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

The superplastic deformation behavior of Ti3Al based (alpha(2)+beta) alloy was studied with respect to the volume fraction of alpha(2)/beta. Three alloys containing 21, 50 and 77% in volume fractions of beta exhibited large tensile elongations of over 500% at 970 degrees C with a strain rate of 2.5x10(-4) sec(-1). The largest elongation was observed in the alloy with 21% of beta. As the volume fraction of beta phase increased, the flow stress and correspondingly, the strain-rate sensitivity values decreased. Due to the higher diffusivity of Ti in beta phase than in alpha(2) phase, the increase in beta 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.