Ultra-grain refinement and enhanced low-temperature superplasticity in a friction stir-processed Ti-6Al-4V alloy

An ultrafine microstructure consisting of alpha grains (similar to 0.51 mu m) and a small amount of beta phase was successfully achieved in a friction stir-processed (FSPed) Ti-6Al-4V alloy. The fraction of high angle grain boundaries (HAGBs) with random crystallographic orientations reached 89.3% revealed that dynamic recrystallization was responsible for the ultra-grain refinement mechanism during friction stir processing (FSP). Low-temperature superplasticity (LTSP) of such an ultrafine microstructure was demonstrated in the temperature range of 550-650 degrees C and strain rates of 1 x 10(-4)-3 x 10(-3) s(-1). Specifically, an extremely superior LTSP of 1130% was achieved at 600 degrees C and 3 x 10(-4) s(-1), which was explained by means of the ultrafine equiaxed grains, a large proportion of HAGBs with random orientations as well as the presence of beta phase. The predominant superplastic deformation mechanism was considered as grain boundary sliding associated with grain boundary diffusion.