tusMan

Keywords:Near- Ti alloy

oleS (bilisingmp

predominant nucleation from grain boundaries. Detailed electron backscatter diffraction (EBSD)characterisation was carried out on the quenched and slowly cooled samples in order to comparethe predicted texture based on the texture measurements assuming no variant selection with the

etallicallograpre ande so ca1 1

hingmentreakectedriant

The purpose of heat treatment is to recrystallise the phase

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Materials Science

Materials Science & Engineering A 576 (2013) 272279Considering the importance of neighbouring grain orientations,E-mail address: michael.preuss@manchester.ac.uk (M. Preuss).microstructure. The cleanliness of typical + titanium alloys(Ti6Al4V or Ti-6246), i.e. absence of carbides or other hightemperature non-metallic precipitates, means that heat treatmentin the single-phase region leads to very rapid grain growth.

0921-5093 & 2013 The Authors. Published by Elsevier B.V.http://dx.doi.org/10.1016/j.msea.2013.04.018

n Corresponding author. Tel.: +44 161 306 3601; fax: +44 161 306 3586.

Open access under CC BY license.slow cooling rates, the to phase transformation is diffusioncontrolled resulting in either a colony structure growing from the

selection mechanism, might be difcult to break up increasing thelikelihood of so-called macrozone formation.transformation, such heat treatment is expected to result in a relativelyweak texture. In practice, treat treatments have often been reportedto yield much stronger textures than expected [24], which has beenrelated to texture memory [23,57] or variant selection mechanisms[811]. Considering industrially relevant cooling rates, i.e. relatively

with almost identical crystallographic orientation stretcacross neighbouring grains pairs [4,1011]. The heat treatis usually followed by + thermomechanical processing to bup the coarse columnar microstructure. However, it is suspthat particularly large columnar regions, as created by the vathe low temperature phase while the high temperature phase is bcc.As a result of this relationship, a single grain can assume 6 different orientations during heating while during cooling 12 possible orienta-tion variants exist when a grain transforms to [1]. In the case thatall variants occur with equal probability during -- phase

extremely soft around the transus [15], which makes the build up ofsignicant elastic strain as a result of the transformation very unlikely.

The industrial signicance of the variant selection mechanismrelated to neighbouring grain orientations is not necessarily thelack of texture randomisation but the formation of large colonies1. Introduction

In many important engineering mzirconium and titanium alloys, a crystship exists between the low temperatuIn the case of Ti and Zr alloys, it is thwhere {0 0 0 2}{1 1 0} and 1 1 2 0measured textures. A strong correlation was found between coarsening and level of variant selection.It was also found that the grain coarsening is driven by the predominant growth of low energy grainboundaries, which strengthen specic texture components that are part of the 1 1 1ND bre.Finally, it was possible to demonstrate that the strengthened texture components promote grain pairswith a common 110, which is known to enhance variant selection when nucleates from grainboundaries.

& 2013 The Authors. Published by Elsevier B.V.

materials, such as steel,hic orientation relation-high temperature phase.lled Burgers relationship1with being hcp and

grain boundaries (or grain boundary) or a Widmansttten (basketweave) microstructure. In the former case, it has been demonstratedthat near parallel (110) planes in two neighbouring grains results in variant selection [10,11], which strengthens the texture [4]. It shouldbe mentioned at this stage that variant selection mechanisms are alsooften discussed in view of transformation strains both in bcc and hcpmaterials [8,1214]. However, in most Ti (and Zr) alloys the phase is

Open access under CC BY license.Texture evolutionVariant selectionThe effect of grain coarsening on variannear- Ti alloy

G.C Obasi a, J.Quinta da Fonseca a, D. Rugg b, M. Prea Manchester Materials Science Centre, The University of Manchester, Grosvenor street,b Rolls-Royce plc, P.O. Box 31, Derby DE24 8BJ, UK

a r t i c l e i n f o

Article history:Received 21 January 2013Received in revised form2 April 2013Accepted 7 April 2013Available online 12 April 2013

a b s t r a c t

In the present study, the rnear-titanium alloy Ti21processed in a fully stacontrolled grain-coarsenwas retained at room teselection and texture evolution in a

