Effect of β volume fraction on the dynamic grain growth during superplastic deformation of Ti₃Al-based alloys

The superplastic deformation behavior of Ti₂Al based (α₂ +β) alloy was studied with respect to the volume fraction of α₂/β. Three alloys containing 21, 50 and 77% in volume fractions of β exhibited large tensile elongations of over 500% at 970°C with a strain rate of 2.5×10^-4 sec^-1. The largest elongation was observed in the alloy with 21% of β. As the volume fraction of β phase increased, the flow stress and correspondingly, the strain-rate sensitivity values decreased. Due to the higher diffusivity of Ti in β phase than in α₂ phase, the increase in β volume fraction from 21% to 77% accelerated the dynamic grain growth, and degraded the superplasticity of the Ti₃Al-based alloys. The strain-based grain growth behavior was quantitatively analyzed and incorporated into a constitutive equation. The calculated flow curves are in agreement with the experimental ones in the stable deformation region.