 750A (Figure 3(d)) 850A (Figure 3(f)) 650Q(Figure 3(c)) and 750Q (Figure 3(e)) All of these imagesexhibit equiaxed 120572 phase surrounded by a 120573 phase boundaryThere is a slight difference between 850Q (Figure 3(g)) andothers where the 120573 phase boundary appears thicker In950A (Figure 3(h)) an equiaxed microstructure with pri-mary 120572 phase and a partial 120573 phase boundary still existsbut some transformation from the 120573 phase boundary tolamellar 120572 + 120573 phase has occurred At 950Q (Figure 3(i)) anequiaxed microstructure with primary 120572 phase and a partial120573 phase boundary is observed which is similar to 950AThe difference is that there is no lamellar 120572 + 120573 phase butinstead a region of metastable 120573 phase (dark region) thathas been transformed from the 120573 phase boundary regions isobserved Figure 3(j) (1050A) shows a typical basket-weaveWidmanstatten microstructure with an 120572 grain boundary inthe prior 120573 grains from annealed 1050∘C samples Figure 3(k)(1050Q) shows needle-like 1205721015840martensite microstructure with120573 phase between martensitic laths The grain size tends tobe coarse The results of quantitative microscopy giving thevolume fraction of phases for all different heat treatments aresummarized in Table 1
The SEM backscattered electron images of AR material(equiaxed) 950A (bimodal) 1050A (Widmanstatten) and1050Q (martensite) are shown in Figure 4
Table 1 Volume fraction results of 120572120573 1205731015840 phases
X-ray diffraction patterns of PM 950A 1050A and 1050Qare shown in Figure 5 respectively Reflections of 120572 and120573 phase were detected in diffraction patterns of equiaxedbimodal Widmanstatten and martensite It should be notedthat 120573 reflections are rather weak suggesting a relatively lowvolume fraction of the 120573 phase (Table 1) in PM and 950A
32 Mechanical Properties Hardness testing was conductedon heat-treated Ti6Al4V samples Figure 6 shows the resultsof hardness examination of Ti6Al4V heat-treated at 650∘C750∘C 850∘C 950∘C and 1050∘C for 1 h with annealing (A)and water quenching (Q) respectivelyThere is no significantchange of hardness values among 650A 750A 850A 650Qand 750Q Following the annealing process hardness valuesincreased from 850∘C to 1050∘C the highest hardness valuefor an annealed sample occurred at 1050∘C (741 higher thanfor the AR sample) Following water quenching hardnessvalues increased from between 750∘C and 850∘C to 1050∘Cthe highest hardness value is 1558 higher than that for theAR sample
The trends (Figure 7) of 02 proof stress values ofheat-treated Ti6Al4V following both annealing and waterquenching are very similar to those of the hardness valuesThe highest proof stress value occurred at a 1050∘C water-quenching condition which is 1378 higher than that of theAR sample
The value of elongation to failure of samples deterio-rated following annealing or water quenching following heattreatment in the temperature range 650∘Cndash1050∘C as seen inFigure 7 There is no significant change of elongation valuesamong 650A 750A 850A 650Q and 750Q In annealingelongation values decrease between 850∘C and 1050∘C thelowest elongation value is 2438 less than that of the ARmaterial In the water-quenched condition elongation valuesdropped off more quickly from a point between 750∘C and850∘C and 1050∘C The lowest elongation value was 4273smaller than that of the AR sample
Stress-strain curve of heat-treated Ti6Al4V followingboth annealing and water quenching from different temper-atures are shown in Figure 8
33 Fatigue Behavior Figure 9 shows the high-cycle fatiguefracture lives of the AR material martensite microstructureWidmanstatten microstructure and bimodal microstructuretransformed by 1050Q 1050A and 950A respectively Fatiguestrength (at 107 cycles) of the AR material is 180MPawhich was the highest for all microstructures 1050Q hadthe highest 02 proof stress but its fatigue strength was
4 Advances in Materials Science and Engineering
(a) PM
(b) 650A (c) 650Q
(d) 750A (e) 750Q
(f) 850A (g) 850Q
(h) 950A (i) 950Q
(j) 1050A (k) 1050Q
Grain orientation 3
Grain orientation 1Grain orientation 2
transformed from prior120573 phase boundary
Start transformation
Typicalbasket-wave
structuregrain
boundary
Needle-like structure
Primary 120572120573 phase boundary
200120583m
Prior 120573
Metastable 120573
120572 grain boundary
200120583m
Primary 120572
120573 phase boundary area
950A
950Q
Figure 3 Optical microstructure of AR material and heat-treated Ti6Al4V The number indicates the heat-treatment temperature ldquoArdquoindicates annealing (furnace-cooled) and ldquoQrdquo indicates quenching
(a) PM (b) 950A
(c) 1050A (d) 1050Q
Figure 4 SEM backscattered electron images of AR-PM 950A 1050A and 1050Q
Advances in Materials Science and Engineering 5
120572 pahse120573 pahse
0
50
100
150
200
250
30 35 40 45 50
2120579 (deg)
1050Q
1050A
950A
PM
Figure 5 X-ray diffraction pattern of PM 950A 1050A and 1050Q
Har
dnes
s (H
V)
Heat treatment temperature (∘C)
QuenchAnneal
440
420
400
380
360
600 650 700 750 800 850 900 950 1000 1050 1100
Figure 6 Effect of annealing temperatures and cooling scheduleson hardness of Ti6Al4V
lowest (135MPa) The fatigue strength of 1050A (158MPa)was slightly higher than that of 950A (153MPa)
34 Ti6Al4V Fractographs Figure 10 shows SEM fracto-graphs of fatigue crack propagation areas of AR mate-rial 1050Q (martensite microstructure) 950A (bimodalmicrostructure) and 1050A (Widmanstatten microstruc-ture)The fractograph of ARmaterial (Figure 10(a)) is flat anduniform Fine dimple patterns were observed However thefracture surface of the martensite microstructure is rougheras shown in Figure 10(b)The appearance of mountain-shapepatterns indicates typical brittle fatigue fracture The fracturesurface of the bimodal microstructure is flat but not as
Heat treatment temperature (∘C)600 700 800 900 1000 1100
02
p
roof
stre
ss (M
Pa)
ElongationAnnealing AnnealingWater quench Water quench
02 proof stress
Elon
gatio
n (
)
15
12
9
6
3
0
1300
1250
1200
1150
1100
1050
1000
950
Figure 7 Effect of different heat-treatment temperatures on 02proof stress and elongation of Ti6Al4V
0
200
400
600
800
1000
1200
1400
0 1 3 5 7 9 11 13 15
Stre
ss (M
Pa)
Strain ()650A650Q750A750Q850A
850Q950A950Q1050A1050Q
Figure 8 Stress-strain curve of heat-treated Ti6Al4V followingboth annealing and water quenching from different temperatures
uniform as exhibited in ARmaterial (Figure 10(c)) River pat-terns are observed in the fractograph of the Widmanstattenmicrostructure (Figure 10(d))
4 Discussion
Thedifferentmicrostructures are generated by heat treatmentof the AR material [10 22 23] Generally most of thecommercially used Ti6Al4V has been heat-treated via acomplex sequence of solution heat treatment deformationaging and annealing [5] for stress relief recrystallization
6 Advances in Materials Science and Engineering
Stre
ss am
plitu
de (M
Pa)
Life (cycle)As-received material950A bimodal
1050A Widmanst atten1050Q martensite
300
250
200
150
100
104 105 106 107 108
Figure 9 Stress amplitude versus life curves of AR material martensite Widmanstatten and bimodal microstructures of Ti6Al4V
(a) (b)
(c) (d)
Figure 10 SEM fractographs of the high-cycle fatigue samples of (a) AR material (b) 1050Q (c) 950A and (d) 1050A
and globularization Temperatures of 650∘Cndash850∘C are nor-mally used for these As shown in Figures 3(b)ndash3(f) themicrostructures were not modified There is no significantdifference in themechanical properties (hardness 02 proofstress and elongation) associated with heat treatment atthese temperatures In industrial application stress reliefrecrystallization and globularization can be realized byannealing at 650∘Cndash850∘C however the associated oxidationlayer thickness increases at the same time [24]
Water quenching from 850∘C to 1050∘C tends to increasethe strength owing to higher 120573 phase content [10] Waterquenching at 1050∘C (Figure 3(k)) transformed the equiaxedmicrostructure into acicular 1205721015840 martensite microstructureThe martensite sample exhibited the highest increases inhardness (419HV) and 02 proof stress (138) comparedto the AR sample but at the cost of a reduction in ductility(427 lower elongation compared to theAR sample) Similar
results for mechanical properties of acicular 1205721015840 martensitemicrostructure of Ti6Al4V were obtained by Jovanovic et al[10] By comparing the results of this paper with Jovanovicet alrsquos data (02 proof stress 1400MPa elongation 15water quenching from 1100∘C) it is obvious that the valuesof 02 proof stress (1114MPa) of acicular 1205721015840 martensitemicrostructure in this study are lower but elongation (78)is rather higher Jovanovic et al suggested two reasons forthese differences (1)Their ARmaterials wereWidmanstattenwith a massive 120573 phase boundary that exhibited higherstrength (1100MPa) but lower ductility (lt15) and (2) thepresence of TiC (formed from carbon diffused into the meltforming carbides with titanium and vanadium during thecasting process) raised the tensile strength but compromisedductility The fatigue strength of the acicular 1205721015840 martensitemicrostructure is the lowest (135MPa) being lower thanthe AR material by 45MPa The term ldquofatiguerdquo relates to
Advances in Materials Science and Engineering 7
Table 2 Summary of the study showing absolute values and variance (Δ) of properties of AR materials
Widmanstatten (1050A)(lamellar width of 10120583m +gt400 120583m120572 grain boundary)
1041 642 103 minus244 153 minus15
Martensite (1050Q)(200 120583m grain size with insideacicular 1205721015840)
1114 138 78 minus427 135 minus25