s a,n

chester M13 9PL, UK

of grain coarsening on variant selection has been investigated in theTi15Mo3Nb3Al0.21Si). The material was rst thermomechanicallyed condition in order to obtain a ne grain size before undertakingheat treatments. Two different cooling regimes ensured that either all erature or signicant formation was achieved during cooling with

sevier.com/locate/msea

& Engineering A

recent variant selection studies have focused on the effect of grain coarsening [4,16]. In this case, grain coarsening wascontrolled in Ti6Al4V by adding Yttrium that formed Yttria.The texture was either studied indirectly by reconstructing

was heat treated for 1 h at 900 1C in argon atmosphere followed by

rotation of Euler angles taking into account the crystal symmetriesof the parent and the daughter crystals. TEXTAN III applies thetransformation to experimental data by using the spherical har-monic method proposed by Bunge [23]. In this way it was possibleto describe the disagreement between the predicted and mea-sured texture and to identify the level of variant selection. Thereference systems for the Euler angles are such that the specimencoordinate systems, here the rolling direction (RD), transversedirection (TD) and normal direction (ND), are Cartesian and areparallel to crystal coordinates [1010], [1210] and [0001] respec-tively. This convention was used throughout this work. In the caseof orientation distribution function only the 2451 ODF for and201 for are presented here as they each capture all orienta-tions with relatively high density. Therefore, these slices are

G.C Obasi et al. / Materials Science & Engineering A 576 (2013) 272279 273water quench. This condition is designated here as S1-. Sampleswith dimensions of 12121.5 mm3 were cut from the centreregion of the sheet and subjected to further heat treatments inargon atmosphere. In the rst instance, a heat treatment at820 1C for 20 min (just above the transus) was carried outfollowed by rst cooling at 1 1C/min to 795 1C (just below the transus) and then by 0.1 1C/min cooling rate to room temperature.The purpose of this heat treatment was to obtain lamellaegrowing from the grain boundary without causing any signicant grain coarsening compared to S1-. This condition is designatedS1-. For S2- and S3- the material was exposed for 15 h to 900 1Cand 1000 1C, respectively, followed by water quench. S2- and S3- were equivalent to S2- and S3- but were cooled at 0.1 1C/minagain in order to promote lamellae formation from the grainboundaries. Details of all the heat treatment conditions includingtotal time in the phase eld are depicted in Table 2.

Table 1The nominal chemical compositions of Ti21S.

Elements Mo Nb Al Si C O N H Fe TiWt% 15 3 3 0.21 0.05 0.17 0.05 0.015 0.40 Balgrains from orientation maps recorded at room temperature byEBSD [17] or by in-situ neuron diffraction studies.

In the present work, a more highly stabilised Ti alloy(Ti21STi15Mo3Nb3Al0.2Si) was chosen for such studies,because it displays far slower grain coarsening kinetics above the transus than more conventional Ti alloys, avoiding the need ofsecond phase particles to control grain growth. The relativelymoderate grain coarsening in alloys such as Ti21S has beenattributed to the high Mo content and, to a certain degree, itslow transus [18]. In addition, when Ti21S is cooled relativelyfast, it retains the high temperature microstructure allowing directobservations of the phase at room temperature. The aim of thepresent work is to discuss the texture evolution during graincoarsening and its impact on transformation textures and variantselection when the material is cooled sufciently slowly for phasetransformation to occur.

2. Experimental procedure

2.1. Material

The material used for this study was TIMETAL 21S (Ti15Mo3Nb3Al0.21Si) provided by United States Air Force (USAF) in theform of a 190 mm103 mm cast billet produced by TIMET Co.,USA. Table 1 shows the nominal chemical composition of the alloyprovided by the manufacturer. A block of 14014550 mm3 wascut from the billet for hot and cold rolling. First, the block wasrolled above the -transus at 900 1C with a soaking time beforerolling of 1 h. The nal sample thickness of 4 mm was achievedwith 2 mm reduction per pass with 5 min reheating between eachpass. After hot rolling the sample was pickled at TIMET, UK inWitton to remove the -case that had formed during hot rollingand the material was cold rolled to 1.5 mmwith 0.5 mm reductionper pass. All the heat treatments after rolling were conducted in aprogrammable Vecstar tube furnace with a tube diameter of38 mm, which provides precise temperature control and excellenttemperature uniformity across the tube due to a small tubediameter. In order to rst recrystallise the cold rolled material it2.2. Microstructure and texture analysis

