THE EFFECT OF THERMAL EXPOSURE ON MICROSTRUCTURAL STABILITY AND CREEPRESISTANCE 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 mi crostructure. For short annealing times (less than about 1000 hours), microstructural evolution occurs predominantly by intrapacket terminat ion migration coarsening. For longer annealing times, cylinderization and conventional Ostwald ripening coarsening mechanisms are observed. The activation energy for the rate-controlling diffusion process gover ning intrapacket termination migration coarsening of the lamellar micr ostructure was determined to be 215 kJ/mol. Compression creep tests re veal that the minimum creep rate and primary creep strain of the lamel lar alloy increase with increasing prior annealing time. Furthermore, in contrast to the lamellar microstructure, the globular microstructur e is not susceptible to deformation-induced spheroidization during com pression creep testing. Modeling demonstrates that the increase of the minimum creep rate and primary creep strain as a consequence of annea ling of the lamellar alloy can be accounted for by consideration of tw o factors: the decrease in the work-hardening rate of the lamellar all oy in response to the overall decrease in interphase interfacial area and the decreased mechanical strengthening effect associated with tran sformation from a lamellar to a globular microstructure.