Evolution of microstructure and superplastic deformation mechanism in super α2Ti 3Al base alloys

Abstract

The superplastic characteristics of a super α 2 Ti 3Al based alloy were studied over the range of 900–1000°C and 8×10 −5–2×10 −3 s −1. The maximum superplastic elongation of the 1 and 2 mm sheets was ∼600 and 1500%, respectively, obtained at 960°C and 2×10 −4 s −1. Based on the evolution of grain structures, texture distributions, stress–strain curves, variation of the m-, θ- and R T/ R W values, there seems to be exhibited three stages during superplastic loading. For ε<0.5, the thermally activated dislocation slip process controls the deformation, with m∼0.33 and Q t-400-500 kJ/mole. For 0.5< ε<0.8, the grains become basically equiaxed and GBS starts to play the dominant role. The m-value is around 0.5 and Q t is close to 270 kJ/mole, the latter may correspond to the Ti diffusion in disordered β grains or the interface diffusion. For ε>0.8, the same GBS and associated accommodation mechanisms continuously operate, with strain softening or hardening occurring under different conditions. It is considered that at high temperatures the ordered α 2 grains act as the rigid species and slide and rotate more freely within the β matrix. The disordered β grains are more likely the ones that carry accommodation through dislocation slip and climb.