Effect of Initial Texture on the Evolution of Microstructure and Texture During Rolling of Commercially Pure Titanium at
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HEXAGONAL close-packed (HCP) titanium and its alloys are the material of choice for various structural applications in aerospace, bio-medical and chemical industries[1, 2] due to the high specific strength, excellent fracture toughness and biocompatibility of this class of materials. Due to highly anisotropic plastic behaviour of HCP metals,[3–5] there is a scope to design microstructure and particularly texture to achieve optimum properties like strength, ductility and toughness. Unlike cubic metals,[6] which mostly deform by slip, the
VIVEK KUMAR SAHU and N.P. GURAO are with the Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, India Contact e-mail: [email protected] Shubham Gupta is with the Department of Materials Science and Engineering, Indian Institute of Technology Kanpur and also with the Department of Metallurgical and Materials Engineering, National Institute of Technology Tiruchirappalli, Tiruchirappsalli, 620015, India. Manuscript submitted April 13, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
development of texture of hexagonal titanium depends significantly on deformation twinning at low to intermediate strain after which slip dominates deformation.[7,8] With less than ideal c/a ratio of 1.587, the preferable slip systems of titanium are 1010 1210
prismatic, f0002g 1120 basal and 1101 1120 pyramidal hai slip systems. These slip systems do not accommodate strain along c-axis to maintain strain compatibility between grains according to the von Mises criterion. Therefore, pyramidal hc þ ai 1101 1213 slips system should be active to accommodate the deformation along the c-axis. However, the critical resolved shear stress (CRSS) for 1012 1011 extension twinning is lower than that of pyramidal slip (125 MPa versus 180 MPa) and hence, extension common twinning is favoured in titanium.[9] The most
twin systems of HCP titanium are 1012 1011 exten
sion type I (ET1), 1121 1126 extension type II (ET2)
and 1122 1123 contraction type I (CT1) twin systems. ET1 is characterized by misorientation of
84.78 deg about 1120 axis, ET2 by 35.10 deg about
1010 axis and CT1 is characterized by a misorienta
tion of for 64.62 deg about 1010 axis. Meyers et al.[10] have reported that the activity of these twin systems depend on the orientation of grains, grain size, strain rate, and deformation temperature. In face-centred cubic metals, the propensity of twinning is also governed by the stacking fault energy (SFE);[11] however, the effect of SFE on the nucleation of twins in HCP metals is not well established in the last fifty years.[12–14] Activation of twins in HCP metals is attributed to the unavailability of five independent slip systems to attain compatible deformation in polycrystals.[15] The CRSS for twinning is generally higher compared to that of slip and pile-up of dislocations can aid in overcoming it; hence, twinning has a strong grain size dependence unlike slip with larger grains favouring twinning. Moreover,
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