Effect of annealing on the superplastic properties of ultrafine-grained Ti–5Al–5V–5Mo–1Cr–1Fe alloy

This study explores the superplastic behavior of ultrafine-grained Ti–5Al–5V–5Mo–1Cr–1Fe alloy in the temperature range 773–1023 K depending on its structural-phase state after severe plastic deformation and additional 1-h annealing at temperatures 773, 873, and 973 K. It is shown that the formation of a more equilibrium structural-phase state after the annealing treatments significantly reduces the elongation to failure and causes a shift in the superplasticity temperature to higher deformation temperatures (~100° after 1-h annealing at 973 K). A possible reason for the high elongation to failure (over 1500%) during superplastic deformation of ultrafine-grained Ti–5Al–5V–5Mo–1Cr–1Fe alloy is thought to be the formation of a deformation-induced grain structure which is a mixture of small (<1 μm) and large grains, wherein small grains form interlayers between large grains. The structure evolves in this way at least until 500% strain due to the continuous formation of new less than 1 μm grains. After preliminary annealing of the ultrafine-grained alloy at 973 K under similar superplastic deformation conditions, the fine-grained interlayers do not form. This factor, along with the annealing-induced changes in the state of grain boundaries and density of deformation defects in the grain bulk, is assumed to be the cause of ductility reduction for Ti–5Al–5V–5Mo–1Cr–1Fe alloy under the considered conditions.