Issue 48

A. Ghosh et alii, Frattura ed Integrità Strutturale, 48 (2019) 585-598; DOI: 10.3221/IGF-ESIS.48.57 590 Figure 4 : VPSC simulated stress- strain curve and b) Relative slip/twin activity for tensile samples. R OLE OF TWINS IN ANISOTROPIC TENSILE DEFORMATION BEHAVIOUR he observed anisotropy in strain hardening response during tensile deformation could be understood from the characterisation of deformation twins, using the crystal orientation map shown in Fig. 5a. The deformation microstructure shows grains with 0001 ND  basal orientations, which are twin free in all the three samples. In CP-Titanium off-basal and non-basal orientations are more prone to twinning while 0001 ND  orientations are twin free and exhibit higher hardness [21]. Both contraction twin (CT) and extension twins (ET) are formed as indicated from the grain boundary misorientation plot in Fig. 5b. Sample 0T shows almost equal volume fraction of contraction and extension twin (5.8% CT+ 5%ET) whereas sample 45T (2.6% CT+ 6.5%ET) and 90T (3.8% CT+ 8.6%ET) has higher volume fraction of extension twin. In addition, the morphology and twin variants have been found to be distinctly different in 0T, 45T and 90T. The characterisation of twins in two different parent grain orientations: near basal and off basal shown in Fig. 6 reveals, in 0T sample both the orientations have thin contraction twins with extension twins present inside contraction twins and the contraction twins are multi-variant in nature as indicated from the unfolded IPF-loading axis contour plot. On the other hand, in 45T and 90T sample, twins are relatively thicker than in 0T sample and the extension twins are more prevalent in off basal grain orientation whereas contraction twins are present in near basal grain orientation. Moreover, in 45T sample parallel arrays of single twin variant has been observed while in 90T sample, intersecting twins of multi-variant nature are present. Thus, due to higher prism slip activity dislocation glide through forest dislocation becomes difficult, when deformation proceeds by formation of high volume fraction of thin, intersecting, multi-variant contraction twins, leading to higher strain hardening of 0T compared to 45T and 90T. This also suppresses the activation of pyramidal slip for higher strain and increases ductility in 0T. On the other hand, in 45T and 90T deformation can proceed easily by lateral thickening of extension twins leading to lower strain hardening coefficient, which could be attributed to less elastic stiffness difference between the parent grain matrix and the twinned region as proposed by Sinha et al. [15]. In addition, twinned region gets reoriented to unfavourable prism slip orientation which suppresses prism slip activity. The deformation along c-axis is accommodated by harder <c+a> pyramidal slip which reduces tensile ductility in 45T and 90T. T