the initiation and growth of cracks caused by the repeatedapplication of mechanical stresses or strains and any asso-ciated changes in mechanical properties [21] Sun et al [25]reported that low ductility deteriorated fatigue propagationresistance in tensile mode fracture but high strength didlittle to help fatigue initiation resistance [25 26] Hence thepoor ductility in the acicular 1205721015840 martensite microstructureis one reason why its fatigue strength is so low The coarse-grained structure (Figure 3(k)) can be another reason forbrittle fatigue fracture (Figure 10(b)) It is well known thatfatigue cracking in Ti6Al4V alloys starts at prior 120573 grainboundaries or colony boundaries and 120572120573 interfaces [27]A coarse-grained structure will aggravate dislocations at theprior 120573 grain boundaries [28] and eventually lead to failureA third reason for the lower fracture strength in martensitecould be martensite transformation stress generally it iseasier to initiate cracks at a stress-concentration position [29]Martensite transformation stress may aggravate the stressconcentration
Ti6Al4V heat-treated at 950∘C followed by annealing(Figure 3(h)) exhibits slight lamellar microstructure at prior120573 phase boundaries Such bimodal microstructure (con-sisting partly of equiaxed primary 120572 and a lamellar 120572 +120573 matrix transformed from a prior 120573 phase boundary)results from heat treatment just below the 120573 transus tem-perature [13 30] Tensile test results show (Figure 7) thatbimodal microstructures exhibit well-balanced propertiesbetween 02proof strength (1017MPa) and ductility (118)The Widmanstatten microstructure heat-treated at 1050∘Cfollowed by annealing exhibits a slightly higher strength(1041MPa) than the bimodal microstructure but has lowerductility (103) than the bimodal microstructure Thisresearch indicated that Ti6Al4V with bimodal microstruc-ture has a slightly higher fatigue strength than that withWidmanstattenmicrostructure Similar results were obtainedby Zuo et al [30] The fatigue strengths for their bimodaland Widmanstatten microstructures were 493 and 475MParespectively (with both the stress ratio 119877 = minus1 and frequency= 20 kHz increasing the fatigue strength) Both Zuo et al[30] and Gil et al [5] have concluded that crack initiationrather than crack propagation plays the dominant role incontrolling the total life in high-cycle fatigue when there
are not too many interior defects Equiaxed microstruc-ture benefits fatigue initiation resistance Hence bimodalmicrostructure (with the properties of primary 120572 grain ofslightly lower strength and higher ductility) exhibits higherfatigue life compared with Widmanstatten microstructure(which is a special type of lamellarmicrostructure) Howeverbimodal microstructure exhibits lower fatigue life comparedwith equiaxed microstructure owing to its lower ductilitycompared with AR microstructure The fine dimple patternsobserved in fractographs of AR samplesmay indicate a plasticfatigue fracture process The AR material has an equiaxedmicrostructure that exhibited the highest ductility (136)
The correlations among the microstructures mechanicalproperties and fatigue strengths of 07mmTi6Al4V sheet arelisted together in Table 2
5 Conclusions
This study highlights the role of microstructure on fatiguestrength rather than relying on the broader relationshipbetween ductility and fatigue strength Three key observa-tions are apparent
(1) The 02 proof stress ranking from high to lowfor all types of microstructure of thin Ti6Al4Vsheets observed in this study is as follows aci-cular 1205721015840 martensite equiaxed microstructure withmetastable 120573 Widmanstatten bimodal and equiaxedmicrostructure
(2) The ductility ranking from high to low for all types ofthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and martensite microstructure
(3) The fatigue strength ranking from high to low forthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and acicular 1205721015840 martensite microstruc-ture
The fatigue strength of thin Ti6Al4V sheets decreasesinversely proportionally with the proof stress and propor-tionally with ductility this is in response to the types of
8 Advances in Materials Science and Engineering
microstructure and grain sizes set up in the heat-treatmentprocesses
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the materials supportprovided by Timet UK Limited The work is supportedby Fundamental Research Funds for the Central Universi-ties (2014QNA09) Natural Science Foundation of Jiangsu(BK20150205) and Postdoctoral Science Foundation ofJiangsu (1501029A)The authors also wish to express heartfeltthanks to Professor Philip Shipway and Professor Geoff Tans-ley for encouragement supervision and support throughoutthe work
References
[1] G D Tansley M Cook N Zhang M Chung J Woodard andJ Reizes Complete Passive Suspension of the Ventrassist RotaryBlood Pump Univerisy of Technology Sydney Australia 2000
[2] G Tansley S Vidakovic and J Reizes ldquoFluid dynamic char-acteristics of the VentrAssist rotary blood pumprdquo ArtificialOrgans vol 24 no 6 pp 483ndash487 2000
[3] Y Fan Z Chen C H Zhang and A M Liu ldquoA comparisonof microstructure and mechanical properties of welded thinTi6Al4V with three different types of laserrdquoMaterials ResearchInnovations vol 19 no S4 pp S187ndashS192 2015
[4] L W Tsay and C Y Tsay ldquoEffect of microstructures on thefatigue crack growth in Ti6Al4V laser weldsrdquo InternationalJournal of Fatigue vol 19 no 10 pp 713ndash720 1997
[5] F J Gil M P Ginebra J M Manero and J A PlanellldquoFormation of 120572-Widmanstatten structure effects of grain sizeand cooling rate on the Widmanstatten morphologies and onthe mechanical properties in Ti6Al4V alloyrdquo Journal of Alloysand Compounds vol 329 no 1-2 pp 142ndash152 2001
[6] F Caiazzo F Curcio G Daurelio and F M C MinutololdquoTi6Al4V sheets lap and butt joints carried out by CO
2
lasermechanical and morphological characterizationrdquo Journal ofMaterials Processing Technology vol 149 no 1ndash3 pp 546ndash5522004
[7] I J Polmear Light Alloys From Traditional Alloys to Nanocrys-tals Butterworth-Heinemann Oxford UK 2006
[8] N Poondla T S Srivatsan A Patnaik and M Petraroli ldquoAstudy of the microstructure and hardness of two titaniumalloys commercially pure andTindash6Alndash4Vrdquo Journal of Alloys andCompounds vol 486 no 1-2 pp 162ndash167 2009
[9] M Zitnansky and L Caplovic ldquoEffect of the thermomechanicaltreatment on the structure of titanium alloy Ti6Al4Vrdquo Journal ofMaterials Processing Technology vol 157-158 pp 643ndash649 2004
[10] M T Jovanovic S Tadic S Zec Z Miskovic and I BobicldquoThe effect of annealing temperatures and cooling rates onmicrostructure and mechanical properties of investment castTindash6Alndash4V alloyrdquo Materials and Design vol 27 no 3 pp 192ndash199 2006
[11] D Hardie and S Ouyang ldquoEffect of microstructure and heattreatment on fracture behaviour of smooth and precracked ten-sile specimens of Ti6Al4Vrdquo Materials Science and Technologyvol 15 no 9 pp 1049ndash1057 1999
[12] S Shademan V Sinha A B O Soboyejo and W O SoboyejoldquoAn investigation of the effects of microstructure and stressratio on fatigue crack growth in Tindash6Alndash4V with colony 120572120573microstructuresrdquo Mechanics of Materials vol 36 no 1-2 pp161ndash175 2004
[13] C Leyens and M Peter Titanium and Titanium Alloys Fun-damentals and Applications Wiley-VCH Verlag GmbH amp CoKGaA 2003
[14] C Loier G Thauvin A Hazotte and A Simon ldquoInfluence ofdeformation on the 120573 rarr 120572 + 120573 transformation kinetics of Ti-6 wtAl-4wtV alloyrdquo Journal of the Less Common Metalsvol 108 no 2 pp 295ndash312 1985
[15] T Mohandas D Banerjee and V V K Rao ldquoFusion zonemicrostructure and porosity in electron beam welds of an 120572+120573titanium alloyrdquo Metallurgical amp Materials Transactions A vol30 no 3 pp 789ndash798 1999
[16] T Ahmed and H J Rack ldquoPhase transformations during cool-ing in 120572+120573 titanium alloysrdquo Materials Science and Engineeringvol 243 no 1-2 pp 206ndash211 1998
[17] American Welding Society Recommended Practices for LaserBeamWelding Cutting and Drilling AWSANSI 1998
[18] X Li Research on the Microstructure and Fatigue Propertyof Electron Beam Welding Joint in Titanium Alloy HuazhongUniversity of Science and Technology Wuhan China 2012
[19] C Wu X Li X Huang J Ma and C Cao ldquoRelationship offatigue crack propagation and microstructure for TA15 alloyrdquoRare Metal Materials and Engineering vol 36 no 12 pp 2128ndash2131 2007
[20] European Standard EN10002-1 Metal material Tensile test2001
[21] British Standard 3518-1 Methods of fatigue testing 1993[22] W Sha and Z Guo ldquoPhase evolution of Tindash6A1ndash4V during
continuous heatingrdquo Journal of Alloys and Compounds vol 290no 1-2 pp L3ndashL7 1999
[23] P Sirilar and P Srichandr Grain Refinement of 120572120573 PhaseTi6Al4V Alloy by Thermomechanical Treatment King Mong-kutrsquos University of Technology Thonburi Bangkok Thailand2006
[24] Y Fan G D Tansley P H Shipway and J Niu ldquoThe applicationof laser welding on Left Ventricular Assist Device (LVAD)rdquo inProceedings of the Symposium on Photonics and Optoelectronics(SOPO rsquo11) pp 1ndash3 Wuhan China May 2011
[25] Z Sun L Jiang andZ Ying Failure Analysis Fundamentals andApplications China Machine Press 2005
[26] X L ZhengQuantitativeTheory ofMetal Fatigue NorthwesternPolytechnical University Press 1994
[27] J Oh N J Kim S Lee and E W Lee ldquoCorrelation of fatigueproperties and microstructure in investment cast Tindash6Alndash4Vweldsrdquo Materials Science and Engineering A vol 340 no 1-2pp 232ndash242 2003
[28] F X Gil Mur D Rodrıguez and J A Planell ldquoInfluence of tem-pering temperature and time on the 1205721015840-Ti-6A1-4V martensiterdquoJournal of Alloys and Compounds vol 234 no 2 pp 287ndash2891996
Advances in Materials Science and Engineering 9
[29] S R Lampman and N Dimatteo Fatigue and Fracture ASMInternational Geauga County Ohio USA 1996
[30] J H Zuo Z G Wang and E H Han ldquoEffect of microstructureon ultra-high cycle fatigue behavior of Ti-6Al-4Vrdquo MaterialsScience and Engineering A vol 473 no 1-2 pp 147ndash152 2008
