Issue 48

A. Ghosh et alii, Frattura ed Integrità Strutturale, 48 (2019) 585-598; DOI: 10.3221/IGF-ESIS.48.57 589 Figure 3 : a) True stress-true strain curve b) Cyclic stress-strain curve of 1 st cycle c) Bar chart showing number of cycles to failure and d) Number of cycles to failure vs ratcheting strain. I NFLUENCE OF ORIENTATION ON SLIP ACTIVITY he VPSC simulated stress-strain curve obtained using the voce hardening parameter reported in Table 3 follows similar hardening response as determined experimentally, shown in Fig. 4a. The relative slip/twin activity shown in Fig. 4b indicates prism slip activity is higher while basal slip activity is lower in 0T compared to 45T and 90T which is due to higher Schmid factor for primary prism slip {10 1 0}<11 2 0> for 0T orientation as indicated from the initial texture in Fig. 2c. Since, there is only two independent prism slip system, hence plastic deformation via slip is accompanied by twin activity also. However, with increase in strain, prism slip and extension twin activity decreases at a critical strain value of ~0.16 in 0T which is slightly higher than that of 45T and 90T orientation. Beyond this crossover strain value, <c+a> pyramidal slip activity rises. However, prism slip activity becomes constant and the increase in pyramidal slip is lower in 45T compared to 90T. Thus, due to reduced slip activity at higher strain 45T shows lower ductility and hence lower strain to failure compared to 90T. It is also clear that the activity of pyramidal slip with high critical resolved shear stress (CRSS) compared to prism slip is much higher in 90T compared to 0T and 45T orientation, which results in lower strain hardening coefficient of 90T with high yield strength as reported in Table 4. However, observed relative slip activity from simulated stress-strain curves could not account well for the anisotropy in hardening response. Basically, commercially pure titanium exhibits three stage strain hardening behavior during tensile deformation [20]. First stage is purely slip dominated followed by twinning in the second stage and twin-slip dominated in the final stage of deformation. VPSC model is unable to estimate hardening response very accurately under the combined action of slip and twin activity, hence, very close match between the experimental and simulated hardening response could not be achieved. It is mainly due to orientation change and microstructural inhomogeneity issue especially associated with contraction twin activity. The influence of twin activity on anisotropic response would be depicted from the EBSD based microstructure and micro-texture analysis discussed in the next section. T