Mid sections of the heat-treated samples were preparedfollowing standard grinding/polishing procedures and etched in2% HF, 8% HNO3 and 90% H2O (Kroll's reagent) for 10 s prior tooptical microscopy examination. Electron Backscatter Diffraction(EBSD) was used for the characterisation of microstructure evolu-tion and crystallographic texture analysis. For best Kikuchi patternacquisition, the samples were polished with Oxide PolishingSuspension (OPS) for about 2 h and etched repeatedly duringpolishing. A CamScan, Field Emission Gun Scanning ElectronMicroscope (FEG-SEM), equipped with HKL Channel 5 softwarewas used for EBSD data acquisition and analysis. A step size of8 m was applied and an area of 88 mm2 was covered duringeach scan in order to measure a reasonable number of grains fortexture analysis. In order to compare the grain size of the slowlycooled with the water quenched material, the orientation mapsrecorded on the slowly cooled material were also used to recon-struct the grain structure based on Burger's orientation relationbetween the parent and daughter grains. The software used forthis reconstruction was developed at the University of Shefeldwithin IMMPETUS (Institute for Microstructural and MechanicalProcess Engineering). The software identies for each lath theneighbouring laths that are most likely to have the same parent orientation. To obtain a successful reconstruction a minimumallowable misorientation angle between variants of a common grain is dened, which in the present case was 31. A moredetailed description of the methodology can be found in [17,1921]and details regarding the software in [17]. The information fromthe reconstruction was subsequently imported into Channel 5 toplot the orientation maps. The EBSD orientation data was used toplot pole gures and the Orientation Distribution Functions(ODFs). The textures are represented in form of pole gures andODF using the well-established in-house software TEXTAN IIIdeveloped by Bate [3,4,11,22]. TEXTAN-III was also used to predicttransformation textures assuming no variant selection. This wasachieved by applying the Burgers orientation relationship as a

Table 2Processing conditions and mean grain size.

Condition ProcessingT (1C)

Holdtime(h)

Total timein phaseeld (h)

Cooling rate(1C/min)

Meangrain size(m)

S1- 900 1 1 Water quench (WQ) 60S1- 820 0.33 0.75 1 1C/min to 795 1C,

then 0.1 1C/min to RT64

S2- 900 1 15 WQ 165S3- 1000 15 15 WQ 285S2- 900 15 32.5 0.1 1C/min to RT 170S3- 1000 15 49 0.1 1C/min to RT 295sufcient to describe texture evolution in both phases [24].

3. Result

3.1. Microstructural and texture evolution

The EBSD band contrast maps of the quenched S1-, S2- andS3- are presented in Fig. 1. The maps reveal fully equiaxed

microstructures in all cases with an average grain size of 60 min the case of S1-, 165 m for S2- and 285 m for S3-. The mean grain size was determined using the linear intercept method(ASTM E112) and the values are also included in Table 2. The grainsize distributions (plotted here for grain volume rather thanfrequency) were determined by dening high angle boundaries

Fig. 1. Band contrast EBSD maps recorded on (a) S1-, (b) S2-, (c) S3-, and (d) the grain size distributions of S1-, S2- and S3-.

G.C Obasi et al. / Materials Science & Engineering A 576 (2013) 272279274Fig. 2. Measure (100), (110) and (111) pole gures of (a) S1-, (b) S2- and (c) S3-.

to have a misorientation of at least 151 and can be found in Fig. 1d.The comparison demonstrates that the increase in grain size isobtained by signicant widening of the grain size distribution tolarger grain size. This suggests that certain grains coarsen farmore signicantly than others. Fig. 1d also demonstrates that avery signicant volume of coarse grains occupies the materials assome grains coarsen.

The textures of the three conditions are presented in form of{100}, {110}, {111} pole gures and ODF 2451 sections inFigs. 2 and 3. The {111} pole gures in Fig. 2b and c developed astronger pole along ND compared to Fig. 2a. This strong pole ispart of the 111ND ( bre) texture often found in steel duringrecrystallisation [25] and can be easily identied in the 2451

section of the ODF in Fig. 3c and d. The texture components withinthe bre are signicantly weaker in S1- compared to S2-

Fig. 4. Optical micrographs of (a) S1-, (b) S2- and (c) S3-.