Figure 3 Optical microstructure of AR material and heat-treated Ti6Al4V The number indicates the heat-treatment temperature ldquoArdquoindicates annealing (furnace-cooled) and ldquoQrdquo indicates quenching
(a) PM (b) 950A
(c) 1050A (d) 1050Q
Figure 4 SEM backscattered electron images of AR-PM 950A 1050A and 1050Q
Advances in Materials Science and Engineering 5
120572 pahse120573 pahse
0
50
100
150
200
250
30 35 40 45 50
2120579 (deg)
1050Q
1050A
950A
PM
Figure 5 X-ray diffraction pattern of PM 950A 1050A and 1050Q
Har
dnes
s (H
V)
Heat treatment temperature (∘C)
QuenchAnneal
440
420
400
380
360
600 650 700 750 800 850 900 950 1000 1050 1100
Figure 6 Effect of annealing temperatures and cooling scheduleson hardness of Ti6Al4V
lowest (135MPa) The fatigue strength of 1050A (158MPa)was slightly higher than that of 950A (153MPa)
34 Ti6Al4V Fractographs Figure 10 shows SEM fracto-graphs of fatigue crack propagation areas of AR mate-rial 1050Q (martensite microstructure) 950A (bimodalmicrostructure) and 1050A (Widmanstatten microstruc-ture)The fractograph of ARmaterial (Figure 10(a)) is flat anduniform Fine dimple patterns were observed However thefracture surface of the martensite microstructure is rougheras shown in Figure 10(b)The appearance of mountain-shapepatterns indicates typical brittle fatigue fracture The fracturesurface of the bimodal microstructure is flat but not as
Heat treatment temperature (∘C)600 700 800 900 1000 1100
02
p
roof
stre
ss (M
Pa)
ElongationAnnealing AnnealingWater quench Water quench
02 proof stress
Elon
gatio
n (
)
15
12
9
6
3
0
1300
1250
1200
1150
1100
1050
1000
950
Figure 7 Effect of different heat-treatment temperatures on 02proof stress and elongation of Ti6Al4V
0
200
400
600
800
1000
1200
1400
0 1 3 5 7 9 11 13 15
Stre
ss (M
Pa)
Strain ()650A650Q750A750Q850A
850Q950A950Q1050A1050Q
Figure 8 Stress-strain curve of heat-treated Ti6Al4V followingboth annealing and water quenching from different temperatures
uniform as exhibited in ARmaterial (Figure 10(c)) River pat-terns are observed in the fractograph of the Widmanstattenmicrostructure (Figure 10(d))
4 Discussion
Thedifferentmicrostructures are generated by heat treatmentof the AR material [10 22 23] Generally most of thecommercially used Ti6Al4V has been heat-treated via acomplex sequence of solution heat treatment deformationaging and annealing [5] for stress relief recrystallization
6 Advances in Materials Science and Engineering
Stre
ss am
plitu
de (M
Pa)
Life (cycle)As-received material950A bimodal
1050A Widmanst atten1050Q martensite
300
250
200
150
100
104 105 106 107 108
Figure 9 Stress amplitude versus life curves of AR material martensite Widmanstatten and bimodal microstructures of Ti6Al4V
(a) (b)
(c) (d)
Figure 10 SEM fractographs of the high-cycle fatigue samples of (a) AR material (b) 1050Q (c) 950A and (d) 1050A
and globularization Temperatures of 650∘Cndash850∘C are nor-mally used for these As shown in Figures 3(b)ndash3(f) themicrostructures were not modified There is no significantdifference in themechanical properties (hardness 02 proofstress and elongation) associated with heat treatment atthese temperatures In industrial application stress reliefrecrystallization and globularization can be realized byannealing at 650∘Cndash850∘C however the associated oxidationlayer thickness increases at the same time [24]
Water quenching from 850∘C to 1050∘C tends to increasethe strength owing to higher 120573 phase content [10] Waterquenching at 1050∘C (Figure 3(k)) transformed the equiaxedmicrostructure into acicular 1205721015840 martensite microstructureThe martensite sample exhibited the highest increases inhardness (419HV) and 02 proof stress (138) comparedto the AR sample but at the cost of a reduction in ductility(427 lower elongation compared to theAR sample) Similar
results for mechanical properties of acicular 1205721015840 martensitemicrostructure of Ti6Al4V were obtained by Jovanovic et al[10] By comparing the results of this paper with Jovanovicet alrsquos data (02 proof stress 1400MPa elongation 15water quenching from 1100∘C) it is obvious that the valuesof 02 proof stress (1114MPa) of acicular 1205721015840 martensitemicrostructure in this study are lower but elongation (78)is rather higher Jovanovic et al suggested two reasons forthese differences (1)Their ARmaterials wereWidmanstattenwith a massive 120573 phase boundary that exhibited higherstrength (1100MPa) but lower ductility (lt15) and (2) thepresence of TiC (formed from carbon diffused into the meltforming carbides with titanium and vanadium during thecasting process) raised the tensile strength but compromisedductility The fatigue strength of the acicular 1205721015840 martensitemicrostructure is the lowest (135MPa) being lower thanthe AR material by 45MPa The term ldquofatiguerdquo relates to
Advances in Materials Science and Engineering 7
Table 2 Summary of the study showing absolute values and variance (Δ) of properties of AR materials
Widmanstatten (1050A)(lamellar width of 10120583m +gt400 120583m120572 grain boundary)
1041 642 103 minus244 153 minus15
Martensite (1050Q)(200 120583m grain size with insideacicular 1205721015840)
1114 138 78 minus427 135 minus25
the initiation and growth of cracks caused by the repeatedapplication of mechanical stresses or strains and any asso-ciated changes in mechanical properties [21] Sun et al [25]reported that low ductility deteriorated fatigue propagationresistance in tensile mode fracture but high strength didlittle to help fatigue initiation resistance [25 26] Hence thepoor ductility in the acicular 1205721015840 martensite microstructureis one reason why its fatigue strength is so low The coarse-grained structure (Figure 3(k)) can be another reason forbrittle fatigue fracture (Figure 10(b)) It is well known thatfatigue cracking in Ti6Al4V alloys starts at prior 120573 grainboundaries or colony boundaries and 120572120573 interfaces [27]A coarse-grained structure will aggravate dislocations at theprior 120573 grain boundaries [28] and eventually lead to failureA third reason for the lower fracture strength in martensitecould be martensite transformation stress generally it iseasier to initiate cracks at a stress-concentration position [29]Martensite transformation stress may aggravate the stressconcentration
Ti6Al4V heat-treated at 950∘C followed by annealing(Figure 3(h)) exhibits slight lamellar microstructure at prior120573 phase boundaries Such bimodal microstructure (con-sisting partly of equiaxed primary 120572 and a lamellar 120572 +120573 matrix transformed from a prior 120573 phase boundary)results from heat treatment just below the 120573 transus tem-perature [13 30] Tensile test results show (Figure 7) thatbimodal microstructures exhibit well-balanced propertiesbetween 02proof strength (1017MPa) and ductility (118)The Widmanstatten microstructure heat-treated at 1050∘Cfollowed by annealing exhibits a slightly higher strength(1041MPa) than the bimodal microstructure but has lowerductility (103) than the bimodal microstructure Thisresearch indicated that Ti6Al4V with bimodal microstruc-ture has a slightly higher fatigue strength than that withWidmanstattenmicrostructure Similar results were obtainedby Zuo et al [30] The fatigue strengths for their bimodaland Widmanstatten microstructures were 493 and 475MParespectively (with both the stress ratio 119877 = minus1 and frequency= 20 kHz increasing the fatigue strength) Both Zuo et al[30] and Gil et al [5] have concluded that crack initiationrather than crack propagation plays the dominant role incontrolling the total life in high-cycle fatigue when there
are not too many interior defects Equiaxed microstruc-ture benefits fatigue initiation resistance Hence bimodalmicrostructure (with the properties of primary 120572 grain ofslightly lower strength and higher ductility) exhibits higherfatigue life compared with Widmanstatten microstructure(which is a special type of lamellarmicrostructure) Howeverbimodal microstructure exhibits lower fatigue life comparedwith equiaxed microstructure owing to its lower ductilitycompared with AR microstructure The fine dimple patternsobserved in fractographs of AR samplesmay indicate a plasticfatigue fracture process The AR material has an equiaxedmicrostructure that exhibited the highest ductility (136)
The correlations among the microstructures mechanicalproperties and fatigue strengths of 07mmTi6Al4V sheet arelisted together in Table 2
5 Conclusions
This study highlights the role of microstructure on fatiguestrength rather than relying on the broader relationshipbetween ductility and fatigue strength Three key observa-tions are apparent
(1) The 02 proof stress ranking from high to lowfor all types of microstructure of thin Ti6Al4Vsheets observed in this study is as follows aci-cular 1205721015840 martensite equiaxed microstructure withmetastable 120573 Widmanstatten bimodal and equiaxedmicrostructure
(2) The ductility ranking from high to low for all types ofthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and martensite microstructure
(3) The fatigue strength ranking from high to low forthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and acicular 1205721015840 martensite microstruc-ture
The fatigue strength of thin Ti6Al4V sheets decreasesinversely proportionally with the proof stress and propor-tionally with ductility this is in response to the types of
8 Advances in Materials Science and Engineering
microstructure and grain sizes set up in the heat-treatmentprocesses
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the materials supportprovided by Timet UK Limited The work is supportedby Fundamental Research Funds for the Central Universi-ties (2014QNA09) Natural Science Foundation of Jiangsu(BK20150205) and Postdoctoral Science Foundation ofJiangsu (1501029A)The authors also wish to express heartfeltthanks to Professor Philip Shipway and Professor Geoff Tans-ley for encouragement supervision and support throughoutthe work
References
[1] G D Tansley M Cook N Zhang M Chung J Woodard andJ Reizes Complete Passive Suspension of the Ventrassist RotaryBlood Pump Univerisy of Technology Sydney Australia 2000
