MICROSTRUCTURE, CREEP, AND TENSILE DEFORMATION IN Ti-6Al-2Nb-1Ta-0. 8Mo.

A variety of microstructures simulating the heat affected zone of a weld as well as the as-received structure were tested in a temperature range of 298 K to 873 K. Beta-annealed martensitic microstructures displayed enhanced creep resistance, out-performing other recrystallized HAZ structures. Above 778 K the activation energy of creep is close to that for self-diffusion in titanium, suggesting that diffusion-controlled dislocation mechanisms are the rate-controlling processes at elevated temperatures. Creep rupture at elevated temperatures occurred by microvoid nucleation and growth. Fracture occurred along colony boundaries in the as-received structure but appear to be intergranular with the crack propagating along grain boundary alpha/matrix interfaces in the equiaxed microstructures. Sliding along alpha/beta interfaces, colony boundaries, prior beta grain boundaries, and slip traces contributed to the creep strain and rupture process. Cyclic creep with a loading-unloading sequence was also performed at room temperature and cyclic creep acceleration was observed.