Effect of surface nanocrystallization on fatigue properties of Ti?6Al?4V alloys with bimodal and lamellar structure

High-energy shot peening (HESP) was employed to obtain a nanocrystalline surface layer in Ti?6Al?4V alloy with bimodal and lamellar structure. The microstructure and residual stress after HESP treatment were characterized by transmission electron microscope (TEM) and X-ray diffractometer (XRD), respectively. The fracture morphology of fatigue was analyzed by scanning electron microscope (SEM). The results showed that the average grain sizes of Ti?6Al?4V alloy with bimodal and lamellar structure were 18.9 nm and 17.1 nm at the topmost surface layer, respectively. The maximum compressive residual stress of the bimodal structure was -906MPa, which was higher than that of the lamellar structure (-859 MPa). The fatigue limits of the HESP-treated Ti?6Al?4V alloy with the bimodal and lamellar structure were increased by 13.5% and 32.8%, respectively. Finally, a Goodman equation was employed to calculate the local fatigue strength and analyze the residual stressfatigue strength correlation. Highly defected substructure, reduced effective slip length and improved yield strength were important factors for the improvement of fatigue strength. The synergy effect of surface gradient nanostructure and compressive residual stress improved the fatigue limit of the HESP-treated Ti?6Al?4V alloy with bimodal and lamellar structure.