Phase transformation in the β phase of super α 2 Ti 3Al base alloys during static annealing and superplastic deformation at 700–1000 °C

Abstract

Ti 3Al based alloys have admirable superplasticity with elongation greater than 1000% at temperatures between 950 and 1000 °C. In this study, the phase evolution and mechanical behavior of a super α 2 alloy Ti–25%Al–10%Nb–2%V–1%Mo (in at.%) at lower temperatures of 700–900 °C are further examined, and the reasons for the reduced elongations are examined. The initial grain size was around 2 μm. Although the super α 2 alloy exhibited superior superplastic elongations of 1500% at 960 °C, the elongation dropped to 600% at 900 °C, 330% at 850 °C and 140% at 750 °C. The β→α 2 ′ transformation occurred in accordance with the equilibrium phase diagram during static annealing, but the transformation was enhanced during dynamic straining at temperatures from 750 to 900 °C. The β→α 2 ′ transformation in the two-phase region is shown to be diffusion controlled, and not a massive or martensitic transformation. With the fine acicular α 2 ′ plates inside the β grains, the grain boundary sliding accommodation process across the BCC β grains was impeded, leading to lower tensile elongations.