On the superplastic deformation mechanisms of near-α TNW700 titanium alloy

Abstract

The new near-α TNW700 titanium alloy is a potential candidate material for high performance ultrasonic/hypersonic aircrafts, which is designed for short-term service at 700°C. This study systematically investigated the superplastic deformation microstructure evolution and mechanism of TNW700 alloy at different strain rates and true strains at 925°C. Results show that TNW700 alloy exhibits excellent superplastic behavior in a constant strain rate range of 0.0005–0.005 s−1 with elongation above 400%. The peak stress decreases with decreasing strain rate, which is related to the increase of β-phase volume fraction caused by the increase of thermal exposure time. In addition, significant strain hardening is observed in early-middle stage of superplastic deformation, and flow softening is followed in middle-late stage. To rationalize these complex flow behaviors, electron backscatter diffraction (EBSD) and high resolution transmission electron microscopy (HRTEM) were used to characterize the microstructure. Strain hardening is correlated to the synergistic effect of β grain growth, dislocation accumulation, silicide precipitate, and solid solution strengthening of α phase. Continuous dynamic recrystallization (CDRX) induced the fragmentation of primary α grains in middle-late stage of superplastic deformation, and the refinement of α grains, the increase of β phase volume fraction and dynamic dislocation recovery are main causes of high strain softening. In addition, EBSD and TEM observations confirmed texture randomization, fine equiaxed primary α grains and intragranular dislocation movement, indicating that grain boundary sliding (GBS) accommodated by dislocation sliding / climb is the dominant superplastic deformation mechanism of TNW700 alloy.