Face-centered titanium induced by ultrasonic surface rolling process in Ti-6Al-4V alloy and its tensile behavior

The phase transformation of face-centered cubic (fcc) titanium (Ti), transformed from hexagonal close-packed (hcp) Ti, in Ti-6Al-4V alloy subjected to ultrasonic surface rolling process (USRP) and the tensile behavior were investigated. A gradient nanostructure of fcc-Ti was formed from the depth of 440 μm to the top surface. The stress concentration at the phase interface of α phase and β phase promoted hcp to fcc phase transformation in low strain and strain rate region (adjacent to the substrate) while the extremely high strain and strain rate induced this phase transformation in the surface to sub-surface regions. USRP increased the tensile strength of Ti-6Al-4V alloy slightly but decreased the yield strength dramatically, and kept a good combination strength and ductility of Ti-6Al-4V alloy mainly due to the formation of the gradient nanostructure surface layer and hcp to fcc phase transformation.