G.C Obasi et al. / Materials Science & Engineering A 576 (2013) 272279 275Fig. 3. Measured textures ODF (2451) section of materials (a) more sensitive

scale ODF plot of S1-, (b) S1-, (c) S2- and (d) S3-. and S3-. In order to demonstrate their presence an additional

-, (

G.C Obasi et al. / Materials Science & Engineering A 576 (2013) 272279276ODF was plotted using a more sensitive scale, Fig. 3a. It isimportant to point out at this stage that the texture clearlystrengthens with increasing mean grain size, i.e. widening grain

Fig. 5. ODF (2451) sections of phase measured (a) in S1- (b) in S2size distribution.Optical micrographs of the transformed lamellar microstruc-

tures, S1-, S2- and S3-, are presented in Fig. 4(ac). Asintended, the microstructures are populated by lamellar colo-nies. It can also be seen that the prior grain boundaries are easilyidentied by continuous lm of grain boundary (GB), which wasused to determine the grain size. Table 2 demonstrates thatdespite the slow cooling rates the mean grain size is very similarto the corresponding quenched conditions. EBSD analysisshowed that the and phase fraction was about 50 vol% whenthe material had been very slowly cooled, which allowed thedirect determination of the texture, Fig. 5(ac). In addition,reconstructed maps from were used to calculate the texture,Fig. 5(df). As previously mentioned reconstruction softwareidenties for each lath the neighbouring laths that are mostlikely to have the same parent orientation. To obtain a successfulreconstruction a minimum allowable misorientation anglebetween variants of a common grain is dened, which in thepresent case was 31. It can be seen that, as one would expect, thecorresponding textures are practically identical. Similarly, goodagreement is seen between the corresponding water quenched(Fig. 3bd) and slowly cooled conditions although the strength ofindividual texture components was slightly different.

The transformation texture for S1-, S2- and S3- are shownin the ODF sections at 201 in Fig. 6. The measurements of thethree conditions show similar texture components but signicantdifferences in texture intensity. The textures are composed oftexture components that are part of a 0001 bre texture and asecond texture component with Euler angles {1, , 2} {901, 301,01} i.e. (1215)[51056]. Comparing the and texture evolutions, itseems that the observed strengthening of the 111ND bre inthe phase also results in strengthening of texture components.Therefore, there seems to be a link between texture strengthen-ing and the development of a 111ND bre texture.

c) in S3-, (d) reconstructed from in S1-, (e) in S2- and (f) in S3-.3.2. Variant selection

In order to better understand the inuence of the 111ND bre texture on variant selection during to phase transforma-tion, the predicted textures are shown for S1-1, S2-2 and S3-3in Fig. 7 and can be compared with Fig. 6. The expected texture,assuming no occurrence of variant selection during the -transformation, was modelled from the texture (recorded fromwater quench samples, representing high temperature phase) byemploying the Burgers orientation relationship as described in theexperimental section [22,23]. The discrepancy between the pre-dicted and measured texture for the S2- and S3- conditions areclearly visible when comparing Fig. 6 with Fig. 7. The intensities ofthe main texture components from the predicted textures areabout 3 random intensity for the three conditions while themeasured texture showed 3 random intensity for S1-, 5random intensity for S2- and 7 random intensity for S3-. Itcan be stated that as the grains coarsen, the discrepancy betweenmeasured and predicted texture increases.

In summary, the results of the texture measurements clearlyindicate that there is a strong correlation between the develop-ment of specic texture components that are part of the 111ND bre texture (related to grain coarsening as result of heattreatment temperature and holding time) and level of variantselection. In the following section, the possible root causes forsuch correlation are discussed.

