Microstructural development and creep deformation in equiaxed γ,γ + α2 and γ + α2 + B2 titanium aluminides

The development of microstructure and its influence on creep properties have been studied for structures including equiaxed γ, duplex, and other structures of varying α₂ morphology in two Ti-48Al-2Cr-2Nb alloys. Heat treatment at 1125°C have been utilized to produce equiaxed γ microstructures in alloys with or without Mo additions. The γ→α transformation produces α₂ plates with several orientation variants with γ grains during subsequent annealing of the equiaxed γ microstructures below the α transus. Formation of this α₂ 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 γ/α₂ grains are shown to be minimal. The presence of Mo has also resulted in improved creep resistance in equiaxed γ and γ + α₂ + B2 structures, as compared to similar microstructures in the Ti-48Al-2Cr-2Nb alloy. Deformation during creep at 760 °C 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. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlandom, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.