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Materials Science and Engineering A 398 (2005) 209–219 Effect of deformation twinning on microstructure and textureevolution during cold rolling of CP-titanium Y.B. Chun, S.H. Yu, S.L. Semiatin, S.K. Hwang School of Materials Science and Engineering, Inha University, 253 Yonghyun-Dong, Nam-Gu, Incheon 402-751, South Korea b Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLM, Wright-Patterson Air Force Base, OH 45433, USA Received 22 November 2004; received in revised form 10 March 2005; accepted 16 March 2005
Abstract The evolution of microstructure and textureduring cold rolling of commercial-purity titanium(CP-Ti)was studied with particular reference to deformation twinning and dislocation slip. For low to intermediate deformation up to 40% in thickness reduction, the external strain wasaccommodated by slip and deformation twinning. In this stage, both compressive ({11¯22}11¯2¯3) and tensile ({10¯12}10¯1¯1) twins,as well as, secondary twins and tertiary twins were activated in the grains of favorable orientation, and this resulted in a heterogeneousmicrostructure in which grains were refined in local areas. For heavy deformation, between 60 and 90%, slip overrode twinning and shearbands developed. Thecrystaltextureof deformed specimens was weakened by twinning butwas strengthened by slip, resulting in asplit-basaltexture in heavily deformed specimens.© 2005 Elsevier B.V. All rights reserved.	Abstract Эволюция микроструктуры и текстру при холодной прокатке CP-Ti изучали с особым вниманием деформационному двойникованию и скольжению дислокация. При низкой до средней деформации (до 40% уменьшения толщины), внешние деформации вызывали скольжение и деформационное двойникование. На этом этапе, как сжимающие ({11 ¯ 22}  11 ¯ ¯ 2 3 ) так и растягивающие ({10 ¯ 12}  10 ¯ 1 ¯ 1 ) двойники, а также, вторичные и третичные двойники были активизированы в зернах с благоприятной ориентацией, и это привело к формированию гетерогенной структуры, в которой были области, совбодные от дефектов. Для больших деформаций, от 60 до 90%, скольжения перодолело двойникование и развивались полосы сдвига. Кристаллическая текстура деформированных образцов была ослаблена двойникования но была усилена скольжения, в результате чего сплит-базальной текстуры в сильно деформированных образцов.
1Introduction Plastic deformation of metals is usually governed by the activation of slip or deformation twinning. The specific deformation mechanisms in metals with a hexagonal close packed(hcp) crystal structure are less well understood than those in cubic metals which usually have a large number of indepen-dent slip systems. In pure titanium, for example, slip occurs most easily via the activation of dislocations withatype Burgers vector primarily on prism planes, to some extent onbasal planes and least on pyramidal planes[1]. Becauseaslip alone cannot provide five independent slip systems, as required to accommodate an external strain imposed on the grains of a polycrystalline aggregate, deformation byc+aslip(onpyramidalplanes)orbytwinningusuallymustbeac-tivatedinadditiontoaslip[2–6].Inthisrespect,ithasbeen suggested on a theoretical basis that twinning can account for a maximum strain of only 0.1[3],or a value considerably less than the ductility of pure titanium[1].Despite such assertions, there are reports that twinning plays an essential role in deformation and texture formation for titanium[7–9].Other research has shown that heavy cold rolling of high-purity titanium results in the development of a split-basal texture Ti[10–15], where as a normal basal texture forms in less pure Ti containing alloying element such as Al[7]. The difference in texture development for the different types of Ti has been attributed to the effect of composition on the activation of deformation twinning[8]. Due to the tedious nature of the determining deformation twins via transmission electron microscopy (TEM) in early work; however, a quantitative explanation has not been developed to describewhich twin systems become active under specific modes of deformation, how twinning contributes to microstructure re-finement or how twinning affects the resultant texture. Recent advances in electron-back-scattered-diffraction(EBSD) techniques provide a powerful method for charac-terizing local texture, twin relationships, etc. and thus offer significant promise to provide answers to such questions[16].The objective of the present study, therefore, was to utilize such techniques in order to obtain a firm understanding of the details of deformation twinning systems in commercial-purity titanium (CP-Ti) under cold rolling conditions and to establish how twinning affects the formation of basal and other types of textures	1 Введение Пластическая деформация металлов обычно регулируется активацией скольжения или деформационного двойникования. Конкретные механизмы деформации в металлах с гексагональной плотноупакованной (ГПУ) кристаллической структурой менее понятны, чем в кубических металлах, которые обычно имеют большое число независимых систем скольжения. В чистом