[2] G Tansley S Vidakovic and J Reizes ldquoFluid dynamic char-acteristics of the VentrAssist rotary blood pumprdquo ArtificialOrgans vol 24 no 6 pp 483ndash487 2000
[3] Y Fan Z Chen C H Zhang and A M Liu ldquoA comparisonof microstructure and mechanical properties of welded thinTi6Al4V with three different types of laserrdquoMaterials ResearchInnovations vol 19 no S4 pp S187ndashS192 2015
[4] L W Tsay and C Y Tsay ldquoEffect of microstructures on thefatigue crack growth in Ti6Al4V laser weldsrdquo InternationalJournal of Fatigue vol 19 no 10 pp 713ndash720 1997
[5] F J Gil M P Ginebra J M Manero and J A PlanellldquoFormation of 120572-Widmanstatten structure effects of grain sizeand cooling rate on the Widmanstatten morphologies and onthe mechanical properties in Ti6Al4V alloyrdquo Journal of Alloysand Compounds vol 329 no 1-2 pp 142ndash152 2001
[6] F Caiazzo F Curcio G Daurelio and F M C MinutololdquoTi6Al4V sheets lap and butt joints carried out by CO
2
lasermechanical and morphological characterizationrdquo Journal ofMaterials Processing Technology vol 149 no 1ndash3 pp 546ndash5522004
[7] I J Polmear Light Alloys From Traditional Alloys to Nanocrys-tals Butterworth-Heinemann Oxford UK 2006
[8] N Poondla T S Srivatsan A Patnaik and M Petraroli ldquoAstudy of the microstructure and hardness of two titaniumalloys commercially pure andTindash6Alndash4Vrdquo Journal of Alloys andCompounds vol 486 no 1-2 pp 162ndash167 2009
[9] M Zitnansky and L Caplovic ldquoEffect of the thermomechanicaltreatment on the structure of titanium alloy Ti6Al4Vrdquo Journal ofMaterials Processing Technology vol 157-158 pp 643ndash649 2004
[10] M T Jovanovic S Tadic S Zec Z Miskovic and I BobicldquoThe effect of annealing temperatures and cooling rates onmicrostructure and mechanical properties of investment castTindash6Alndash4V alloyrdquo Materials and Design vol 27 no 3 pp 192ndash199 2006
[11] D Hardie and S Ouyang ldquoEffect of microstructure and heattreatment on fracture behaviour of smooth and precracked ten-sile specimens of Ti6Al4Vrdquo Materials Science and Technologyvol 15 no 9 pp 1049ndash1057 1999
[12] S Shademan V Sinha A B O Soboyejo and W O SoboyejoldquoAn investigation of the effects of microstructure and stressratio on fatigue crack growth in Tindash6Alndash4V with colony 120572120573microstructuresrdquo Mechanics of Materials vol 36 no 1-2 pp161ndash175 2004
[13] C Leyens and M Peter Titanium and Titanium Alloys Fun-damentals and Applications Wiley-VCH Verlag GmbH amp CoKGaA 2003
[14] C Loier G Thauvin A Hazotte and A Simon ldquoInfluence ofdeformation on the 120573 rarr 120572 + 120573 transformation kinetics of Ti-6 wtAl-4wtV alloyrdquo Journal of the Less Common Metalsvol 108 no 2 pp 295ndash312 1985
[15] T Mohandas D Banerjee and V V K Rao ldquoFusion zonemicrostructure and porosity in electron beam welds of an 120572+120573titanium alloyrdquo Metallurgical amp Materials Transactions A vol30 no 3 pp 789ndash798 1999
[16] T Ahmed and H J Rack ldquoPhase transformations during cool-ing in 120572+120573 titanium alloysrdquo Materials Science and Engineeringvol 243 no 1-2 pp 206ndash211 1998
[17] American Welding Society Recommended Practices for LaserBeamWelding Cutting and Drilling AWSANSI 1998
[18] X Li Research on the Microstructure and Fatigue Propertyof Electron Beam Welding Joint in Titanium Alloy HuazhongUniversity of Science and Technology Wuhan China 2012
[19] C Wu X Li X Huang J Ma and C Cao ldquoRelationship offatigue crack propagation and microstructure for TA15 alloyrdquoRare Metal Materials and Engineering vol 36 no 12 pp 2128ndash2131 2007
[20] European Standard EN10002-1 Metal material Tensile test2001
[21] British Standard 3518-1 Methods of fatigue testing 1993[22] W Sha and Z Guo ldquoPhase evolution of Tindash6A1ndash4V during
continuous heatingrdquo Journal of Alloys and Compounds vol 290no 1-2 pp L3ndashL7 1999
[23] P Sirilar and P Srichandr Grain Refinement of 120572120573 PhaseTi6Al4V Alloy by Thermomechanical Treatment King Mong-kutrsquos University of Technology Thonburi Bangkok Thailand2006
[24] Y Fan G D Tansley P H Shipway and J Niu ldquoThe applicationof laser welding on Left Ventricular Assist Device (LVAD)rdquo inProceedings of the Symposium on Photonics and Optoelectronics(SOPO rsquo11) pp 1ndash3 Wuhan China May 2011
[25] Z Sun L Jiang andZ Ying Failure Analysis Fundamentals andApplications China Machine Press 2005
[26] X L ZhengQuantitativeTheory ofMetal Fatigue NorthwesternPolytechnical University Press 1994
[27] J Oh N J Kim S Lee and E W Lee ldquoCorrelation of fatigueproperties and microstructure in investment cast Tindash6Alndash4Vweldsrdquo Materials Science and Engineering A vol 340 no 1-2pp 232ndash242 2003
[28] F X Gil Mur D Rodrıguez and J A Planell ldquoInfluence of tem-pering temperature and time on the 1205721015840-Ti-6A1-4V martensiterdquoJournal of Alloys and Compounds vol 234 no 2 pp 287ndash2891996
Advances in Materials Science and Engineering 9
[29] S R Lampman and N Dimatteo Fatigue and Fracture ASMInternational Geauga County Ohio USA 1996
[30] J H Zuo Z G Wang and E H Han ldquoEffect of microstructureon ultra-high cycle fatigue behavior of Ti-6Al-4Vrdquo MaterialsScience and Engineering A vol 473 no 1-2 pp 147ndash152 2008
Figure 5 X-ray diffraction pattern of PM 950A 1050A and 1050Q
Har
dnes
s (H
V)
Heat treatment temperature (∘C)
QuenchAnneal
440
420
400
380
360
600 650 700 750 800 850 900 950 1000 1050 1100
Figure 6 Effect of annealing temperatures and cooling scheduleson hardness of Ti6Al4V
lowest (135MPa) The fatigue strength of 1050A (158MPa)was slightly higher than that of 950A (153MPa)
34 Ti6Al4V Fractographs Figure 10 shows SEM fracto-graphs of fatigue crack propagation areas of AR mate-rial 1050Q (martensite microstructure) 950A (bimodalmicrostructure) and 1050A (Widmanstatten microstruc-ture)The fractograph of ARmaterial (Figure 10(a)) is flat anduniform Fine dimple patterns were observed However thefracture surface of the martensite microstructure is rougheras shown in Figure 10(b)The appearance of mountain-shapepatterns indicates typical brittle fatigue fracture The fracturesurface of the bimodal microstructure is flat but not as
Heat treatment temperature (∘C)600 700 800 900 1000 1100
02
p
roof
stre
ss (M
Pa)
ElongationAnnealing AnnealingWater quench Water quench
02 proof stress
Elon
gatio
n (
)
15
12
9
6
3
0
1300
1250
1200
1150
1100
1050
1000
950
Figure 7 Effect of different heat-treatment temperatures on 02proof stress and elongation of Ti6Al4V
0
200
400
600
800
1000
1200
1400
0 1 3 5 7 9 11 13 15
Stre
ss (M
Pa)
Strain ()650A650Q750A750Q850A
850Q950A950Q1050A1050Q
Figure 8 Stress-strain curve of heat-treated Ti6Al4V followingboth annealing and water quenching from different temperatures
uniform as exhibited in ARmaterial (Figure 10(c)) River pat-terns are observed in the fractograph of the Widmanstattenmicrostructure (Figure 10(d))
4 Discussion
Thedifferentmicrostructures are generated by heat treatmentof the AR material [10 22 23] Generally most of thecommercially used Ti6Al4V has been heat-treated via acomplex sequence of solution heat treatment deformationaging and annealing [5] for stress relief recrystallization
6 Advances in Materials Science and Engineering
Stre
ss am
plitu
de (M
Pa)
Life (cycle)As-received material950A bimodal
1050A Widmanst atten1050Q martensite
300
250
200
150
100
104 105 106 107 108
Figure 9 Stress amplitude versus life curves of AR material martensite Widmanstatten and bimodal microstructures of Ti6Al4V
(a) (b)
(c) (d)
Figure 10 SEM fractographs of the high-cycle fatigue samples of (a) AR material (b) 1050Q (c) 950A and (d) 1050A
and globularization Temperatures of 650∘Cndash850∘C are nor-mally used for these As shown in Figures 3(b)ndash3(f) themicrostructures were not modified There is no significantdifference in themechanical properties (hardness 02 proofstress and elongation) associated with heat treatment atthese temperatures In industrial application stress reliefrecrystallization and globularization can be realized byannealing at 650∘Cndash850∘C however the associated oxidationlayer thickness increases at the same time [24]
Water quenching from 850∘C to 1050∘C tends to increasethe strength owing to higher 120573 phase content [10] Waterquenching at 1050∘C (Figure 3(k)) transformed the equiaxedmicrostructure into acicular 1205721015840 martensite microstructureThe martensite sample exhibited the highest increases inhardness (419HV) and 02 proof stress (138) comparedto the AR sample but at the cost of a reduction in ductility(427 lower elongation compared to theAR sample) Similar
results for mechanical properties of acicular 1205721015840 martensitemicrostructure of Ti6Al4V were obtained by Jovanovic et al[10] By comparing the results of this paper with Jovanovicet alrsquos data (02 proof stress 1400MPa elongation 15water quenching from 1100∘C) it is obvious that the valuesof 02 proof stress (1114MPa) of acicular 1205721015840 martensitemicrostructure in this study are lower but elongation (78)is rather higher Jovanovic et al suggested two reasons forthese differences (1)Their ARmaterials wereWidmanstattenwith a massive 120573 phase boundary that exhibited higherstrength (1100MPa) but lower ductility (lt15) and (2) thepresence of TiC (formed from carbon diffused into the meltforming carbides with titanium and vanadium during thecasting process) raised the tensile strength but compromisedductility The fatigue strength of the acicular 1205721015840 martensitemicrostructure is the lowest (135MPa) being lower thanthe AR material by 45MPa The term ldquofatiguerdquo relates to
Advances in Materials Science and Engineering 7
Table 2 Summary of the study showing absolute values and variance (Δ) of properties of AR materials
Widmanstatten (1050A)(lamellar width of 10120583m +gt400 120583m120572 grain boundary)
1041 642 103 minus244 153 minus15
