Hot deformation and dynamic recrystallization behavior of TiAl-based alloy

Hot compression and dynamic recrystallization (DRX) behavior of TiAl alloy (Ti-44Al-4Nb-1.0Mo-0.1B-0.01Y) were investigated by hot compression tests. The tests were conducted at temperature range of 1120–1240 °C with strain rate of 10⁻³∼1 s⁻¹. Results show that the flow stress during hot compression is affected by deformation temperature and strain rate. The stress exponent n and activation energy Q are measured to be 2.711 and 524.67 kJ/mol, respectively. Critical strain for DRX initiation is measured, and the relationship between critical strain ε_c and peak strain ε_p can be expressed as: ε_c = (0.7–0.9) ε_p. It is found that crack in β phase as well as the decomposition of β phase result from inhomogeneous deformation between γ phase and β phase. With the decreasing of lnZ, volume fraction (referred to V_f) of lamellar colony decreases and α phase gradually “invade” from the periphery to the center part of lamellar colony. Two DRX models during the hot deformation of TiAl alloy are developed. Type I relies on the nucleation within lamellae. Type II is the DRX of α phase. If lnZ>39.6, only type I can be found in the microstructure. If 36.1<lnZ<39.6, the microstructure contains the mixture of type I and type II DRX grains. While lnZ<36.1, only type II DRX grains are left. The guiding principle can well predict the deformed microstructure of TiAl alloy.