THE EFFECT OF THERMAL EXPOSURE ON MICROSTRUCTURAL STABILITY AND CREEP RESISTANCE OF A 2-PHASE TIAL/TI3AL LAMELLAR ALLOY

During annealing of a two-phase TiAl/Ti3Al lamellar alloy at 1273 and 1323 K, the lamellar microstructure evolves into a course, globular microstructure. For short annealing times (less than about 1000 hours), microstructural evolution occurs predominantly by intrapacket termination migration coarsening. For longer annealing times, cylinderization and conventional Ostwald ripening coarsening mechanisms are observed. The activation energy for the rate-controlling diffusion process governing intrapacket termination migration coarsening of the lamellar microstructure was determined to be 215 kJ/mol. Compression creep tests reveal that the minimum creep rate and primary creep strain of the lamellar alloy increase with increasing prior annealing time. Furthermore, in contrast to the lamellar microstructure, the globular microstructure is not susceptible to deformation-induced spheroidization during compression creep testing. Modeling demonstrates that the increase of the minimum creep rate and primary creep strain as a consequence of annealing of the lamellar alloy can be accounted for by consideration of two factors: the decrease in the work-hardening rate of the lamellar alloy in response to the overall decrease in interphase interfacial area and the decreased mechanical strengthening effect associated with transformation from a lamellar to a globular microstructure.