Microstructural changes and estimated strengthening contributions in a gamma alloy Ti-45Al-5Nb pack-rolled sheet

Thin sheets made of a gamma-titanium aluminide alloy, Ti-45Al-5Nb, produced by a pack-rolling process, were evaluated for microstructure variation and evolution taking place during aging and annealing treatments. The as-received sheet material was characterized by remarkably high yield strength, 810 MPa, and a complex bimodal microstructure. The microstructure consisted of a matrix of twinned gamma-phase grains and fine-lath lamellar grain microconstituent, together with a dispersed ultra-fine-grained gamma + alpha-2 mixture microconstituent. High-temperature isothermal aging treatments changed the microstructure to a stable mixture of gamma-phase grains (matrix) and coarse alpha-2-phase particles, having size distributions and volume fractions that were specific to the aging temperature. A concurrent strength loss reflects this trend and results in a stable strength level of 550 MPa upon aging at 1000 °C for 144 h. Using composition estimates from the phase-boundary shifts that occur from the Nb addition to a Ti-45Al base alloy and, the rule of mixtures, an analysis was made to show that the gamma-phase matrix has an intrinsic strength of 178 MPa. This is a significant intrinsic strength level, well over that of ∼70 MPa for the Ti-45Al binary alloy. This is rationalized as the solid-solution strengthening effect from shifts of the Ti and Nb levels in the gamma phase and, by an added effect due to increased oxygen solubility in the gamma phase. The overall strength of Ti-45Al-5Nb, however, is roughly the same as that of Ti-45Al, and this is explained by a drastic reduction in the volume fraction of alpha-2-phase in Ti-45Al-5Nb alloy, which is a result of the Nb-induced phase-boundary shifts.