Nanocrystallization mechanism of beta phase in Ti-6Al-4V subjected to severe plastic deformation

Nanocrystallization mechanism of beta phase in the bulk coarse-grained Ti-6Al-4V by high energy shot peening was investigated via high-resolution transmission electron microscopy. The results suggested that dislocation gliding was first initiated in beta phase at and around the intersections of the phase boundary with the high-density dislocation structures in alpha phase followed by the formation of dislocation tangles and dislocation walls. As the short axis sizes of alpha phase approached that of beta phase, the dislocation tangles and dislocation walls gradually evolved into the high-angle grain boundaries and subdivided the original grains into the equiaxed ultrafine grains. Under the ultrahigh strain and strain rate, the equiaxed ultrafine grains were eventually refined to randomly oriented nanograins via dynamic recrystallization. In addition, the nanograins would be further refined via dislocation motion upon further straining.