Part II. The creep behavior of Ti-Al-Nb O+bcc orthorhombic alloys

Abstract

The intermediate-temperature (650 °C to 760 °C) creep behavior of orthorhombic (O)+bcc alloys containing 50 at. pct Ti was studied. Ti-25A1-25Nb, Ti-23Al-27Nb, and Ti-12Al-38Nb ingots were processed and heat treated to obtain a wide variety of microstructures. Creep deformation mechanisms and the effects of grain size, phase volume fraction, tension vs compression and aging on creep rates were examined. Unaged microstructures, which transformed during the creep experiments, exhibited larger primary creep strains than transformed microstructures, which were crept after long-term aging. The deformation observations and calculated creep exponents and activation energies suggested that separate creep mechanisms, dependent on the applied stress level, were dominating the secondary creep behavior. Coble creep characteristics, including relatively low activation energies and dislocation densities as well as stress exponents close to unity, were exhibited at low applied stresses. Experiments on fiducially marked specimens indicated that grain-boundary sliding was occurring for intermediate applied stresses. In this regime, the minimum creep rates were proportional to the applied stress squared and inversely proportional to the grain size. Dislocation-controlled creep characteristics, including stress exponents of greater than or equal to 3.5 and relatively high activation energies and dislocation densities, were exhibited at high stresses. Overall, the minimum creep rates were dependent on microstructure and stress. Within the low-to-intermediate stress regimes, subtransus processed and heat-treated microstructures, which contained much finer grain sizes than supertransus microstructures, exhibited the poorest creep resistance. The influence of grain size was not as significant within the high-stress regime. It is shown that for low-to-intermediate stress levels, grain size is the dominant microstructural feature influencing the creep behavior of O+bcc alloys.