High-temperature deformation processing of Ti-24Al-20Nb

Abstract

Power dissipation maps have been generated in the temperature range of 900°C to 1150°C and strain rate range of 10^-3 to 10 s^-1 for a cast aluminide alloy Ti-24Al-20Nb using dynamic material model. The results define two distinct regimes of temperature and strain rate in which efficiency of power dissipation is maximum. The first region, centered around 975°C/0.1 s^-1, is shown to correspond to dynamic recrystallization of the α₂ phase and the second, centered around 1150°C/0.001 s^-1, corresponds to dynamic recovery and superplastic deformation of the β phase. Thermal activation analysis using the power law creep equation yielded apparent activation energies of 854 and 627 kJ/mol for the first and second regimes, respectively. Reanalyzing the data by alternate methods yielded activation energies in the range of 170 to 220 kJ/mol and 220 to 270 kJ/mol for the first and second regimes, respectively. Cross slip was shown to constitute the activation barrier in both cases. Two distinct regimes of processing instability-one at high strain rates and the other at the low strain rates in the lower temperature regions-have been identified, within which shear bands are formed.