Abstract: Focusing on the poor hot-deformability of TiAl alloy, the thermal deformation behavior of Ti-40Al-6V-1Cr-0.3Ni at deformation temperature of 1 100-1 225 ℃, strain rate of 0.01-0.5 s^-1 and engineering strain of 70% is studied by Gleeble-3800 thermal simulation test machine. The constitutive equation of the alloy is deduced based on the Arrhenius hyperbolic sinusoidal function model. The activation energy and stress index are 464.74 kJ / mol and 2.50 respectively, lower than that of the present deforming TiAl alloy. Based on the dynamic material model, the hot working diagram of the alloy with the engineering strain of 70% is established. The microstructure analysis results show that the multiphase TiAl alloy could fully recrystallize at high temperature and low strain rate, showing a similar behavior to that of the alloy with high stacking fault energy. The addition of β phase stable elements widen the hot working window of the alloy. β Phase plays an important role in relieving work hardening during high-temperature deformation. The full recrystallization and phase transformation decomposition of β phase should be the main ways to alleviate stress concentration. At the same time, it acted as a high-temperature lubricant to alleviate the stress at grain boundary and phase boundary and coordinate the difficult deformation α phase at high temperature. As a result, the thermoplastic deformation ability of the material is improved. According to the hot working diagram and microstructure feature, the appropriate deformation process parameters are proposed and successfully applied to the nearly isothermal forging of TiAl alloy.

Key words: multiphase TiAl, thermoplastic deformation, phase transition, dynamic recrystallization, softening mechanism