4. Discussion

When uniaxially rolling materials with a bcc crystal structure,in absence of recrystallisation, an bre texture is typically

G.C Obasi et al. / Materials Science & Engineering A 576 (2013) 272279 277expected to be generated while the development of a bretexture indicates recrystallisation [25]. The latter is generallyattributed to high stored energy nucleation mechanism, whichsuggests that the new nuclei appear preferentially at sites exhibit-ing a low value of stored energy [25]. In the present work, arelatively weak 1 1 1ND bre texture that developed after therolled material had been annealed for 1 h at 900 1C exhibited askeleton line that was slightly curved with respect to ideal1 1 1ND . Further grain growth observed when the materialhad been heat treated at 900 1C and 1000 1C for 15 h gave rise to astrengthening of {1, , 2} {601, 871, 451} and {1, , 2} {241,501, 451} texture components that are part of the 1 1 1ND breas shown in Fig. 3c and d. The review work in [26] has shown thatthe evolution of microstructure and micro-texture are related tothe misorientation angle distribution when applying differentprocessing regimes. When these processing regimes lead to grain

Fig. 6. Measured textures represented by ODF (201) section (a) S1-, (b) S2-and (c) S3-.coarsening, the material develops a specic misorientation dis-tribution. Fig. 8 compares misorientation angle distributionsrecorded for Ti21S in the different heat-treated conditions(S1-, S2- and S3-) to a theoretical Mackenzie distribution whenthe material has a perfectly random texture [26]. It can be seenthat an increase in heat treatment time or temperature results inmisorientation distributions increasingly skewed towards lowangle and 3 (o1114601) boundaries, i.e. low energy stateboundaries [27], compared to the theoretical distribution. Thismeans that the highest level of low angle and 3 boundaries isfound in the case of the coarsest grain size. Hence, the driver toreduce grain boundary energy does not only result in graincoarsening but also preferred growth of certain grains, whichresults in an overall increase in low energy, i.e. low angle and 3,boundaries. As a result of the preferred growth of certain grainorientations the 1 1 1ND bre texture components strengthenwith increasing grain size as observed in Fig. 3. Similar observa-tions were also obtained in [28,29] showing the dominance of lowangle and special grain boundaries in materials, which wereprocessed to have strong or specic textures and heat-treatedresulting in grain coarsening. It should be mentioned that thispresent work conrms our previous ndings in Ti6Al4V withsignicant grain growth, once heat treated above the transustemperature. It was concluded that the coarsening and asso-ciated texture strengthening are the driving force for variant

Fig. 7. Predicted transformation textures represented by ODF (201) section for(a) S1-1, (b) S2-2 and (c) S3-3.

G.C Obasi et al. / Materials Science & Engineering A 576 (2013) 272279278selection in the case the material is cooled slowly from above the transus [4,16].

The experimental textures of phase formed in S1- are verysimilar with the predicted textures from the phase texturebefore transformation based on the Burgers orientation relation-ship. This demonstrates that limited grain coarsening is accom-panied by the formation of a weak 1 1 1ND bre texture,which results in almost no variant selection during cooling. Incontrast, the long heat treatments (S2- and S3-) result insignicant further grain coarsening and strengthening of specic texture components that are part of the 1 1 1ND bre. Theresulting texture after cooling coincides qualitatively but notquantitatively with the predicted textures from the high tem-perature phase texture. Hence, variant selection increases with grain coarsening that is accompanied by strengthening of specic texture components. The comparison especially between Fig. 6cand Fig. 7c indicates very strong preferential selection of variantsforming 0001 bre components and (1215)[51056] texturecomponents. The two strong texture components develop inconjunction with a high level of low energy grain boundaries, i.e.low angle and 3 grain boundaries. Similar to previous work[16,30], the high level of variant selection observed in S2- and S3- seems to be related to the combination of specic 1 1 1ND

Fig. 8. Misorientation angle distribution of S1-, S2- and S3- compared to aMackenzie random distribution (data adapted from Ref. [26]).texture components making it more likely to obtain pairs of grains with closely aligned (1 1 0) planes. It has been demon-strated previously that such grain pairs result in early nucleationwith common 0 0 0 2 poles on both sides of the grain boundaryduring cooling due to the reduced nucleation energy [4,10,30].

In order to provide evidence that the strengthening of the two texture components along the 1 1 1ND bre enhance thelikelihood of grain pairs with closely aligned 1 1 0 poles, astatistical analysis was carried out. The textures were recreatedby using TEXTAN-III to output randomly 1000 Euler angles fromthe ODFs of the phase shown in Fig. 3. The similarity of therecreated ODFs in Fig. 9 with the measured phase in Fig. 3conrms that 1000 Euler angles were sufcient to accuratelyrepresent the measured textures.