Martensite (1050Q)(200 120583m grain size with insideacicular 1205721015840)
1114 138 78 minus427 135 minus25
the initiation and growth of cracks caused by the repeatedapplication of mechanical stresses or strains and any asso-ciated changes in mechanical properties [21] Sun et al [25]reported that low ductility deteriorated fatigue propagationresistance in tensile mode fracture but high strength didlittle to help fatigue initiation resistance [25 26] Hence thepoor ductility in the acicular 1205721015840 martensite microstructureis one reason why its fatigue strength is so low The coarse-grained structure (Figure 3(k)) can be another reason forbrittle fatigue fracture (Figure 10(b)) It is well known thatfatigue cracking in Ti6Al4V alloys starts at prior 120573 grainboundaries or colony boundaries and 120572120573 interfaces [27]A coarse-grained structure will aggravate dislocations at theprior 120573 grain boundaries [28] and eventually lead to failureA third reason for the lower fracture strength in martensitecould be martensite transformation stress generally it iseasier to initiate cracks at a stress-concentration position [29]Martensite transformation stress may aggravate the stressconcentration
Ti6Al4V heat-treated at 950∘C followed by annealing(Figure 3(h)) exhibits slight lamellar microstructure at prior120573 phase boundaries Such bimodal microstructure (con-sisting partly of equiaxed primary 120572 and a lamellar 120572 +120573 matrix transformed from a prior 120573 phase boundary)results from heat treatment just below the 120573 transus tem-perature [13 30] Tensile test results show (Figure 7) thatbimodal microstructures exhibit well-balanced propertiesbetween 02proof strength (1017MPa) and ductility (118)The Widmanstatten microstructure heat-treated at 1050∘Cfollowed by annealing exhibits a slightly higher strength(1041MPa) than the bimodal microstructure but has lowerductility (103) than the bimodal microstructure Thisresearch indicated that Ti6Al4V with bimodal microstruc-ture has a slightly higher fatigue strength than that withWidmanstattenmicrostructure Similar results were obtainedby Zuo et al [30] The fatigue strengths for their bimodaland Widmanstatten microstructures were 493 and 475MParespectively (with both the stress ratio 119877 = minus1 and frequency= 20 kHz increasing the fatigue strength) Both Zuo et al[30] and Gil et al [5] have concluded that crack initiationrather than crack propagation plays the dominant role incontrolling the total life in high-cycle fatigue when there
are not too many interior defects Equiaxed microstruc-ture benefits fatigue initiation resistance Hence bimodalmicrostructure (with the properties of primary 120572 grain ofslightly lower strength and higher ductility) exhibits higherfatigue life compared with Widmanstatten microstructure(which is a special type of lamellarmicrostructure) Howeverbimodal microstructure exhibits lower fatigue life comparedwith equiaxed microstructure owing to its lower ductilitycompared with AR microstructure The fine dimple patternsobserved in fractographs of AR samplesmay indicate a plasticfatigue fracture process The AR material has an equiaxedmicrostructure that exhibited the highest ductility (136)
The correlations among the microstructures mechanicalproperties and fatigue strengths of 07mmTi6Al4V sheet arelisted together in Table 2
5 Conclusions
This study highlights the role of microstructure on fatiguestrength rather than relying on the broader relationshipbetween ductility and fatigue strength Three key observa-tions are apparent
(1) The 02 proof stress ranking from high to lowfor all types of microstructure of thin Ti6Al4Vsheets observed in this study is as follows aci-cular 1205721015840 martensite equiaxed microstructure withmetastable 120573 Widmanstatten bimodal and equiaxedmicrostructure
(2) The ductility ranking from high to low for all types ofthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and martensite microstructure
(3) The fatigue strength ranking from high to low forthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and acicular 1205721015840 martensite microstruc-ture
The fatigue strength of thin Ti6Al4V sheets decreasesinversely proportionally with the proof stress and propor-tionally with ductility this is in response to the types of
8 Advances in Materials Science and Engineering
microstructure and grain sizes set up in the heat-treatmentprocesses
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the materials supportprovided by Timet UK Limited The work is supportedby Fundamental Research Funds for the Central Universi-ties (2014QNA09) Natural Science Foundation of Jiangsu(BK20150205) and Postdoctoral Science Foundation ofJiangsu (1501029A)The authors also wish to express heartfeltthanks to Professor Philip Shipway and Professor Geoff Tans-ley for encouragement supervision and support throughoutthe work
References
[1] G D Tansley M Cook N Zhang M Chung J Woodard andJ Reizes Complete Passive Suspension of the Ventrassist RotaryBlood Pump Univerisy of Technology Sydney Australia 2000
[2] G Tansley S Vidakovic and J Reizes ldquoFluid dynamic char-acteristics of the VentrAssist rotary blood pumprdquo ArtificialOrgans vol 24 no 6 pp 483ndash487 2000
[3] Y Fan Z Chen C H Zhang and A M Liu ldquoA comparisonof microstructure and mechanical properties of welded thinTi6Al4V with three different types of laserrdquoMaterials ResearchInnovations vol 19 no S4 pp S187ndashS192 2015
[4] L W Tsay and C Y Tsay ldquoEffect of microstructures on thefatigue crack growth in Ti6Al4V laser weldsrdquo InternationalJournal of Fatigue vol 19 no 10 pp 713ndash720 1997
[5] F J Gil M P Ginebra J M Manero and J A PlanellldquoFormation of 120572-Widmanstatten structure effects of grain sizeand cooling rate on the Widmanstatten morphologies and onthe mechanical properties in Ti6Al4V alloyrdquo Journal of Alloysand Compounds vol 329 no 1-2 pp 142ndash152 2001
[6] F Caiazzo F Curcio G Daurelio and F M C MinutololdquoTi6Al4V sheets lap and butt joints carried out by CO
2
lasermechanical and morphological characterizationrdquo Journal ofMaterials Processing Technology vol 149 no 1ndash3 pp 546ndash5522004
[7] I J Polmear Light Alloys From Traditional Alloys to Nanocrys-tals Butterworth-Heinemann Oxford UK 2006
[8] N Poondla T S Srivatsan A Patnaik and M Petraroli ldquoAstudy of the microstructure and hardness of two titaniumalloys commercially pure andTindash6Alndash4Vrdquo Journal of Alloys andCompounds vol 486 no 1-2 pp 162ndash167 2009
[9] M Zitnansky and L Caplovic ldquoEffect of the thermomechanicaltreatment on the structure of titanium alloy Ti6Al4Vrdquo Journal ofMaterials Processing Technology vol 157-158 pp 643ndash649 2004
[10] M T Jovanovic S Tadic S Zec Z Miskovic and I BobicldquoThe effect of annealing temperatures and cooling rates onmicrostructure and mechanical properties of investment castTindash6Alndash4V alloyrdquo Materials and Design vol 27 no 3 pp 192ndash199 2006
[11] D Hardie and S Ouyang ldquoEffect of microstructure and heattreatment on fracture behaviour of smooth and precracked ten-sile specimens of Ti6Al4Vrdquo Materials Science and Technologyvol 15 no 9 pp 1049ndash1057 1999
[12] S Shademan V Sinha A B O Soboyejo and W O SoboyejoldquoAn investigation of the effects of microstructure and stressratio on fatigue crack growth in Tindash6Alndash4V with colony 120572120573microstructuresrdquo Mechanics of Materials vol 36 no 1-2 pp161ndash175 2004
[13] C Leyens and M Peter Titanium and Titanium Alloys Fun-damentals and Applications Wiley-VCH Verlag GmbH amp CoKGaA 2003
[14] C Loier G Thauvin A Hazotte and A Simon ldquoInfluence ofdeformation on the 120573 rarr 120572 + 120573 transformation kinetics of Ti-6 wtAl-4wtV alloyrdquo Journal of the Less Common Metalsvol 108 no 2 pp 295ndash312 1985
[15] T Mohandas D Banerjee and V V K Rao ldquoFusion zonemicrostructure and porosity in electron beam welds of an 120572+120573titanium alloyrdquo Metallurgical amp Materials Transactions A vol30 no 3 pp 789ndash798 1999
[16] T Ahmed and H J Rack ldquoPhase transformations during cool-ing in 120572+120573 titanium alloysrdquo Materials Science and Engineeringvol 243 no 1-2 pp 206ndash211 1998
[17] American Welding Society Recommended Practices for LaserBeamWelding Cutting and Drilling AWSANSI 1998
[18] X Li Research on the Microstructure and Fatigue Propertyof Electron Beam Welding Joint in Titanium Alloy HuazhongUniversity of Science and Technology Wuhan China 2012
[19] C Wu X Li X Huang J Ma and C Cao ldquoRelationship offatigue crack propagation and microstructure for TA15 alloyrdquoRare Metal Materials and Engineering vol 36 no 12 pp 2128ndash2131 2007
[20] European Standard EN10002-1 Metal material Tensile test2001
[21] British Standard 3518-1 Methods of fatigue testing 1993[22] W Sha and Z Guo ldquoPhase evolution of Tindash6A1ndash4V during
continuous heatingrdquo Journal of Alloys and Compounds vol 290no 1-2 pp L3ndashL7 1999
[23] P Sirilar and P Srichandr Grain Refinement of 120572120573 PhaseTi6Al4V Alloy by Thermomechanical Treatment King Mong-kutrsquos University of Technology Thonburi Bangkok Thailand2006
[24] Y Fan G D Tansley P H Shipway and J Niu ldquoThe applicationof laser welding on Left Ventricular Assist Device (LVAD)rdquo inProceedings of the Symposium on Photonics and Optoelectronics(SOPO rsquo11) pp 1ndash3 Wuhan China May 2011
[25] Z Sun L Jiang andZ Ying Failure Analysis Fundamentals andApplications China Machine Press 2005
[26] X L ZhengQuantitativeTheory ofMetal Fatigue NorthwesternPolytechnical University Press 1994
[27] J Oh N J Kim S Lee and E W Lee ldquoCorrelation of fatigueproperties and microstructure in investment cast Tindash6Alndash4Vweldsrdquo Materials Science and Engineering A vol 340 no 1-2pp 232ndash242 2003
[28] F X Gil Mur D Rodrıguez and J A Planell ldquoInfluence of tem-pering temperature and time on the 1205721015840-Ti-6A1-4V martensiterdquoJournal of Alloys and Compounds vol 234 no 2 pp 287ndash2891996
