HIGH-TEMPERATURE DEFORMATION PROCESSING OF TI-24AL-20NB

Power dissipation maps have been generated in the temperature range of 900 degrees C to 1150 degrees 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 stra in rate in which efficiency of power dissipation is maximum. The first region, centered around 975 degrees C/0.1 s(-1), is shown to correspo nd to dynamic recrystallization of the alpha(2) phase and the second, centered around 1150 degrees C/0.001 s(-1), corresponds to dynamic rec overy and superplastic deformation of the beta phase. Thermal activati on analysis using the power law creep equation yielded apparent activa tion energies of 854 and 627 kJ/mol for the first and second regimes, respectively. Reanalyzing the data by alternate methods yielded activa tion 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 t o constitute the activation barrier in both cases. Two distinct regime s of processing instability-one at high strain rates and the other at the low strain rates in the lower temperature regions-have been identi fied, within which shear bands are formed.