Characterization of hot deformation behavior of as-cast TC21 titanium alloy using processing map

The hot deformation behavior of as-cast TC21 titanium alloy has been investigated in the isothermal compression of the alloy using processing maps conducted at the deformation temperature ranging from 1000°C to 1150°C, the strain rate ranging from 0.01 to 10s^-1, and the height reduction of 60%. The constitutive equation of as-cast TC21 alloy is developed in the form of the exponential law and the deformation activation energy is calculated to be about 185.51kJ/mol. Based on the dynamic materials model and Prasad's instability criterion, the processing maps of as-cast TC21 titanium alloy are constructed at strains of 0.4 and 0.6. Deformation mechanisms in the stable and unstable regions are verified through the microstructure observation. The maps exhibit a stable domain with the temperature range of 1000-1150°C and strain rate range of 0.01-0.1s^-1 with two peak efficiencies of power dissipation, one is 48% occurring at 1000°C/0.01s^-1 that is associated with dynamic recovery, the other is 38% occurring at 1150°C/0.01s^-1, which displays the feature of continuous recrystallization. Therefore, the optimal processing condition of the alloy is 1150°C/0.01s^-1. In addition, the material undergoes flow localization at strain rates higher than 1s^-1 which has to be avoided in hot processing for obtaining satisfactory properties. Thus the applicable cogging temperatures and cogging strain rates for as-cast TC21 alloy are 1000-1150°C and 0.01-0.5s^-1, respectively.