Advances in Materials Science and Engineering 9
[29] S R Lampman and N Dimatteo Fatigue and Fracture ASMInternational Geauga County Ohio USA 1996
[30] J H Zuo Z G Wang and E H Han ldquoEffect of microstructureon ultra-high cycle fatigue behavior of Ti-6Al-4Vrdquo MaterialsScience and Engineering A vol 473 no 1-2 pp 147ndash152 2008
Figure 9 Stress amplitude versus life curves of AR material martensite Widmanstatten and bimodal microstructures of Ti6Al4V
(a) (b)
(c) (d)
Figure 10 SEM fractographs of the high-cycle fatigue samples of (a) AR material (b) 1050Q (c) 950A and (d) 1050A
and globularization Temperatures of 650∘Cndash850∘C are nor-mally used for these As shown in Figures 3(b)ndash3(f) themicrostructures were not modified There is no significantdifference in themechanical properties (hardness 02 proofstress and elongation) associated with heat treatment atthese temperatures In industrial application stress reliefrecrystallization and globularization can be realized byannealing at 650∘Cndash850∘C however the associated oxidationlayer thickness increases at the same time [24]
Water quenching from 850∘C to 1050∘C tends to increasethe strength owing to higher 120573 phase content [10] Waterquenching at 1050∘C (Figure 3(k)) transformed the equiaxedmicrostructure into acicular 1205721015840 martensite microstructureThe martensite sample exhibited the highest increases inhardness (419HV) and 02 proof stress (138) comparedto the AR sample but at the cost of a reduction in ductility(427 lower elongation compared to theAR sample) Similar
results for mechanical properties of acicular 1205721015840 martensitemicrostructure of Ti6Al4V were obtained by Jovanovic et al[10] By comparing the results of this paper with Jovanovicet alrsquos data (02 proof stress 1400MPa elongation 15water quenching from 1100∘C) it is obvious that the valuesof 02 proof stress (1114MPa) of acicular 1205721015840 martensitemicrostructure in this study are lower but elongation (78)is rather higher Jovanovic et al suggested two reasons forthese differences (1)Their ARmaterials wereWidmanstattenwith a massive 120573 phase boundary that exhibited higherstrength (1100MPa) but lower ductility (lt15) and (2) thepresence of TiC (formed from carbon diffused into the meltforming carbides with titanium and vanadium during thecasting process) raised the tensile strength but compromisedductility The fatigue strength of the acicular 1205721015840 martensitemicrostructure is the lowest (135MPa) being lower thanthe AR material by 45MPa The term ldquofatiguerdquo relates to
Advances in Materials Science and Engineering 7
Table 2 Summary of the study showing absolute values and variance (Δ) of properties of AR materials
Widmanstatten (1050A)(lamellar width of 10120583m +gt400 120583m120572 grain boundary)
1041 642 103 minus244 153 minus15
Martensite (1050Q)(200 120583m grain size with insideacicular 1205721015840)
1114 138 78 minus427 135 minus25
the initiation and growth of cracks caused by the repeatedapplication of mechanical stresses or strains and any asso-ciated changes in mechanical properties [21] Sun et al [25]reported that low ductility deteriorated fatigue propagationresistance in tensile mode fracture but high strength didlittle to help fatigue initiation resistance [25 26] Hence thepoor ductility in the acicular 1205721015840 martensite microstructureis one reason why its fatigue strength is so low The coarse-grained structure (Figure 3(k)) can be another reason forbrittle fatigue fracture (Figure 10(b)) It is well known thatfatigue cracking in Ti6Al4V alloys starts at prior 120573 grainboundaries or colony boundaries and 120572120573 interfaces [27]A coarse-grained structure will aggravate dislocations at theprior 120573 grain boundaries [28] and eventually lead to failureA third reason for the lower fracture strength in martensitecould be martensite transformation stress generally it iseasier to initiate cracks at a stress-concentration position [29]Martensite transformation stress may aggravate the stressconcentration
Ti6Al4V heat-treated at 950∘C followed by annealing(Figure 3(h)) exhibits slight lamellar microstructure at prior120573 phase boundaries Such bimodal microstructure (con-sisting partly of equiaxed primary 120572 and a lamellar 120572 +120573 matrix transformed from a prior 120573 phase boundary)results from heat treatment just below the 120573 transus tem-perature [13 30] Tensile test results show (Figure 7) thatbimodal microstructures exhibit well-balanced propertiesbetween 02proof strength (1017MPa) and ductility (118)The Widmanstatten microstructure heat-treated at 1050∘Cfollowed by annealing exhibits a slightly higher strength(1041MPa) than the bimodal microstructure but has lowerductility (103) than the bimodal microstructure Thisresearch indicated that Ti6Al4V with bimodal microstruc-ture has a slightly higher fatigue strength than that withWidmanstattenmicrostructure Similar results were obtainedby Zuo et al [30] The fatigue strengths for their bimodaland Widmanstatten microstructures were 493 and 475MParespectively (with both the stress ratio 119877 = minus1 and frequency= 20 kHz increasing the fatigue strength) Both Zuo et al[30] and Gil et al [5] have concluded that crack initiationrather than crack propagation plays the dominant role incontrolling the total life in high-cycle fatigue when there
are not too many interior defects Equiaxed microstruc-ture benefits fatigue initiation resistance Hence bimodalmicrostructure (with the properties of primary 120572 grain ofslightly lower strength and higher ductility) exhibits higherfatigue life compared with Widmanstatten microstructure(which is a special type of lamellarmicrostructure) Howeverbimodal microstructure exhibits lower fatigue life comparedwith equiaxed microstructure owing to its lower ductilitycompared with AR microstructure The fine dimple patternsobserved in fractographs of AR samplesmay indicate a plasticfatigue fracture process The AR material has an equiaxedmicrostructure that exhibited the highest ductility (136)
The correlations among the microstructures mechanicalproperties and fatigue strengths of 07mmTi6Al4V sheet arelisted together in Table 2
5 Conclusions
This study highlights the role of microstructure on fatiguestrength rather than relying on the broader relationshipbetween ductility and fatigue strength Three key observa-tions are apparent
(1) The 02 proof stress ranking from high to lowfor all types of microstructure of thin Ti6Al4Vsheets observed in this study is as follows aci-cular 1205721015840 martensite equiaxed microstructure withmetastable 120573 Widmanstatten bimodal and equiaxedmicrostructure
(2) The ductility ranking from high to low for all types ofthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and martensite microstructure
(3) The fatigue strength ranking from high to low forthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and acicular 1205721015840 martensite microstruc-ture
The fatigue strength of thin Ti6Al4V sheets decreasesinversely proportionally with the proof stress and propor-tionally with ductility this is in response to the types of
8 Advances in Materials Science and Engineering
microstructure and grain sizes set up in the heat-treatmentprocesses
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the materials supportprovided by Timet UK Limited The work is supportedby Fundamental Research Funds for the Central Universi-ties (2014QNA09) Natural Science Foundation of Jiangsu(BK20150205) and Postdoctoral Science Foundation ofJiangsu (1501029A)The authors also wish to express heartfeltthanks to Professor Philip Shipway and Professor Geoff Tans-ley for encouragement supervision and support throughoutthe work
References
[1] G D Tansley M Cook N Zhang M Chung J Woodard andJ Reizes Complete Passive Suspension of the Ventrassist RotaryBlood Pump Univerisy of Technology Sydney Australia 2000
[2] G Tansley S Vidakovic and J Reizes ldquoFluid dynamic char-acteristics of the VentrAssist rotary blood pumprdquo ArtificialOrgans vol 24 no 6 pp 483ndash487 2000
[3] Y Fan Z Chen C H Zhang and A M Liu ldquoA comparisonof microstructure and mechanical properties of welded thinTi6Al4V with three different types of laserrdquoMaterials ResearchInnovations vol 19 no S4 pp S187ndashS192 2015
[4] L W Tsay and C Y Tsay ldquoEffect of microstructures on thefatigue crack growth in Ti6Al4V laser weldsrdquo InternationalJournal of Fatigue vol 19 no 10 pp 713ndash720 1997
[5] F J Gil M P Ginebra J M Manero and J A PlanellldquoFormation of 120572-Widmanstatten structure effects of grain sizeand cooling rate on the Widmanstatten morphologies and onthe mechanical properties in Ti6Al4V alloyrdquo Journal of Alloysand Compounds vol 329 no 1-2 pp 142ndash152 2001
[6] F Caiazzo F Curcio G Daurelio and F M C MinutololdquoTi6Al4V sheets lap and butt joints carried out by CO
2
lasermechanical and morphological characterizationrdquo Journal ofMaterials Processing Technology vol 149 no 1ndash3 pp 546ndash5522004
[7] I J Polmear Light Alloys From Traditional Alloys to Nanocrys-tals Butterworth-Heinemann Oxford UK 2006
[8] N Poondla T S Srivatsan A Patnaik and M Petraroli ldquoAstudy of the microstructure and hardness of two titaniumalloys commercially pure andTindash6Alndash4Vrdquo Journal of Alloys andCompounds vol 486 no 1-2 pp 162ndash167 2009
[9] M Zitnansky and L Caplovic ldquoEffect of the thermomechanicaltreatment on the structure of titanium alloy Ti6Al4Vrdquo Journal ofMaterials Processing Technology vol 157-158 pp 643ndash649 2004
[10] M T Jovanovic S Tadic S Zec Z Miskovic and I BobicldquoThe effect of annealing temperatures and cooling rates onmicrostructure and mechanical properties of investment castTindash6Alndash4V alloyrdquo Materials and Design vol 27 no 3 pp 192ndash199 2006
[11] D Hardie and S Ouyang ldquoEffect of microstructure and heattreatment on fracture behaviour of smooth and precracked ten-sile specimens of Ti6Al4Vrdquo Materials Science and Technologyvol 15 no 9 pp 1049ndash1057 1999
