Superplastic behavior of TiAl based alloy at
relatively low temperatures ranging from
800℃ to 1075℃
relatively low temperatures ranging from
800℃ to 1075℃
(1. State Key Laboratory for Powder Metallurgy,
Central South University, Changsha 410083, China;
2. Shanghai Jiaotong University, Shanghai 200030, China)
Central South University, Changsha 410083, China;
2. Shanghai Jiaotong University, Shanghai 200030, China)
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