титане, например, скольжение происходит наиболее легко через активацию дислокаций с векторм Бюргерса типа  главным образом по призматическим плоскостям, в некоторой степени по базисным и меньшей мере по пирамидальным плоскостям [1]. Так как  скольжения само по себе не может обеспечить пять независимых систем скольжения, которые требуются для внешних деформаций, приложенных к зернам поликристаллического образца, деформации  CA  скольжения (на пирамидальных плоскостях) или двойникования как правило, должны быть активированы в дополнение к  скольжения [2-6]. В связи с этим было предложено на теоретическом основании, что двойникование может составлятьмаксимум напряжение только 0,1 [3 ], или его значение значительно меньше, чем пластичность чистого титана [1]. Несмотря на такие утверждения, есть сведения, что двойникования играет существенную роль в деформации и формирование текстуры для титана [7-9]. Другие исследования показали, что тяжелые холодной прокатки высокочистого титана результатов в развитиисплит- базальной текстуры Ti [ 10-15 ], где в качестве нормальной формы базальной текстуры в меньшей чистый Ti, содержащих легирующий элемент, такой как Al [7]. Различие в текстуре развивающейся при различных типах Ti было обусловлено влияниес структуры на активацию деформационного двойникования [8]. В связи со сложным характером определения деформационных двойников с помощью просвечивающей электронной микроскопии (ПЭМ) в начале работы, но количественное объяснение не была разработана, чтобы describewhich двойной системы становятся активными при определенных режимов деформации, как двойникование способствует микроструктура повторного конфайнмента или как двойникования влияет на полученную текстуру. Последние достижения в области электронно- обратно - рассеянных дифракции (ЭИ) методы обеспечивают мощный метод характеризующих местные текстуры, две отношения и т.д. и таким образом предлагают значительные перспективы, чтобы дать ответы на такие вопросы [16].Цель настоящего исследование, таким образом, было использовать такие методы, чтобы получить твердое пониманиедетали систем деформации двойникование в коммерческих чистого титана (CP- Ti) в холодных условиях прокатки и установить, как двойникование влияет на формирование базальной и другие типы текстур
2. Experimental procedures Thematerialusedinthisworkwascommercial-puritytita-nium received as 12-mm-thick hot-rolled and annealed platewhose measured composition is given inTable 1. Samplesmeasuring 150mm×200mm were cold rolled by reversingthe rolling direction between each pass at room temperatureto a total thickness reduction of 90% in a two-high mill with220mm diameter rolls using a rolling speed of 13.8m/min.During each pass, the thickness was reduced by 0.2mm withthe aid of oil lubrication.Following cold working, optical microscopy, EBSD anal-ysis and TEM were conducted on transverse cross-sectionscut from the rolled samples. For optical microscopy andEBSD analysis, specimens were mechanically polished andthen electro-polished in a solution consisting of 5ml per-chloric acid and 95ml methanol at 30V and−40◦C. Subse-quently, the samples were etched with a solution consistingof 1ml HNO3, 2ml HF and 40ml H2O.Grain-boundary character distributions (GBCD) in therolled specimens were established via EBSD using a Hi-tachi 3400S field emission gun scanning electron micro-scope (FEG-SEM) and TSL-OIMTM software. The statisti-cal certainty of the EBSD analysis, especially for the highlystrained materials, is significantly affected by the level of confidence index (CI) for which the software allowed duringpost-processing of measured EBSD data. Preliminary EBSDexperiment for cold rolled-Ti revealed that the fractionsof random high angle boundaries decreased with increasingCI min (the minimum CI allowed in EBSD post-processing)in the range of CInminnfrom 0 to 0.1. This is mainly due torandom orientation relationship between incorrectly indexedpoints(generallyhavinglowCI)andtheirneighboringpoints.In the range of CI min higher than 0.1; however, the overallaspectofmisorientationangledistributionwasunaffectedbyCI min. Based on these, any measured points whose CI is lessthan 0.1 were excluded from the analysis of the EBSD datain the present study.To determine the substructure developed during rolling,TEM analysis was performed using a Philips CM200 trans-mission electron microscope. Specimens for TEM werethinned to 60m and then twin-jet electro-polished at 30Vand−40◦C using the solution previously described.The textures developed during rolling were quantified us-ing a Rigaku RINT2500 X-ray diffractometer. For this pur-pose, five pole figures ((10¯10), (0002), (10¯11), (11¯20)and (10¯12)) were obtained from the plate/sheet surfaceusing the Schulz reflection method. Using the five incom-plete pole figures so obtained, the orientation distributionfunction (ODF) was calculated with the commercial pro-gramLaboTex TM basedonthearbitrarilydefinedcell(ADC)method[17].From the ODFs, complete pole figures were reconstructed. Euler angles were represented with referenceto a crystal coordinate system consisting of X=[2¯1¯10],Y =[01¯10] andZ=[0002].
