Simulation of hot deformation of TiAl based alloy containing high Nb

The hot deformation behavior of Ti-45Al-8.0Nb-0.2B-0.2W-0.02Y(at.%) alloy was studied by simulating isothermal forging tests using 6 mm diameter and 10 mm height compressive specimens in order to find the appropriate deformation processing parameters. The seven deformation temperatures of 900, 1000, 1050, 1100, 1150, 1200 and 1250 °C and three deformation strain rates of 5×10^-4, 1×10^-3 and 5×10^-2 s^-1 were used in the present investigation. The experimental results showed that the deformation resistance decreased and the true stress vs. true strain curves became flatter with the decrement of strain rate at the same deformation temperatures. However, the deformation resistance decreased with increasing temperature, and the sensitivity of true stress to strain rate became smaller as temperature increased at the same deformation strain rates of 5×10^-2, 1×10^-3 or 5×10^-4 s^-1. This indicates that the higher the deformation temperature and the lower the strain rate, the smaller the deformation resistance, which means that the high deformation temperature and low strain rate are beneficial to the hot deformation of high Nb containing TiAl alloys. Using the kinetic rate equation the stress exponent n and the apparent activation energy Q were determined as 4.7 and 366.5 kJ mol^-1, respectively. Considering the durability limit of the forging machine and mold materials for deformation temperature and the deformation microstructure control depending on the deformation degree and ending deformation temperature, an appropriate set of deformation processing parameters of 1250 °C and 1×10^-3 s^-1 are recommended for the present TiAl alloy. These recommended hot deformation processing parameters have been successfully used for the quasi-isothermal canned forging of high Nb containing TiAl alloy billets with the initial dimension of 120 mm in diameter and 180-350 mm in height.