Effect of modified gradient fields on the high-cycle fatigue crack initiation mechanism of Ti6Al4V alloy

In this paper, the high-cycle fatigue (HCF) properties and microcrack initiation mechanism of Ti6Al4V alloy after polishing (P) and after polishing followed by the ultrasonic surface rolling process (P + USRP) treatments are investigated. The results indicate that variations in modified gradient fields would alter the crack initiation location and mode. Specifically, after P treatment, the fatigue crack initiation source occurs approximately 205 mu m from the surface, owing to localized strain formation within the internal alpha p/alpha p grain boundaries under cyclic loading, thereby promoting rapid cleavage at the (0001) twist interface and evolving into microcracks. This result aligns with the crack initiation mechanism of an untreated specimen. Conversely, fatigue crack initiation after P + USRP treatment occurs approximately 425 mu m from the surface, as a result of the deeper modified gradient field, where the crack initiation mechanism results from the competing effects of rapid cleavage at the (0001) twist interface of alpha p/alpha p and the accumulation of numerous microvoids. The difference in strain between the alpha and beta phases causes the alpha/beta interface to become a weak region of dislocation accumulation, leading to the formation of microvoids induced by microvoid aggregation. Both P and P + USRP are beneficial for improving the fatigue performance of the alloy.