Understanding the influence of forming condition on superplasticity elongation for SP700 Ti-alloy

Abstract

The high-temperature tensile tests were conducted for SP700 Titanium alloy sheet under the temperatures of 755°C, 775°C, 795°C, 815°C, and the strain rates of 1×10−2 s−1, 1×10−3 s−1, 1×10−4 s−1, respectively. The microstructure evolution, Superplasticity deformation characteristics and elongation differences under different forming conditions were investigated. Results indicate that the steady-state flow duration is the longest under the deformation conditions of 775°C and a strain rate of 1×10−3 s−1, achieving a maximum elongation of 746 %. At 795°C with the same strain rate, fine porosities can be observed at the α/β phase boundaries in the uniform deformation and fracture zones, with larger porosities at P3 corresponding to more pronounced necking phenomena. At 775°C and a strain rate of 1×10−3 s−1, the average grain size of the α phase is maintained at 3–5μm, accounting for 78.6 % of the microstructure. It is suggested that maintaining an average grain size of approximately 4μm after tensile test can result in an even better superplasticity and elongation rate. Furthermore, at a strain rate of 1×10−3 s−1, the average grain size of the α phase initially increases and then decreases with the temperature increasing. At 795°C and a strain rate of 1×10−3 s−1, the α/β phase boundaries are curved. However, the α/β phase boundaries remain straight after tensile test, under 815°C and a strain rate of 1×10−3 s−1. Moreover, the observed dislocation is mainly in the α phase and only a few in the β phase. It shows that the superplastic deformation of SP700 alloy mainly occurs within the α phase, and the dislocation activity of the β phase may be limited.