Microstructural development and creep deformation in equiaxed γ, γ+α 2, and γ+α 2+B2 titanium aluminides

Abstract

The development of microstructure and its influence on creep properties have been studied for structures including equiaxed ␥, duplex, and other structures of varying ␣ 2 morphology in two Ti-48Al-2Cr-2Nb alloys. Heat treatments at 1125 ЊC have been utilized to produce equiaxed ␥ microstructures in alloys with or without Mo additions. The ␥ → ␣ transformation produces ␣ 2 plates with several orientation variants within ␥ grains during subsequent annealing of the equiaxed ␥ microstructures below the ␣ transus. Formation of this ␣ 2 morphology results from rapid up-quenching (UQ), and this structure persists through annealing, cooling, and creep testing. Differences in minimum creep rates for several microstructures containing varying amounts of multi-or single variant ␥/␣ 2 grains are shown to be minimal. The presence of Mo has also resulted in improved creep resistance in equiaxed ␥ and ␥ ϩ ␣ 2 ϩ B2 structures, as compared to similar microstructures in the Ti-48Al-2Cr-2Nb alloy. Deformation during creep at 760 ЊC at stresses between 200 and 400 MPa occurs by a combination of twinning and dislocation glide without recrystallization, resulting in power-law stress exponents in the range of 6 to 9. Only minimal strain path dependence of the minimum creep rate is detected in a comparison of creep rates in stress jump, stress drop, and single stress tests.