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.