Twin Interactions in Pure Ti Under High Strain Rate Compression
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INTRODUCTION
TWINNING is an important deformation mode for hexagonal close-packed (hcp) titanium with a c/a ratio of 1.587 where independent slip systems are not sufficient to accommodate continuous deformation.[1] So far six types of twins have been reported in titanium, including three types of tension twins of {1012}h1011i, {1121}h1126i and {1123}h1122i, and three types of contraction twins of {1122}h1123i, {1011}h1012i and {1124}h2243i, which can be abbreviated as E1, E2, E3, C1, C2, and C3, respectively.[1–4] Among these twins, E1 and C1 are the most frequently observed types, depending on whether the c-axes of the hcp crystals are subjected to tensile or compressive stresses. With the increase in the deformation strain or strain rate, interactions of twin-slip and twin-twin appear. Dislocations in the parent grain can actively interact with the tip of the twin embryos and proceed in step with the twin lamellae, based on the transmission electron microscopy (TEM) analysis.[5,6] And slip produced in a soft-oriented grain can stimulate the twin nucleation and growth in the neighboring hard grain.[7,8] Furthermore, twin-twin pairs are observed at grain boundaries during bending tests of Ti,[9] depending on the orientations of the parent grain and the geometrical twinning alignment at both sides of the boundary. Twin interactions in titanium deformed at high strain rates
PING ZHOU, Engineer, CHUNLI JIANG, Senior Engineer, and GE SANG, Research Fellow, are with the Science and Technology on Surface Physics and Chemistry Laboratory, Jiangyou 621908, China. Contact emails: [email protected], [email protected] DAWU XIAO and DONGLI ZOU, Associate Research Fellows, are with the Institute of Materials, China Academy of Engineering Physics (CAEP), Jiangyou 621907, China. Manuscript submitted April 21, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
were also observed by TEM analysis.[10] A particular examination has been conducted on the E2 twins using a four-point bending test.[11] It is found that one side of the E2 exhibited irregular twin boundaries, and E2 could interact with E1 and dislocation slip. In some cases, E1 can stimulate the nucleation of E2; in other cases, E2 can induce the nucleation of E1. Besides, the hc + ai dislocations in parent grains could transmute to E2 with the Burgers vector parallel to the twinning plane. In addition, selection of twin variant in hcp structure is also a complex issue since each twin type has six equivalent variants and those with the highest Schmid factors (SFs) may not occur.[12–15] Jonas et al.[15] used the accommodation strains that twins are trying to impose on their neighboring grains to explain the occurrence of low-SF variants in magnesium. Schuman et al.[16,17] proposed a new variant selection criterion to account for the activation of twinning variants in titanium based on the calculation of deformation energy. Both methods could explain the non-Schmid behaviors successfully. Compared with the E1 and C1 twins, investigations focusing on the E2 twins are limited s
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