Effects of grain size and fraction on the deformation modes of a Ti-6Al-2Sn-4Zr-2Mo-Si alloy with equiaxed ( plus ) microstructures: Slip trace analysis and multiscale simulation of polycrystal plasticity

This study investigates the mechanisms behind the great mechanical properties observed at room temperature for a dual-phase Ti-6Al-2Sn-4Zr-2Mo-Si titanium alloy with equiaxed (alpha + beta) microstructures. More precisely, analyzing the material deformation modes and the possible effects of beta fraction and grain size was done to better understand such micromechanisms. With this idea in mind, uniaxial tensile deformation tests were performed at room temperature, and the resulting mechanical behaviors were analyzed. It was observed that increasing beta fraction would enhance the overall ductility and work hardening while conversely decreasing the material resistance. Additionally, the material strengthening due to grain size effect, quantified by the Hall-Petch parameter, was also found to be dependent on beta fraction. Slip trace analysis was conducted to understand the effects of grain size and beta fraction on the activation of the basal , prismatic , and pyramidal slip systems and their critical resolved shear stress (CRSS) ratios were established. The qualitative study of CRSS ratios revealed that at smaller grain sizes, the basal slip systems were dominant (e.g. basal/prismatic CRSS ratio of 0.86 for d=2.98 mu m) whereas the prismatic slip systems were prevalent and more easily activated for coarser grains (e.g. basal/prismatic CRSS ratio of 1.19 for d=4.21 mu m). Such CRSS ratios were then used to identify the material parameters of a self-consistent multiscale model employed to reproduce the tensile behaviors. For a more quantitative analysis, the CRSS values were evaluated and correlated to grain sizes with Hall-Petch relations. Clear correlations regarding grain size and beta fraction were found for the CRSS of prismatic and pyramidal systems. However, special attention was given to the ambiguous results regarding basal slip systems because of the potential manifestation of the compatibility stresses and grain boundary sliding mechanisms due to the higher density of grain boundaries at small grain sizes.