Investigation on superplastic deformation behavior and microstructure evolution of TNW700 titanium alloy

TNW700 titanium is a near-a high temperature titanium alloy, which can be used at 700°C for a short time. The superplasticity behavior and microstructure evolution of TNW700 alloy were investigated by means of isothermal tensile test within the temperature and strain rate ranges from 900° to 975°C and 0.01s^-1 to 0.0005s^-1, respectively. The flow stress-strain curves show that deformation temperature, strain rate and strain all play an important role in the superplasticity test of the TNW700 alloy. Flow stress decreases with increasing temperature and decreasing strain rate, and increases with increasing strain. Meanwhile, the TNW700 alloy exhibits a longer hardening stage, especially at the lower strain rates, which may be related to the presence of (ZrSi) precipitated phases. The microstructure evolution observed by scanning electron microscope (SEM) indicate that (ZrSi) precipitated phases are mainly distributed at a and ß phase boundaries. The volume fraction and the grain size of ß phase gradually increase with increasing temperature and decreasing strain rate, and the influence of temperature on it is greater than that of the strain rate, especially at high temperatures. Grain coarseness caused by dynamic and static growth is the main cause of stress hardening. The strain rate sensitivity exponent (m) was calculated to identify the dominant deformation mechanisms under various deformation parameters.