. Results 3.1. Starting materialOpticalmicroscopyshowedthatthestartingmaterialcomprisedsingle-phase,equiaxed-Tiwithanaveragegrainsizeof 30m (Fig. 1(a)). In addition, XRD analysis revealedpeaksonlyforthe-phase,and back-scatter-electron imagingintheSEMconfirmedthattherewasnosecondphase(suchas-phase). These analytical results indicated that the programmaterial (as-received CP-Ti) was indeed composed solely of -phase despite being commercial grade, most likely due tothe low levels of impurities (Table 1).In particular, the level of iron, a potent-stabilizer in titanium alloys, was approx-imately 200wppm, or only half the maximum solubility of Fe in the-phase (∼400wppm), thus resulting in a very lowprobability for the retention of -phase at room temperature.Hence, the possible effect of second phases on the deforma-tion behavior of CP-Ti can be excluded from consideration.The as-received CP-Ti plate, which had been hot rolledand then annealed in the-phase region, had a moderate tex-ture (Fig. 1(b)). The (0002) pole figure revealed a bimodaldistribution of basal poles, a texture commonly found in coldrolled pure Ti; the maximum intensity (4.4×random) wasfound at locations tilted±35◦from the ND toward the TD.A second, weaker component comprising (11¯20) poles atlocations tilted 15◦from the RD toward the ND suggestedthe development of a recrystallization texture also. In the(10¯10) pole figure, the maximum intensity was found at theRD, indicating that a considerable amount of rolling texture,which had developed during hot rolling, remained. From thepole figure analysis, therefore, it was confirmed that the as-received texture comprised both rolling and recrystallizationcomponents
.2. Microstructure evolution during low-to-mediumlevels of deformation Low-to-medium levels of deformation resulted in the de-velopment of heterogeneous microstructures due to the fragmentationofsomegrainsasaresultoftwinningandtheelon-gationofothergrainsthatdeformedbyslipalone.Asthelevelof deformation increased, the fraction of twins gradually in-creased. For example, the overall microstructure after 10%reductionwasverysimilartothatoftheas-receivedmaterial,exceptfortheformationoftwinsinafewgrains,asshownviaorientation imaging microscopy (OIM) (Fig. 2(a)). At 20% reduction, however, more twinning was activated, and grainswith and without deformation twins were clearly identified(Fig. 2(b)). At yet higher reductions of 30 and 40%, a signif- icant refinement of the microstructure was observed in somegrains (Fig. 2(cand d)); in such instances, the crossing of deformation twins and the generation of secondary and tertiarytwinscausedthedevelopmentofatwinnedlamellarstructurewith a thickness of 1–5m, a significant refinement com-pared to the starting grain size of 30m. In contrast, grainsinwhichtheimposeddeformationwasaccommodatedbysliprather than by twinning (marked ‘NT’ inFig. 2),remained coarse, albeit elongated along the RD, after 40% cold reduc-tion.In order to identify the twinning systems that were ac-tivated during low-to-medium levels of cold deformation,the misorientation angle and rotation axis of each twin rela-tive to the parent (matrix) orientation were determined fromthe EBSD data. This analysis revealed that 65◦10¯10and85◦11¯20boundaries (corresponding to{11¯22}11¯2¯3and{10¯12}10¯1¯1twin families, respectively) were mostfrequently observed. The activation of these two twinningmodes was confirmed statistically from misorientation-angledistributions (Fig. 3), which also showed high fractions nearthe misorientation angles of 65◦and 85◦. The misorienta-tion angle distributions also indicated that boundaries witha misorientation angle near 65◦were predominant at a re-duction of 20%. However, boundaries with misorientationsnear 85◦became comparable to those with the 65◦mis-orientation as the level of deformation increased. This in-dicates that{10¯12}10¯1¯1twinning was more active than{11¯22}11¯2¯3twinning as the cold rolling reduction wasincreased.Themisorientationdistributionsalsoshowedthatthefrac-tion of low angle boundaries (LAB) increased with increas-ing reduction (Fig. 3).Specifically, the fraction of low angle boundaries was 27% after 10% reduction and increased sig-nificantly to 80% after 