[12] S Shademan V Sinha A B O Soboyejo and W O SoboyejoldquoAn investigation of the effects of microstructure and stressratio on fatigue crack growth in Tindash6Alndash4V with colony 120572120573microstructuresrdquo Mechanics of Materials vol 36 no 1-2 pp161ndash175 2004
[13] C Leyens and M Peter Titanium and Titanium Alloys Fun-damentals and Applications Wiley-VCH Verlag GmbH amp CoKGaA 2003
[14] C Loier G Thauvin A Hazotte and A Simon ldquoInfluence ofdeformation on the 120573 rarr 120572 + 120573 transformation kinetics of Ti-6 wtAl-4wtV alloyrdquo Journal of the Less Common Metalsvol 108 no 2 pp 295ndash312 1985
[15] T Mohandas D Banerjee and V V K Rao ldquoFusion zonemicrostructure and porosity in electron beam welds of an 120572+120573titanium alloyrdquo Metallurgical amp Materials Transactions A vol30 no 3 pp 789ndash798 1999
[16] T Ahmed and H J Rack ldquoPhase transformations during cool-ing in 120572+120573 titanium alloysrdquo Materials Science and Engineeringvol 243 no 1-2 pp 206ndash211 1998
[17] American Welding Society Recommended Practices for LaserBeamWelding Cutting and Drilling AWSANSI 1998
[18] X Li Research on the Microstructure and Fatigue Propertyof Electron Beam Welding Joint in Titanium Alloy HuazhongUniversity of Science and Technology Wuhan China 2012
[19] C Wu X Li X Huang J Ma and C Cao ldquoRelationship offatigue crack propagation and microstructure for TA15 alloyrdquoRare Metal Materials and Engineering vol 36 no 12 pp 2128ndash2131 2007
[20] European Standard EN10002-1 Metal material Tensile test2001
[21] British Standard 3518-1 Methods of fatigue testing 1993[22] W Sha and Z Guo ldquoPhase evolution of Tindash6A1ndash4V during
continuous heatingrdquo Journal of Alloys and Compounds vol 290no 1-2 pp L3ndashL7 1999
[23] P Sirilar and P Srichandr Grain Refinement of 120572120573 PhaseTi6Al4V Alloy by Thermomechanical Treatment King Mong-kutrsquos University of Technology Thonburi Bangkok Thailand2006
[24] Y Fan G D Tansley P H Shipway and J Niu ldquoThe applicationof laser welding on Left Ventricular Assist Device (LVAD)rdquo inProceedings of the Symposium on Photonics and Optoelectronics(SOPO rsquo11) pp 1ndash3 Wuhan China May 2011
[25] Z Sun L Jiang andZ Ying Failure Analysis Fundamentals andApplications China Machine Press 2005
[26] X L ZhengQuantitativeTheory ofMetal Fatigue NorthwesternPolytechnical University Press 1994
[27] J Oh N J Kim S Lee and E W Lee ldquoCorrelation of fatigueproperties and microstructure in investment cast Tindash6Alndash4Vweldsrdquo Materials Science and Engineering A vol 340 no 1-2pp 232ndash242 2003
[28] F X Gil Mur D Rodrıguez and J A Planell ldquoInfluence of tem-pering temperature and time on the 1205721015840-Ti-6A1-4V martensiterdquoJournal of Alloys and Compounds vol 234 no 2 pp 287ndash2891996
Advances in Materials Science and Engineering 9
[29] S R Lampman and N Dimatteo Fatigue and Fracture ASMInternational Geauga County Ohio USA 1996
[30] J H Zuo Z G Wang and E H Han ldquoEffect of microstructureon ultra-high cycle fatigue behavior of Ti-6Al-4Vrdquo MaterialsScience and Engineering A vol 473 no 1-2 pp 147ndash152 2008
Widmanstatten (1050A)(lamellar width of 10120583m +gt400 120583m120572 grain boundary)
1041 642 103 minus244 153 minus15
Martensite (1050Q)(200 120583m grain size with insideacicular 1205721015840)
1114 138 78 minus427 135 minus25
the initiation and growth of cracks caused by the repeatedapplication of mechanical stresses or strains and any asso-ciated changes in mechanical properties [21] Sun et al [25]reported that low ductility deteriorated fatigue propagationresistance in tensile mode fracture but high strength didlittle to help fatigue initiation resistance [25 26] Hence thepoor ductility in the acicular 1205721015840 martensite microstructureis one reason why its fatigue strength is so low The coarse-grained structure (Figure 3(k)) can be another reason forbrittle fatigue fracture (Figure 10(b)) It is well known thatfatigue cracking in Ti6Al4V alloys starts at prior 120573 grainboundaries or colony boundaries and 120572120573 interfaces [27]A coarse-grained structure will aggravate dislocations at theprior 120573 grain boundaries [28] and eventually lead to failureA third reason for the lower fracture strength in martensitecould be martensite transformation stress generally it iseasier to initiate cracks at a stress-concentration position [29]Martensite transformation stress may aggravate the stressconcentration
Ti6Al4V heat-treated at 950∘C followed by annealing(Figure 3(h)) exhibits slight lamellar microstructure at prior120573 phase boundaries Such bimodal microstructure (con-sisting partly of equiaxed primary 120572 and a lamellar 120572 +120573 matrix transformed from a prior 120573 phase boundary)results from heat treatment just below the 120573 transus tem-perature [13 30] Tensile test results show (Figure 7) thatbimodal microstructures exhibit well-balanced propertiesbetween 02proof strength (1017MPa) and ductility (118)The Widmanstatten microstructure heat-treated at 1050∘Cfollowed by annealing exhibits a slightly higher strength(1041MPa) than the bimodal microstructure but has lowerductility (103) than the bimodal microstructure Thisresearch indicated that Ti6Al4V with bimodal microstruc-ture has a slightly higher fatigue strength than that withWidmanstattenmicrostructure Similar results were obtainedby Zuo et al [30] The fatigue strengths for their bimodaland Widmanstatten microstructures were 493 and 475MParespectively (with both the stress ratio 119877 = minus1 and frequency= 20 kHz increasing the fatigue strength) Both Zuo et al[30] and Gil et al [5] have concluded that crack initiationrather than crack propagation plays the dominant role incontrolling the total life in high-cycle fatigue when there
are not too many interior defects Equiaxed microstruc-ture benefits fatigue initiation resistance Hence bimodalmicrostructure (with the properties of primary 120572 grain ofslightly lower strength and higher ductility) exhibits higherfatigue life compared with Widmanstatten microstructure(which is a special type of lamellarmicrostructure) Howeverbimodal microstructure exhibits lower fatigue life comparedwith equiaxed microstructure owing to its lower ductilitycompared with AR microstructure The fine dimple patternsobserved in fractographs of AR samplesmay indicate a plasticfatigue fracture process The AR material has an equiaxedmicrostructure that exhibited the highest ductility (136)
The correlations among the microstructures mechanicalproperties and fatigue strengths of 07mmTi6Al4V sheet arelisted together in Table 2
5 Conclusions
This study highlights the role of microstructure on fatiguestrength rather than relying on the broader relationshipbetween ductility and fatigue strength Three key observa-tions are apparent
(1) The 02 proof stress ranking from high to lowfor all types of microstructure of thin Ti6Al4Vsheets observed in this study is as follows aci-cular 1205721015840 martensite equiaxed microstructure withmetastable 120573 Widmanstatten bimodal and equiaxedmicrostructure
(2) The ductility ranking from high to low for all types ofthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and martensite microstructure
(3) The fatigue strength ranking from high to low forthe microstructure of thin Ti6Al4V sheets observedin this study is as follows equiaxed bimodal Wid-manstatten and acicular 1205721015840 martensite microstruc-ture
The fatigue strength of thin Ti6Al4V sheets decreasesinversely proportionally with the proof stress and propor-tionally with ductility this is in response to the types of
8 Advances in Materials Science and Engineering
microstructure and grain sizes set up in the heat-treatmentprocesses
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the materials supportprovided by Timet UK Limited The work is supportedby Fundamental Research Funds for the Central Universi-ties (2014QNA09) Natural Science Foundation of Jiangsu(BK20150205) and Postdoctoral Science Foundation ofJiangsu (1501029A)The authors also wish to express heartfeltthanks to Professor Philip Shipway and Professor Geoff Tans-ley for encouragement supervision and support throughoutthe work
References
[1] G D Tansley M Cook N Zhang M Chung J Woodard andJ Reizes Complete Passive Suspension of the Ventrassist RotaryBlood Pump Univerisy of Technology Sydney Australia 2000
[2] G Tansley S Vidakovic and J Reizes ldquoFluid dynamic char-acteristics of the VentrAssist rotary blood pumprdquo ArtificialOrgans vol 24 no 6 pp 483ndash487 2000
[3] Y Fan Z Chen C H Zhang and A M Liu ldquoA comparisonof microstructure and mechanical properties of welded thinTi6Al4V with three different types of laserrdquoMaterials ResearchInnovations vol 19 no S4 pp S187ndashS192 2015
[4] L W Tsay and C Y Tsay ldquoEffect of microstructures on thefatigue crack growth in Ti6Al4V laser weldsrdquo InternationalJournal of Fatigue vol 19 no 10 pp 713ndash720 1997
[5] F J Gil M P Ginebra J M Manero and J A PlanellldquoFormation of 120572-Widmanstatten structure effects of grain sizeand cooling rate on the Widmanstatten morphologies and onthe mechanical properties in Ti6Al4V alloyrdquo Journal of Alloysand Compounds vol 329 no 1-2 pp 142ndash152 2001
[6] F Caiazzo F Curcio G Daurelio and F M C MinutololdquoTi6Al4V sheets lap and butt joints carried out by CO
2
lasermechanical and morphological characterizationrdquo Journal ofMaterials Processing Technology vol 149 no 1ndash3 pp 546ndash5522004
[7] I J Polmear Light Alloys From Traditional Alloys to Nanocrys-tals Butterworth-Heinemann Oxford UK 2006
[8] N Poondla T S Srivatsan A Patnaik and M Petraroli ldquoAstudy of the microstructure and hardness of two titaniumalloys commercially pure andTindash6Alndash4Vrdquo Journal of Alloys andCompounds vol 486 no 1-2 pp 162ndash167 2009
[9] M Zitnansky and L Caplovic ldquoEffect of the thermomechanicaltreatment on the structure of titanium alloy Ti6Al4Vrdquo Journal ofMaterials Processing Technology vol 157-158 pp 643ndash649 2004
