The effect of morphology of alpha and beta phases on superplastic properties of titanium-(6)aluminum-(4)vanadium alloy

The effects of morphology of alpha and beta phases on the superplastic properties of the Ti-6Al-4V alloy was examined. Tensile testing of cylindrical specimens at constant strain rates of 5 $\times$ 10$\sp{-3}$ to 1 $\times$ 10$\sp{-4}$ s$\sp{-1}$ in the temperature range of 875 to 950$\sp\circ$C were performed to determine the flow properties of beta annealed, beta quenched and equiaxed material with the same overall chemical composition and similar initial volume fractions of alpha and beta phases. Specifically, the total elongation to failure and the change in strain rate sensitivity values with strain were determined. Optical, scanning, and transmission electron microscopy studies were performed on tensile specimens water quenched from the test temperatures to relate the observed superplastic properties to the observed microstructural changes.

It was revealed that high aspect ratio alpha phase microstructures result in strain softening and initially low strain rate sensitivity values. The strain rate sensitivity values increase as the microstructures become more equiaxed with strain. The break-up of the alpha phase is characterized by alpha/alpha boundary separation by beta cusping and increased beta phase continuity. Activation energy and TEM observations indicate that grain boundary diffusion, dislocation slip and grain boundary sliding occur during deformation. The morphology of the alpha and beta phases determine the contributions from each mechanism.