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 degrees after 1-h annealing at 973 K). A possible reason for the high elongation to failure (over 1500%) during superplastic deformation of ultrafinegrained 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 mu m) 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 mu m 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.