Surface nanocrystallization of body-centered cubic beta phase in Ti-6Al-4V alloy subjected to ultrasonic surface rolling process

The nanocrystallization mechanism of a body-centered cubic beta phase in Ti-6Al-4V alloy subjected to ultrasonic surface rolling process was investigated. A gradient nanostructure (thickness: similar to 400 mu m) that the beta grain size in thickness gradually changes from similar to 0.76 mu m in the interior to similar to 36.5 nm at the topmost surface was formed in Ti-6Al-4V alloy surface layer. The gradient nanostructure of the beta phase is formed primarily via dislocation activities without the occurrence of deformation twinning. Dislocations were first generated in beta phase at the phase boundaries where a high density of dislocations occurred in a phase. The coarse beta grains were then gradually transformed into equiaxed nano grains via longitudinal splitting and transverse breakdown, which are induced by dislocation glide, entanglement, accumulation, and rearrangement. Additionally, with increasing strain, the beta nanograins will be further refined via dislocation activities.