[10] M T Jovanovic S Tadic S Zec Z Miskovic and I BobicldquoThe effect of annealing temperatures and cooling rates onmicrostructure and mechanical properties of investment castTindash6Alndash4V alloyrdquo Materials and Design vol 27 no 3 pp 192ndash199 2006
[11] D Hardie and S Ouyang ldquoEffect of microstructure and heattreatment on fracture behaviour of smooth and precracked ten-sile specimens of Ti6Al4Vrdquo Materials Science and Technologyvol 15 no 9 pp 1049ndash1057 1999
[12] S Shademan V Sinha A B O Soboyejo and W O SoboyejoldquoAn investigation of the effects of microstructure and stressratio on fatigue crack growth in Tindash6Alndash4V with colony 120572120573microstructuresrdquo Mechanics of Materials vol 36 no 1-2 pp161ndash175 2004
[13] C Leyens and M Peter Titanium and Titanium Alloys Fun-damentals and Applications Wiley-VCH Verlag GmbH amp CoKGaA 2003
[14] C Loier G Thauvin A Hazotte and A Simon ldquoInfluence ofdeformation on the 120573 rarr 120572 + 120573 transformation kinetics of Ti-6 wtAl-4wtV alloyrdquo Journal of the Less Common Metalsvol 108 no 2 pp 295ndash312 1985
[15] T Mohandas D Banerjee and V V K Rao ldquoFusion zonemicrostructure and porosity in electron beam welds of an 120572+120573titanium alloyrdquo Metallurgical amp Materials Transactions A vol30 no 3 pp 789ndash798 1999
[16] T Ahmed and H J Rack ldquoPhase transformations during cool-ing in 120572+120573 titanium alloysrdquo Materials Science and Engineeringvol 243 no 1-2 pp 206ndash211 1998
[17] American Welding Society Recommended Practices for LaserBeamWelding Cutting and Drilling AWSANSI 1998
[18] X Li Research on the Microstructure and Fatigue Propertyof Electron Beam Welding Joint in Titanium Alloy HuazhongUniversity of Science and Technology Wuhan China 2012
[19] C Wu X Li X Huang J Ma and C Cao ldquoRelationship offatigue crack propagation and microstructure for TA15 alloyrdquoRare Metal Materials and Engineering vol 36 no 12 pp 2128ndash2131 2007
[20] European Standard EN10002-1 Metal material Tensile test2001
[21] British Standard 3518-1 Methods of fatigue testing 1993[22] W Sha and Z Guo ldquoPhase evolution of Tindash6A1ndash4V during
continuous heatingrdquo Journal of Alloys and Compounds vol 290no 1-2 pp L3ndashL7 1999
[23] P Sirilar and P Srichandr Grain Refinement of 120572120573 PhaseTi6Al4V Alloy by Thermomechanical Treatment King Mong-kutrsquos University of Technology Thonburi Bangkok Thailand2006
[24] Y Fan G D Tansley P H Shipway and J Niu ldquoThe applicationof laser welding on Left Ventricular Assist Device (LVAD)rdquo inProceedings of the Symposium on Photonics and Optoelectronics(SOPO rsquo11) pp 1ndash3 Wuhan China May 2011
[25] Z Sun L Jiang andZ Ying Failure Analysis Fundamentals andApplications China Machine Press 2005
[26] X L ZhengQuantitativeTheory ofMetal Fatigue NorthwesternPolytechnical University Press 1994
[27] J Oh N J Kim S Lee and E W Lee ldquoCorrelation of fatigueproperties and microstructure in investment cast Tindash6Alndash4Vweldsrdquo Materials Science and Engineering A vol 340 no 1-2pp 232ndash242 2003
[28] F X Gil Mur D Rodrıguez and J A Planell ldquoInfluence of tem-pering temperature and time on the 1205721015840-Ti-6A1-4V martensiterdquoJournal of Alloys and Compounds vol 234 no 2 pp 287ndash2891996
Advances in Materials Science and Engineering 9
[29] S R Lampman and N Dimatteo Fatigue and Fracture ASMInternational Geauga County Ohio USA 1996
[30] J H Zuo Z G Wang and E H Han ldquoEffect of microstructureon ultra-high cycle fatigue behavior of Ti-6Al-4Vrdquo MaterialsScience and Engineering A vol 473 no 1-2 pp 147ndash152 2008
microstructure and grain sizes set up in the heat-treatmentprocesses
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgments
The authors wish to acknowledge the materials supportprovided by Timet UK Limited The work is supportedby Fundamental Research Funds for the Central Universi-ties (2014QNA09) Natural Science Foundation of Jiangsu(BK20150205) and Postdoctoral Science Foundation ofJiangsu (1501029A)The authors also wish to express heartfeltthanks to Professor Philip Shipway and Professor Geoff Tans-ley for encouragement supervision and support throughoutthe work
References
[1] G D Tansley M Cook N Zhang M Chung J Woodard andJ Reizes Complete Passive Suspension of the Ventrassist RotaryBlood Pump Univerisy of Technology Sydney Australia 2000
[2] G Tansley S Vidakovic and J Reizes ldquoFluid dynamic char-acteristics of the VentrAssist rotary blood pumprdquo ArtificialOrgans vol 24 no 6 pp 483ndash487 2000
[3] Y Fan Z Chen C H Zhang and A M Liu ldquoA comparisonof microstructure and mechanical properties of welded thinTi6Al4V with three different types of laserrdquoMaterials ResearchInnovations vol 19 no S4 pp S187ndashS192 2015
[4] L W Tsay and C Y Tsay ldquoEffect of microstructures on thefatigue crack growth in Ti6Al4V laser weldsrdquo InternationalJournal of Fatigue vol 19 no 10 pp 713ndash720 1997
[5] F J Gil M P Ginebra J M Manero and J A PlanellldquoFormation of 120572-Widmanstatten structure effects of grain sizeand cooling rate on the Widmanstatten morphologies and onthe mechanical properties in Ti6Al4V alloyrdquo Journal of Alloysand Compounds vol 329 no 1-2 pp 142ndash152 2001
[6] F Caiazzo F Curcio G Daurelio and F M C MinutololdquoTi6Al4V sheets lap and butt joints carried out by CO
2
lasermechanical and morphological characterizationrdquo Journal ofMaterials Processing Technology vol 149 no 1ndash3 pp 546ndash5522004
[7] I J Polmear Light Alloys From Traditional Alloys to Nanocrys-tals Butterworth-Heinemann Oxford UK 2006
[8] N Poondla T S Srivatsan A Patnaik and M Petraroli ldquoAstudy of the microstructure and hardness of two titaniumalloys commercially pure andTindash6Alndash4Vrdquo Journal of Alloys andCompounds vol 486 no 1-2 pp 162ndash167 2009
[9] M Zitnansky and L Caplovic ldquoEffect of the thermomechanicaltreatment on the structure of titanium alloy Ti6Al4Vrdquo Journal ofMaterials Processing Technology vol 157-158 pp 643ndash649 2004
[10] M T Jovanovic S Tadic S Zec Z Miskovic and I BobicldquoThe effect of annealing temperatures and cooling rates onmicrostructure and mechanical properties of investment castTindash6Alndash4V alloyrdquo Materials and Design vol 27 no 3 pp 192ndash199 2006
[11] D Hardie and S Ouyang ldquoEffect of microstructure and heattreatment on fracture behaviour of smooth and precracked ten-sile specimens of Ti6Al4Vrdquo Materials Science and Technologyvol 15 no 9 pp 1049ndash1057 1999
[12] S Shademan V Sinha A B O Soboyejo and W O SoboyejoldquoAn investigation of the effects of microstructure and stressratio on fatigue crack growth in Tindash6Alndash4V with colony 120572120573microstructuresrdquo Mechanics of Materials vol 36 no 1-2 pp161ndash175 2004
[13] C Leyens and M Peter Titanium and Titanium Alloys Fun-damentals and Applications Wiley-VCH Verlag GmbH amp CoKGaA 2003
[14] C Loier G Thauvin A Hazotte and A Simon ldquoInfluence ofdeformation on the 120573 rarr 120572 + 120573 transformation kinetics of Ti-6 wtAl-4wtV alloyrdquo Journal of the Less Common Metalsvol 108 no 2 pp 295ndash312 1985
[15] T Mohandas D Banerjee and V V K Rao ldquoFusion zonemicrostructure and porosity in electron beam welds of an 120572+120573titanium alloyrdquo Metallurgical amp Materials Transactions A vol30 no 3 pp 789ndash798 1999
[16] T Ahmed and H J Rack ldquoPhase transformations during cool-ing in 120572+120573 titanium alloysrdquo Materials Science and Engineeringvol 243 no 1-2 pp 206ndash211 1998
[17] American Welding Society Recommended Practices for LaserBeamWelding Cutting and Drilling AWSANSI 1998
[18] X Li Research on the Microstructure and Fatigue Propertyof Electron Beam Welding Joint in Titanium Alloy HuazhongUniversity of Science and Technology Wuhan China 2012
[19] C Wu X Li X Huang J Ma and C Cao ldquoRelationship offatigue crack propagation and microstructure for TA15 alloyrdquoRare Metal Materials and Engineering vol 36 no 12 pp 2128ndash2131 2007
[20] European Standard EN10002-1 Metal material Tensile test2001
[21] British Standard 3518-1 Methods of fatigue testing 1993[22] W Sha and Z Guo ldquoPhase evolution of Tindash6A1ndash4V during
continuous heatingrdquo Journal of Alloys and Compounds vol 290no 1-2 pp L3ndashL7 1999
[23] P Sirilar and P Srichandr Grain Refinement of 120572120573 PhaseTi6Al4V Alloy by Thermomechanical Treatment King Mong-kutrsquos University of Technology Thonburi Bangkok Thailand2006
[24] Y Fan G D Tansley P H Shipway and J Niu ldquoThe applicationof laser welding on Left Ventricular Assist Device (LVAD)rdquo inProceedings of the Symposium on Photonics and Optoelectronics(SOPO rsquo11) pp 1ndash3 Wuhan China May 2011
[25] Z Sun L Jiang andZ Ying Failure Analysis Fundamentals andApplications China Machine Press 2005
[26] X L ZhengQuantitativeTheory ofMetal Fatigue NorthwesternPolytechnical University Press 1994
[27] J Oh N J Kim S Lee and E W Lee ldquoCorrelation of fatigueproperties and microstructure in investment cast Tindash6Alndash4Vweldsrdquo Materials Science and Engineering A vol 340 no 1-2pp 232ndash242 2003
[28] F X Gil Mur D Rodrıguez and J A Planell ldquoInfluence of tem-pering temperature and time on the 1205721015840-Ti-6A1-4V martensiterdquoJournal of Alloys and Compounds vol 234 no 2 pp 287ndash2891996
Advances in Materials Science and Engineering 9
[29] S R Lampman and N Dimatteo Fatigue and Fracture ASMInternational Geauga County Ohio USA 1996
[30] J H Zuo Z G Wang and E H Han ldquoEffect of microstructureon ultra-high cycle fatigue behavior of Ti-6Al-4Vrdquo MaterialsScience and Engineering A vol 473 no 1-2 pp 147ndash152 2008
[29] S R Lampman and N Dimatteo Fatigue and Fracture ASMInternational Geauga County Ohio USA 1996
[30] J H Zuo Z G Wang and E H Han ldquoEffect of microstructureon ultra-high cycle fatigue behavior of Ti-6Al-4Vrdquo MaterialsScience and Engineering A vol 473 no 1-2 pp 147ndash152 2008
