Microstructural aspects of superplasticity in Ti-6Al-4V alloy

It is generally accepted that fine-grained and equiaxed microstructure enables superplastic forming of two-phase titanium alloys. Appropriate microstructure is usually developed in the thermomechanical process with careful selection of the parameters of plastic deformation and heat treatment. The results of previous studies proved that β solution treatment preceding plastic deformation in thermomechanical process distinctly increased superplasticity of Ti-6Al-4V alloy despite the fact that obtained microstructure considerably differed from equiaxed one. In current studies fine-grained microstructure of the Ti-6Al-4V alloy was obtained by several variants of thermomechanical process. It was found that the alloy after thermomechanical process comprising β solution treatment followed by forging in α+β temperature range exhibited high superplastic deformability. Metallographic examination of the alloy was carried out at various stages of superplastic tensile test in order to analyse microstructure evolution during superplastic deformation. Tensile tests were interrupted at the strain values corresponding to characteristic points of stress-strain curve, i.e. maximum and steady-state plastic flow stress. Digital image analysis was employed to quantify the changes of strereological parameters of the microstructure of Ti-6Al-4V alloy. The effect of spheroidization and growth of α grains on superplastic behaviour of the alloy was identified.