Influences of recovery and recrystallization on the superelastic behavior of a beta titanium alloy made by suction casting

The influences of cold rolling (CR), recovery and recrystallization on the microstructure, mechanical properties and superelasticity of the suction cast Ti-7.5Nb-4Mo-2Sn alloy were investigated. It has been found that, with the increase of the CR thickness reduction rate, the volume fraction of the straininduced martensite transiting from the b phase increases. During annealing treatment, the precipitation of a and u phases occurred at 700 and 800 C, respectively, and the volume fraction of the a and u phases increases with increasing CR thickness reduction rate. The high density of dislocations introduced by cold rolling can accelerate the course of a and u transformation. The cold rolled alloy after annealing at 700 C exhibits excellent superelasticity with a high first yielding stress (sSIM), a large recoverable strain (3recoverable) and a high strain recovery rate (h) due to the retention of massive dislocations and a phase precipitates in the recovery process. The grain size shows a strong effect on the martensitic transformation, and the relationship between the critical stress for the martensite transformation and grain size follows the Hall-Petch equation with a slope of approximately 0.27 MN m3/2