Superplastic Deformation Behavior and Cavity Formation of Ti55 Titanium Alloy

Ti55 is a type of new near-α titanium alloy featuring good heat resistance and thermal stability at a temperature below 550°C. This paper explores the superplastic behaviors of Ti55 titanium alloy sheets at high temperature. The results showed that at a strain rate between 8.3×10-4 and 1.32×10-2s-1 and a deformation temperature between 885 and 935°C, this material exhibited favorable superplasticity. A 872% tensile elongation was achieved even at a high temperature of 925°C and a high strain rate of 1.32×10-2s-1. For a deformation temperature of 925°C and a strain rate range of 8.3×10-4～1.32×10-2s-1, the strain rate sensitivity index (m) was equal to or higher than 0.38, which depends on the specific strain rate and reached its peak at 6.64×10-3s-1. The study showed that tiny cavities tended to appear around the rare earth phase particles. As the deformation temperature and the deformation strain rate increased, both size and volume fraction of the cavities decreased significantly. When the plastic deformation further increased, the cavities experienced not only longitudinal aggregation along the tensile strain direction, but also transverse aggregation normal to the tensile strain direction. The typical high-temperature ductile fracture morphology is related to both aggregation and growth of the large number of microscopic cavities and dimples.