Gradient nanostructure evolution and phase transformation of α phase in Ti-6Al-4V alloy induced by ultrasonic surface rolling process

The gradient nanostructure evolution and the mechanism governing this evolution of α phase in Ti-6Al-4V alloy induced by ultrasonic surface rolling process were investigated. A gradient nanostructure consisting of a roughly equiaxed nanograin layer, an elongated nano-lamellar layer, an elongated ultrafine lamellar layer, a refined grain layer, and a low-strain coarse-grained layer was formed with a thickness of more than 400 µm. The formation of gradient nanostructure of α phase was dominated by complex dislocation activities in hcp grains without twins occurring, supplemented by hexagonal close-packed (hcp) titanium (Ti) to face-centered cubic (fcc) Ti phase transformation. During the microstructural evolution, the coarse hcp-Ti grains were first elongated into lamellae. Then, these sub-micron lamellae were gradually transformed into roughly equiaxed nanograins via two deformation modes of longitudinal splitting and transverse breakdown, accompanied by dynamic recovery. The fcc-Ti grains were deformed mainly via twin-twin intersections and twin-dislocation interactions, accompanied by longitudinal splitting and transverse breakdown, resulted in refinement of the micron-scale fcc-Ti grains to roughly equiaxed nanograins. The interaction of hcp and fcc phases influenced and synergistically promoted the microstructural evolution process. In addition, the microhardness improvement in the surface layer of Ti-6Al-4V alloy was attributed to the increase of dislocation density, grain refinement and the occurrence of deformation twinning in fcc-Ti grains.