Fig. 10 plots the relative frequency of misorientation between{1 1 0} in the range of 0151. Details of this calculation are found in[30]. As mentioned above, previous work has shown that mis-alignment between 1 1 0 poles of less than 101 can lead to variant selection [11]. Fig. 10 conrms that the texture evolutionassociated with grain coarsening is associated with having anincreasing number of neighbouring grains with closely aligned(1 1 0) planes. Consequently, variant selection is intensied.5. Conclusion

The present work has investigated the effect of graincoarsening on variant selection in a highly stabilised titaniumalloys, which displays comparatively slow coarsening rates

Fig. 9. Representative ODFs of phase for (a) S1-, (b) S2- and (c) S3-.

comPreallo

variants. In the present case, the slow grain growth in Ti21S

Finally, even though Ti21S is a much higher stabilised alloythan Ti6Al4V and displayed a slightly different texturebefore cooling from above the transus, the present workconrms previous ndings related to Ti6Al4V that coarsen-ing and associated texture strengthening are the drivingfactors for variant selection in the case the material is cooled

mis

G.C Obasi et al. / Materials Science & Engineering A 576 (2013) 272279 279allowed to study the effect of grain coarsening without thecomplication of pinning particles. The main ndings can besummarised as follow:

The processing and recrystallisation heat treatment of Ti21Sresulted in the formation of a 1 1 1ND bre. This is incontrast to work carried out on Ti6Al4V thermomechanicallyprocessed in the two phase regime prior to the heat treat-arg

pared to conventional + titanium alloys such as Ti6Al4V.vious investigations in this area, using conventional titaniumys, required adding Yttria particles [4,16], which one mightue have potentially an additional effect on the nucleation of

10. Comparison of calculated frequency of grain pairs with low (less than 151)orientation between one of their 110.Fig.ment, where the phase generally displayed a rotated1 1 1ND bre.It was found that after cold rolling stabilised Ti21S and aninitial recrystallisation heat treatment followed by quenching,only a weak texture was produced with a very slight buthomogeneous bre. Additional heat treatments to coarsen the grains resulted in selective growth of grains leading tostrengthening of particular bre texture components.It was demonstrated that grain coarsening and the strength-ening of particular texture components that are part of the1 1 1ND bre is driven by the relative increase of low angleand 3 grain boundaries, which are associated with low grainboundary energy. It was also shown that the strengthening ofthe two texture components increase the likelihood of having grain pairs with common 1 1 0 poles, which is known to bean important factor in terms of variant selection when nucleation starts from grain boundaries. Consequently, variant selection resulting in texture strengthening becameincreasingly important with the growth of grain size.

[22] P. Bate TEXTAN III, 1990.

[29] F. Caleyo, T. Baudin, R. Penelle, V. Venegas, Mater. Sci. Eng. A 298 (2001)227234.[30] G.C. Obasi, S. Birosca, D.G. Leo Prakash, J. Quinta da Fonseca, M. Preuss, Acta.Mater. 60 (2012) 60136024.[23] H. Bunge, Texture analysis in Materials Science, Butter-worths, London, 1982.[24] Gideon Obasi, Doctorate Dissertation, School of Materials, The university of

Manchester, 2011.[25] L. Kestens, K. Verbeken, J.J Jonas, Recrystallisation and Grain Growth, in:

G. Gottstein, D.A. Molodov (Eds.), Proceedings of First Joint InternationalConference, Springer-Verlag, Berlin, 2001.

[26] V. Randle, Int. Mater. Rev. 49 (1) (2004).[27] B. Alexandreanu, B. Capell, G.S Was, Mater. Sci. Eng. A300 (2001) 94104.[28] W.E King, A.J Schwartz, Scr. Mater. 38 (3) (1998) 449455.Acknowledgements

The authors are grateful for material provision from thelaboratories of the United States Air Force (USAF). The authorsare also indebted to Professor Pete Bate for useful discussions andinput and funding by EPSRC.

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258264.slowly from above the transus.

The effect of grain coarsening on variant selection and texture evolution in a near- Ti alloyIntroductionExperimental procedureMaterialMicrostructure and texture analysis

ResultMicrostructural and texture evolutionVariant selection

DiscussionConclusionAcknowledgementsReferences