40% reduction. While the increase inLAB fraction with increased reduction led to a correspond-ing decrease in the overall fraction of high angle boundaries,the fraction of boundaries near 65◦and 85◦was still higherthan the fraction of other high angle boundaries. However,the misorientation angle distribution, which exhibited sharppeaks around 65◦and 85◦after 10% reduction (Fig. 3(a)),began to spread with increased reduction. This trend sug-gests that the ideal twin–matrix orientation relationship wasdestroyed due to deformation via slip in both the twin andmatrix in order to accommodate the deformation imposedfollowing twin formation.TheEBSDresultsalsorevealedthattheactivationoftwin-ning was dependent on the local crystallographic orientationof each matrix grain. For the few, remnant coarse grains thatwere observed to have accommodated deformation mainlyby slip (Fig. 2),the basal poles were located approximately 40◦to90◦fromtheNDtowardtheTD,whiletheprismpoleswereratherrandomlydistributed(Fig.4(a)).Suchgrainswere thuswellorientedforaccommodatingtheplane-strainrollingdeformation via prism slip. On the other hand, EBSD anal-ysis of 62 grains that had undergone twinning by a reduc-tionof20%reductionrevealedthat {11¯22}11¯2¯3twinningwas observed only in grains whose basal poles were locatedwithin approximately 50◦from the ND (Fig. 4(b)). Such observationsthusindicatethattheactivationof {11¯22}11¯2¯3compressivetwins was most likely dependent on the ori-entation of the matrix and the difficulty of accommodatingcompression near thec-axis via slip processes. In addition,{10¯12}10¯1¯1tensiletwinning appeared to have been acti-vated without a noticeable dependence on matrix orientation(Fig. 4(c)). The formation tendency of particular twins in a grain was also affected by the orientations of the surround-ing grains in addition to that of the matrix grain because, asshown inFig. 4(b and c), either compressive twins or tensiletwins were generated in similarly oriented grains. For thick-ness reductions higher than 20%; however, the dependence of the activation of {11¯22}11¯2¯3twins on the matrix ori-entation decreased. At the same time, other types of twins,suchas{11¯21}¯1¯126and{10¯11}10¯1¯2,wereobservedoccasionally.Secondary twins were observed for thickness reduc-tions above 20%. When the primary twins were of the{11¯22}11¯2¯3compressive type, the secondary twinswithin the primary twins were of the{10¯12}10¯1¯1ten-sile type (Fig. 5), thus also indicating a dependence of twinactivity on parent orientation.Pole figures determined from X-ray diffraction (XRD)measurements revealed that the initial split-basal texture wastransformed to a basal texture as the reduction was increasedto 40%. After 20% reduction, the original basal poles of thebimodaldistributionalongtheND–TDbegantobedispersedtoward the ND (Fig. 6(b)). As a result, the maximum basal- pole intensity after 30–40% reduction was observed parallelto the ND (Fig. 6(c and d)). Unlike the distribution of the basal poles, the maximum intensities for the prism poles, al-though not very strong, were found along the RD and werenot affected noticeably by the level of cold reduction
3.3. Microstructure evolution during higher levels of deformation At yet higher levels of thickness reduction (≤90%), themicrostructurebecamemorerefined,butmoreheterogeneousas well. After 60% reduction, elongated, coarse grains (witha thickness of 10m) were interspersed with fine grains (de-veloped at lower reductions due to twinning), as shown inFig. 7.Because the as-received CP-Ti was equiaxed with anaverage grain size of 30m, the aspect ratio of the coarse grains reflected the amount of deformation imposed dur-ing cold rolling. EBSD analysis showed that the elongatedcoarsegrains(Fig.7(c)),hadorientationsintherange ϕ1=0◦,Φ=30–90◦and ϕ2=30◦. Also, a small amount of macro-scopic shear banding which had not been found during reductionsequaltoorbelow40%wasnotedathighreductions.After 90% cold reduction, the microstructure becamemuch more refined and the macroscopic shear banding wasmore evident (Fig. 7(b)). The thickness of the elongated coarse grains had been reduced to 3m. The orientation im-age for the sample rolled to 90% reduction (Fig. 7(d)) alsoshowed that the