Investigation of flow behavior and microstructure of Ti–6Al–4V with annealing treatment during superplastic forming

The flow stress and microstructure evolution of Ti–6Al–4V alloy with annealing treatment are identified. Isothermal tensile tests of material, obtained with the various volume fraction of β-phase, were performed at 870 °C and the strain rate of 10−3/s. Stress hardening was found in the titanium alloys with annealing treatment. Subsequently, the microstructures were investigated by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Electron Back-scattered Diffraction (EBSD) observation. Annealing treatment has little effect on the dislocation density of titanium alloy before tensile tests. Contrary to the initial material, the volume fractions of β-phase are higher in alloys with β-annealing treatment and lower for α-annealing. Significant grain growth related to heat treatment is detected. Especially, the static recrystallization and grain elongation are observed in β-annealed titanium alloy. In addition, a physic constitutive model based on the microstructure evolution is established, and an effective optimization method is applied to determine the material constants. The calculated results indicate that the effect of grain growth due to annealing treatment can't be ignored, and the static recrystallization is also an important mechanism in grain refinement for β-annealed titanium alloy. Besides, grain boundary sliding is still the dominant deformation mechanism in the Ti–6Al–4V with annealing treatment. •The microstructure evolution of annealed Ti-6Al-4V alloy during superplastic forming is investigated.•The flow behavior of titanium with annealing treatment is put up.•A multi-scales constitutive model is established to predict the flow behavior and microstructure evolution.•An efficient path to determine the material constants is given.•The dominant mechanism for Ti–6Al–4V with annealing treatment is discussed.