Effect of thermomechanical processing on superplasticity in Ti–Al–Mo–Zr alloy

Abstract

Superplasticity in terms of total tensile elongation was studied in a titanium alloy of nominal composition Ti–6·5Al–3·3Mo–1·6Zr (wt-%) for three strain rates (1·04 × 10−3, 2·1 × 10−3, and 4·2 × 10−3s−1) and in the temperature range 1123–1223 K for microstructures obtained by different processing schedules. Fine equiaxed microstructure with a low aspect ratio of 1·15 was accomplished in this alloy by combining two types of deformation. While the first step consists of heavy deformations for refining and intermixing the phases, a second step, consisting of light homogeneous reductions in several stages, was necessary to remove the banding that developed during the first step. The resulting microstructure underwent enormous tensile elongation (1700–1725%), even under relatively high strain rates (1·04 × 10−3 and 2·1 × 10−3s−1), making this alloy most suitable for commercial superplastic forming. The present investigation also revealed that the usual sheet rolling practice of heavy reductions to refine the microstructure leads to localised banding which could not be removed by annealing; therefore, the tensile elongation was limited to 770% only. The reason for this may be attributed to the resistance in grain boundary sliding and rotation encountered in microstructures with shear bands and grains with high aspect ratio. Strain enhanced grain growth was also greater in these microstructures.MST/555