lattice was so severely deformed that it wasimpossible to analyze approximately 70% of the data pointsvia EBSD. The orientation of the elongated coarse grainsin the sample rolled to 90% reduction was in the range of ϕ1=0◦,Φ=30–50◦and ϕ2=30◦, thus indicating that thebasal poles near the TD moved toward the ND as the amountof deformation increased.TEM analysis of CP-Ti samples rolled to 60% reductionrevealed a fine lamellar structure with high dislocation density in regions which had been difficult to analyze with opti-cal microscopy or EBSD. Deformation bands composed of alamellar-typemicrostructurewithathicknessof100–150nmwere observed (Fig. 8(a)); a generally high dislocation den- sity was found within the deformation bands[18,19]. In an-other region of the same sample, grains elongated parallelto the RD with a thickness of 100–500nm were observed; ahigh dislocation density was also found inside these grains(Fig. 8(b)). The ring-like selected area diffraction patterns(SADP) (upper right-hand corner of Fig. 8(b)) indicated thatthe grain-boundary character in this region was high an-gle.Elongatedcoarsegrainswithhomogeneouslydistributeddislocations were also observed (upper left-hand corner of Fig. 8(a)), and similar observation was made earlier by Wag-ner et al.[15].Thesplit-basaltexturereappearedathigherlevelsofdeformation.Thebasalpoleshadanintensityof3.7(×random)after60%reduction(Fig.9(a)).Thesplit-basaltexturestrength- ened with yet further cold reduction, reaching an intensity of 5.9 (×random) at locations tilted±35◦from the ND towardthe TD after 90% reduction (Fig. 9(b)). While the maximum intensity in CP-Ti rolled to a 40% reduction was observedin the (0002) pole figure, the maximum intensity of 4.6(×random)wasobservedintheRDofthe(10¯10)polefigurefor material rolled to 60% reduction (Fig. 9(a)). The inten- sity of prism poles was also strengthened by increasing theamount of reduction, reaching∼7.7 (×random) after 90%reduction.
4. Discussion 4.1. Deformation twinning at low-to-medium levels of deformation The activation of twinning in the present material exhib-ited a strong dependence on the level of deformation. Forthickness reductions less than or equal to 40%, twinning wasactive, whereas for higher deformations dislocation slip wasthe sole mechanism of deformation. The occurrence of twin-ning was confirmed by the peaks in the misorientation distri-bution at 65◦and 85◦, which correspond to{11¯22}11¯2¯3compressive twinning and{10¯12}10¯1¯1tensile twinning,respectively (Fig. 3).Fig. 3(a and b) also reveal that com- pressive twinning was more prevalent than tensile twinningat reductions less than 20%. This result does not necessar-ily mean that the critical shear stresses for the two twin-ning systems were different, for the imposed deformationand the crystallographic orientation of the grains also playa key role in the activation of a particular twinning system.For the undeformed CP-Ti program material, the basal poleswere preferentially distributed along the ND, which is sub- jected to a compressive strain during rolling (Fig. 1(b)); very few grains had basal poles parallel to the RD along which atensile strain is imposed. Therefore, the combination of theinitialtextureandthestateofdeformationimposedduringflatrolling resulted in the preferential activation of compressivetwins.An explanation for the activation of {10¯12}10¯1¯1ten-sile twins, despite the unfavorable texture, focuses on thevalueofthecriticalshearstressforsuchadeformationmode.In related work, for example, Tenckhoff [20]established thetwinning activity in pure zirconium by determining the ini-tial orientation and lattice rotations of 19 grains during coldrolling. In this earlier work,{11¯22}11¯2¯3compressivetwins and{10¯12}10¯1¯1tensile twins were found to beactivated in grains with their basal poles inclined by 0–50◦and 50–90◦, respectively, to the ND. A similar analysis inthe present work, using EBSD and focusing on a much largernumber of grains (62 grains) (Fig. 4(b)), confirmed Tenck- hoff’s observation for the case of {11¯22}11¯23compres-sive twins. The{10¯12}10¯1¯1tensile twins, however, wereactivated in grains which did not have an obvious orien-tation relationship to the imposed plane-strain