The Influence of Temperature and Strain Rate on the Superplastic Deformation Behavior and Microstructure Evolution of TNW700 Alloy

TNW700 titanium, as a new near-α high-temperature titanium alloy, is designed to work at 700 °C for a short-term service. Superplastic deformation behavior of TNW700 alloy at 900–975 °C and strain rate of 0.0005–0.01 s⁻¹ was investigated to identify the optimum deformation temperature and strain rate. The microstructure evolution after high temperature tensile was investigated using scanning electron microscope. The research found that TNW700 alloy has an excellent superplasticity. The elongation exceeds 200% at various deformation conditions except 975 °C with higher strain rate of 0.005 and 0.01 s⁻¹, and the maximum elongation of 613% was obtained at a temperature of 925 °C and strain rate of 0.001 s⁻¹. The flow stress is sensitive to temperature and strain rate, and it increases with decreasing temperature and increasing strain rate. In addition, the flow stress exhibits strong work hardening with increase in true strain, and the instantaneous work hardening exponent n as well as critical hardening strain is accelerated as the strain rate decreases. The strain rate sensitivity exponent (m) is higher than 0.4 when the temperature is lower than 975 °C, which corresponds to dynamic recrystallization and grain boundary sliding mechanisms. The m is 0.229 at temperature of 975 °C, corresponding to dynamic grains growth mechanism. The deformed microstructure of TNW700 alloy consists of β grains and equiaxed α grains. Increasing the temperature is beneficial to the transformation of α phase into the β phase, which resulted in an increase in the volume fraction of the β phase. The β grains grow rapidly at higher temperatures and lower strain rates due to the higher diffusion coefficient.