Elucidating the influence of temperature and strain rate on superplasticity and microstructure evolution of near-α TNW700 titanium alloy

Abstract

The superplastic deformation and microstructure evolution of a new near-α TNW700 titanium alloy with short-term service temperature up to 700 °C were systematically investigated in the temperature range of 900–975 °C and strain rate range of 0.0005–0.01s−1. The results illustrate that TNW700 alloy has excellent superplasticity with elongation above 300% and m value above 0.41 at 900–950 °C, and the maximum elongation of 523% is obtained at 950°C-0.001s−1 due to the appropriate grain size and phase ratio. In addition, TNW700 alloy exhibits high temperature softening and strain rate hardening behaviors. The increase of β phase volume fraction induced by dynamic phase transformation is a crucial cause for flow softening, and this phenomenon is accelerated with increasing temperature and decreasing strain rate. The refinement of primary α grains and the inconspicuous growth of β grains at lower temperatures are favorable for grain boundary sliding and superplasticity enhance, whereas the overgrowth of β grains at 975 °C results in superplasticity absence. The decrease of dislocation density in primary α grains caused by strain partitioning, dynamic recovery (DRV) or dynamic recrystallization (DRX) is another important factor for high temperature softening; the increase of dislocation density and velocity, and the decrease of annihilation time contribute to strain rate hardening. The appearance of random texture indicates the activation of grain boundary sliding (GBS) and grain rotation. GBS is the overarching mechanism of superplastic deformation of TNW700 alloy, accompanied by grain rotation, dislocation movement, dynamic phase transformation and grain refinement or coarsening.