deformation(Fig. 4(c)). This result may be interpreted to be a result of a comparatively low critical shear stress for tensile twinningcompared to that required for compressive twinning and per-hapsaslip. This hypothesis was confirmed by the nature of the secondarytwins. As shown by EBSD analysis (Fig. 5),secondarytwinsofthe{10¯12}10¯1¯1typenucleatedwithinthe primary compressive twins of the{11¯22}11¯23typewhose thickness ranged from 1 to 5m. It is well known thatthe propensity for twin formation is significantly reduced asgrainsizedecreases[8,21,22].Therefore,theformationofthesecondary tensile twins within the fine primary twins wouldbefeasibleonlyifthecriticalshearstressforthetensiletwinswere very small.According to Paton and Backofen[23], the formation ten-dency of particular twins in-Ti is affected by temperature:{11¯22}11¯23type compressive twins at room tempera-ture whereas{10¯11}10¯1¯2type compressive twins above400◦C.Thepresentresultisinsupportofthisearlierreport.Incase of the tensile twins, the critical shear stress is reportedto be low for the{11¯21}¯1¯126type twins compared tothe{10¯12}10¯1¯1type[24].In the contrary, however, the latter type tensile twins were dominant in the present result.ChristianandMahajan[22]suggestedthatthefavorablecon- ditions for twin formation are a low twin shear and a smallextent of atomic shuffling. Yoo[25]calculated the two pa- rameters for the hcp crystals, the result of which is shown inTable 2.The type of twins found in the present result can be explainedintermsofthetwoparameters:the{10¯11}10¯1¯2type compressive twins and the{11¯21}¯1¯126type tensiletwins are difficult to activate in Ti due to the large shufflingparameter and the high twinning shear, respectively. In con-trast, the{11¯22}11¯23type compressive twins and the{10¯12}10¯1¯1type tensile twins are easily activated be-cause of their small shuffling parameter and low twin shear,respectively.The formation of deformation twins during cold rollingcontributed to the significant refinement in microstructureand hence reduced the effective slip length. The initial CP-Ti material used in this work presentedafavorableconditionfordeformation twinning because the grain size was relativelylarge. Consequently, the number density of twins increasednwith the imposed deformation. Formation of numerous me-chanical twins and intersection among these twins divide thegrain interior, resulting in microstructural refinement. Addi-tional grain refinement occurred by the almost simultaneousformation of secondary and the tertiary twins in addition tosubdivisionoftwinsduetocrossingtwins.Twinningbecamesaturated at 40% thickness reduction (Fig. 2(d)) at which theeffective grain size had been reduced to such a large extentthattwinningwasimpossible.Incontrast,grainswhosebasalpoles were inclined from the ND toward the TD by 40–90◦were not susceptible to twinning (Fig. 4(a)). These grains deformed mainly by dislocation slip and, as a result, becameelongatedgrains,whichwerecomparativelylargerthanthosethat underwent twinning. The coexistence of fine twinnedgrains and large grains that had undergone slip alone, there-fore, resulted in an inhomogeneous microstructure in CP-Ticold rolled to low-to-medium levels of reduction. This inho-mogeneity in microstructure persisted to high deformation(Fig. 7)
4.2. Texture evolution Slip in titanium occurs most readily along theadirec-tion on prism and basal planes. However,aslip alone cannot satisfy the von Mises requirement of five independentdeformation modes to accommodate an externally imposedstrain[26,27].Although the activation of twinning accom- modates plastic deformation along thecdirection, heavydeformation above 40% suppresses further twin formationdue to the reduced grain size introduced by prior twinning.The absence of additional twinning during the large defor-mation of titanium has also been reported by Philippe et al.[28]andMullinsandPatchett[29].Therefore,anotherdefor- mation mechanism is required to accommodate strain above40% thickness reduction. Otherwise, it would be impossi-ble to accommodate uniform plane-strain deformation in allcrystallitesduringrolling.Inthepresentwork,itappearsthatthe latter case pertained in that non-uniform, macroscopicshear banding was activated as shown inFig. 