An age hardening study of a near gamma titanium aluminide Ti-47 Al-2Nb-2Mn-0.08vol% TiB2 alloyed with 0.1wt% carbon

A classical age hardening study on a two-phase near gamma titanium aluminide with a composition Ti-47Al-2Nb-2Mn(at%)-0.08vol%TiB2 +0.1wt%C has revealed an increase in primary creep resistance with increasing aging parameters. Solutionization experiments were conducted that revealed a solution temperature of 1300°C for 8 hours. Aging temperatures of 950°C, 1000°C, and 1100°C for 2 and 40 hours were conducted to study the effect of aging on precipitate size, morphology, distribution, and its effects on creep resistance. An increase in primary creep resistance with increasing precipitate size was observed with increasing aging time and temperature. Aging for 2 hours at each temperature produced microstructures yielding exceptional creep resistance compared to 40 hour aging times. Variation in primary creep resistance was traced to specimen location within each test bar. Specimens machined from the top of the test bar had greater primary creep resistance than specimens machined from the bottom of the test bar. This was correlated with microscopy that revealed large volume fractions of equiaxed gamma grains in the bottom 2/3 of cast test bars resulting from the closure of porosity during the HIP (hot isostatic pressing) heat treatment. Optical, scanning electron (SEM), and transmission electron microscopy (TEM) were used to characterize the microstructure resulting from the various aging schemes.