On the Plasticity of Bimodal Structures in Titanium Alloys

The onset of plasticity in engineering titanium alloys with bimodal microstructures depends on the activation of slip in the equiaxed α grains. We use protocols based on slip trace offset analysis to measure the ratios of critical resolved shear strengths (CRSS) in the equiaxed α grains in bimodal microstructures of Ti 6242 and Ti 6246 alloys. The resulting CRSS ratios for prismatic 〈a〉/basal 〈a〉 and pyramidal 〈a〉/basal 〈a〉 for Ti 6242 vary depending on whether the probability of slip is estimated at high Schmid factors for these slip systems or average Schmid factors over the range of orientations of equiaxed α used in the dataset. We compare these ratios to the existing data on these and other titanium alloys. We then examine slip in transformed β in these bimodal structures and conclude that deformation in transformed β is influenced strongly by slip transfer from equiaxed α arising from orientation relationships between equiaxed α and β rather than global Schmid factors associated with the load axis.