7. In speci-mens reduced by 60% or more, numerous shear bands werepresent. The particular shear deformation is known to occurwhen the grain orientation is unfavorable for slip or where afine lamellar structure is predominant[30–32]. Consideringthe fine deformed microstructure and the lack of sufficientslip systems in CP-Ti, the observed deformation via shearbanding during heavy deformation is as expected.In the present work, two principal types of textures werefound: a basal texture (Fig. 6(d)), developed during low-to-intermediate rolling reductions (≤40%), and a split-basaltexture (Fig. 9), found at high reductions (to 90%). Using a Taylor-type (isostrain) crystal-plasticity model, Thornburgand Piehler[7]suggested that the basal texture originatedfrom a combination of prismaand pyramidalc+aslip.As shown in the present work, however, the probability of pyramidalc+aslip seems to be low for low-to-mediumrolling reductions because twinning can accommodate thestrain along thecaxis as well as the fact that the criticalresolved shear stress forc+aslip is relatively high. There-fore, it is expected that the main slip systems would be theprismaand the basala. During large deformation (>40% thickness reduction),a split-basal texture was formed (Fig. 9). Thornburgh andPiehler[7]concluded that such a texture results from theactivation of both slip and twinning. The present result, how-ever, does not support this conclusion in as much as no ad-ditional twinning was found for reductions above 40%. Thisimplies that twinning did not contribute to the separation of thebasalpolesfromtheNDtowardtheTD.Therefore,itmaybe hypothesized that either pyramidalc+aslip (activatedto accommodate deformation along thecaxis when twin-ning is not feasible) or the shift in strain path associated withshearbandingmayhavecontributedtothesplit-basaltexture.The development of texture during cold rolling was alsointerpretedintermsoftheϕ2=30◦sectionoforientationdis-tribution function maps (Fig. 10).The location of the maxi- mum f (g)progressedfrom(ϕ1=20◦,Φ=35◦andϕ2=30◦)inthe initial (undeformed) condition toward (ϕ1=0◦,Φ=35◦and ϕ2=30◦) inthefinal90%coldrolledcondition.TheODFresults showed that the typical cold rolling texture compo-nent (ϕ1=0◦,Φ=35◦and ϕ2=30◦) started to form at low reductions. With increasing cold reduction, however, the in-tensity of this component was weakened when twinning wasactivated in addition to slip, as shown inFig. 11.By contrast, duringheavycoldrollingduringwhichslipwaspredominant, the intensity of the cold rolling texture component increased.Therefore, it is concluded that slip intensifies the cold rollingtexture, but twinning weakens it by randomizing the crystalorientations
5. Conclusions Microstructure and texture evolution during cold rollingof CP-Ti were studied via optical microscopy, OIM-EBSDand TEM. The following conclusions were drawn:1. Deformation comprising low-to-moderate thickness re-ductions (≤40%) was accommodated by slip and twinning,whereasslippredominatedathigherreductions.Theprimarytwinningsystemsactivatedwere{11¯22}11¯2¯3compressivetwinsand {10¯12}10¯1¯1tensiletwins.Sec-ondary twins, mainly of the tensile type, were also acti-vated, thus indicating that the critical shear stresses of these twins is probably relatively low.2. The activation of deformation twinning results in grainrefinement due to intersection of twins and the formationof secondary and tertiary twins. Grain refinement due totwinning leads to increased difficulty for twin activity,leading to saturation in twinning at modest reductions.Furthermore, an inhomogeneous grain structure can begenerated because some grains may be oriented to ac-commodate the imposed strain via slip alone.3. During heavy deformation (thickness reductions >40%),macroscopic shear bands develop because of the absenceof twinning and the difficulty of accommodating the im-posed plain-strain deformation viaaslip alone.4. The characteristic rolling texture of (ϕ1=0◦,Φ=35◦and ϕ2=30◦), a split-basal texture, is a consequence of deformation by slip. Twinning weakens this particulartexture component by randomizing the orientations of crystals

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