Abstract:Superplastic behaviors of TiAl based alloy with initial grain size of about 2μm obtained by multistep forging were investigated at 800～1075℃  with strain rates of 8×10^-5s^-1～2×10^-3s^-1. The results show that the material exhibits excellent low temperature superplasticity. Flow softing resulting from dynamic recrystallization is observed at relatively low temperatures (≤1000℃) or at higher strain rates (≥2×10^-4s^-1). Continuous strain hardening resulting from strain-enhanced grain growth occurs at higher temperatures or at lower strain rates. A maximum elongation of 533% is obtained at 800℃ with strain rate of 2×10^-5s^-1, and at 1050℃, a maximum elongation of 570% is obtained at strain rate of 8×10^-5s^-1. At a fixed strain rate of 2×10^-4s^-1, when the alloy is deformed at 850℃, the microstructure is refined, however at 1050℃, is coarsened. The as-deformed microstructure shows relatively high strain rate sensitivity value and it keeps nearly stable during deformation. The activation energy is calculated to be 290kJ/mol at 950～1075℃, with the grain size exponent, p=2, and 224kJ/mol at 800～900℃ with p=3. Therefore, it is suggested that the dominant mechanism during superplastic deformation at 800～900℃ is grain boundary sliding controlled by grain boundary diffusion; however at 950～1075℃ is grain boundary sliding controlled by lattice diffusion.

Key words: TiAl based alloy; superplasticity; activation energy; grain size exponent