MECHANICAL-BEHAVIOR AND MICROSTRUCTURAL EVOLUTION DURING SUPERPLASTIC DEFORMATION OF TI-6AL-4V

The superplastic deformation of fine grained Ti-6Al-4V has been studied under uniaxial tensile test condition at temperatures of 750, 800 and 850-degrees-C and at strain rates in the range of 5 x 10(-6)-2 x 10(-4) s-1. Constant strain rate and stepped true strain rate tensile tests were performed to determine the mechanical behavior of this alloy. Strain hardening and softening have been observed in these tensile tests. At high temperatures and/or low strain rates, the stress-strain curves showed strain hardening. In contrast, at lower temperatures and/or higher strain rates, strain softening was observed. Microstructural study of the alloy during superplastic deformation by scanning electron microscopy and transmission electron microscopy has been carried out to get a better insight into the mechanisms and mechanical characteristics of the titanium alloy in the context of strain enhanced grain growth and dynamic recrystallization. A deformation model for this alloy has also been suggested. The model predicts that certain amount of beta-phase is required to effectively coat alpha-phases and accommodate grain boundary sliding so that maximum superplasticity can be manifested.