ON THE CREEP-BEHAVIOR OF THE TI3AL TITANIUM ALUMINIDE TI-25AL-10NB-3V-1MO

The temperature and stress dependence of the steady-state creep behavior of the Ti3Al alloy Ti-25Al-10Nb-3V-1Mo (at.%) has been evaluated. Two microstructural conditions were evaluated as follows: As processed (rolled) consisting of the fine grained (approx. 6-10-mu-m) beta plus ordered alpha-2 phase and beta heat treated consisting of coarse grained (approx. 150-mu-m) retained ordered B2 phase with a fine Widmanstatten structure within the grain interiors. The steady-state creep behavior of both microstructural conditions was studied over the temperature range of 650-815-degrees-C. The apparent creep activation energies and stress exponents were measured for both microstructural conditions. The temperature and stress dependence of the steady-state creep rate of both microstructures can be described well by the power law creep equation suggesting dislocation motion as the operative deformation mechanism. Over the temperature-stress regime of the present study, the creep deformation of the fine grained microstructure possibly breaks down into a low temperature (dislocation core diffusion controlled) regime and a high temperature (bulk diffusion controlled) regime within the power law creep region as indicated by the apparent creep activation energies measured. Upon beta heat treatment, creep deformation is found to be governed by a single rate limiting process. At temperatures and stress levels where a direct comparison can be made, the steady-state creep rates of the beta heat treated Ti-25-10-3-1 exhibit an order of magnitude decrease over those of the processed material. This suggests the possibility of some mechanism other than power law creep controlling within the regime corresponding to the low apparent activation energy of the fine grained microstructure.