Superplasticity of an Ultrafine-Grained Ti–4% Al–1% V–3% Mo Alloy

Abstract

This paper investigates the microstructure and superplastic indicators of industrial titanium sheets (Ti–4% Al–1% V–3% Mo alloy) in a temperature range of 750–900°C and strain-rate range of 1 × 10^–5 to 1 × 10^–2 s^–1. The results show that the deformation temperature of 875°C and strain rates 6 × 10^–4 to 2 × 10^–4 s^–1 provide a relative elongation above 700% and a strain-rate sensitivity index m > 0.4. Under these deformation conditions, at the initial stage there is a decrease in the grain size of the α and β phases by 1.5–2 times, and a strong softening in the stress–strain curves can be observed. The dynamic recrystallization explains these effects. As the degree of deformation increases to above 50%, a steady-flow stage and a weak growth of the α-phase grains are observed; the grain size of the β phase remains unchanged. The calculated values of the effective activation energy Q were 194–237 kJ/mol, which suggests that the superplastic flow under the investigated temperature-rate deformation conditions is controlled by the grain-boundary diffusion.