Microstructure Evolution of Ti-6Al-4V Alloy during Superplastic Deformation

The microstructure evolution and morphologies of Ti-6Al-4V alloy fracture after superplastic tensile testing were investigated by the optical microscope and the scanning electron microscope. The results show that, the ultrafine-grained Ti-6Al-4V alloy displays quasi ductile fracture in room temperature testing, while the ductile dimples-voids gathered type fracture in the superplastic tensile testing. In the same initial strain-rate, the grains dimension and beta phase contents increase, and the voids decrease with increasing of the tensile temperature. From 840 degrees C to 930 degrees C, the grains still retain in equiaxed state. At the higher temperature (960 degrees C), the grains are coarsed and exhibit lamellar crystal at some sections. At the same tensile temperature, the grains dimension and beta phase contents increase and the voids decrease with decreasing of the initial strain-rate. At elevated temperature (above 960 degrees C), beta phases have good plasticity and low hardness. More beta phase contents are favorable to the grain boundary sliding and the voids are difficult to generate.