Post-forming tensile properties of superplastically bulge formed high strength α/β Ti–Al–Mo–Sn–Si alloy (IMI 550)

Abstract

The effect of biaxial superplastic deformation at 900°C on ambient temperature tensile properties and texture of high strength α/β titanium alloy sheet of nominal composition Ti–4Al–4Mo–2Sn–0·5Si (IMI 550) has been examined. Superplastic straining led to significant decreases in both proof stress and tensile strength values. Heat cycling studies on as received sheet material showed that the decreases in strength were in part due to the influence of temperature, but this had little effect on elongation. The further losses in strength after superplastic forming were attributable to grain growth. The decreases in elongation after superplastic deformation were believed to be due mainly to changes of tensile specimen geometry, while the more isotropic tensile behaviour which was observed was due to the gradual removal of the relatively low level of texture in the as received material. Application of the standard heat treatment to the as received IMI 550 sheet material led to increases in proof stress and tensile strength values of ~70 and ~170 MN m−2, respectively, and to a slight decrease in elongation. Heat treatment of heat cycled and superplastically bulge formed specimens increased the proof stress and tensile strength values almost to the levels attained in the as received material.MST/684