The study of flow behavior and governing mechanisms of a titanium alloy during superplastic forming

Abstract TA15 (Ti–6Al–2Zr–1Mo–1V) is a near-α titanium alloy and has wide applications in the aerospace industry because of its high strength to mass ratio, good weldability, and superior creep resistance at high temperatures up to 550 °C, compared to other titanium alloys. This study investigates the flow behavior and microstructural evolution as functions of temperatures and strain rates during deformations under the superplastic conditions at 880 °C/0.01s−1, 900 °C/0.01s−1, 880 °C/0.001s−1, and 920 °C/0.0005s−1. Results showed that this alloy exhibit excellent superplastic behavior for all selected temperatures and strain rates. The maximum tensile elongation of 1450% is achieved at 880 °C with a strain rate of 0.001s−1. Flow softening is observed under deformation conditions of 880 °C/0.01s−1 and 900 °C/0.01s−1, while strain hardening is observed at deformation conditions of 880 °C/0.001s−1 and 920 °C/0.0005s−1. These complex flow behaviors are rationalized by characterizing the underlying microstructures on the interrupted tensile samples using electron backscatter diffraction (EBSD) and backscattered electrons (BSE). The geometrically necessary dislocations (GNDs) density, which is caused by lattice rotation and misorientations and plays a vital role in the plastic constitutive behaviors, was for the first time, systematically revealed. Together with other key microstructures, i.e. grain sizes, texture, phase fractions, the results show that the dominant deformation mode changes at initial, intermediate, and final stages of the deformation. The probable deformation mechanisms, such as grain boundary sliding (GBS) under different deformation conditions, are discussed in terms of grain morphology, GNDs, and texture evolution. Also, it is observed that the β-phase transformation is accelerated during deformation and contributes to the enhancement of superplasticity.