Twin Nucleation by Slip Transfer across Grain Boundaries in Commercial Purity Titanium
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ALTHOUGH titanium and its alloys have been widely used in structural applications for a few decades, knowledge of the microstructure evolution of titanium during plastic deformation is still not well understood. Many studies have shown slip mode in that the primary hexagonal titanium is 10 10 1 210 prismatic slip.[1–3] Experiments on single-crystal high-purity titanium showed that the critical resolved shear stress for prismatic slip is much slip lower than that of other modes: f0001g 1 210 basal slip, 10 11 1 210 pyrami
dal hai slip, and 10 11 2 1 1 3 pyramidal hc+ai slip.[4] Nevertheless, the prismatic slip mode alone is not sufficient to accommodate an arbitrary plastic strain, which, according to the von Mises criterion, requires five independent slip systems.[5] As a result, nonprismatic slip and deformation twinning are also necessary to achieve significant plastic deformation in polycrystalline titanium.[6,7] In polycrystalline Ti, grains with their c-axes nearly perpendicular to the applied uniaxial stress are often L. WANG and Y. YANG, Graduate Students, and T.R. BIELER and M.A. CRIMP, Professors, are with the Chemical Engineering and Materials Science Department, Michigan State University, East Lansing, MI 48824. Contact e-mail: [email protected] P. EISENLOHR, Project Director, is with the Max-Planck-Institut fu¨r Eisenforschung, 40237 Du¨sseldorf, Germany. D.E. MASON, Associate Professor, is with the Mathematics and Computer Science Department, Albion College, Albion, MI 49224. Manuscript submitted May 9, 2009. Article published online November 12, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
referred to as ‘‘soft grains,’’ because of their high Schmid factor for one or more of the three prismatic slip systems.[8] Such soft grains usually display a single set of distinct prismatic slip bands following deformation. In contrast, ‘‘hard grains’’ have their c-axes nearly parallel to the applied stress, and prismatic slip is difficult to activate due to very low Schmid factors. In the present work, a soft grain will be defined to be any grain that has a prismatic slip system with a Schmid factor greater than 0.4, while a hard grain will have all three prismatic slip systems with a Schmid factor less than 0.15. To accommodate the local strains that develop during the deformation of a polycrystal, hard grains often activate pyramidal slip or deformation twinning.[6] In titanium, four deformation twinning modes have been reported,[9] as summarized in Table I. Here, K1 is the twinning plane and g1 is the twinning shear direction. In a given grain, tensile twinning modes (T1 and T2) or compressive twinning modes (C1 and C2) have high Schmid factors when the maximum principal stress direction along its c-axis is tension or compression, respectively. At room temperature, the T1 mode is the predominant twinning mode, which has been attributed to its low shear strain and a simple shuffle mechanism.[10–14] In high-purity titanium, deformation twinning has usually been observed in hard grains
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