Grain egression: A new mechanism of fatigue-crack initiation in Ti-6Al-4V

A new mechanism of fatigue-crack initiation (FCI), grain egression, was observed in the course of investigating corrosion-fatigue crack initiation in Ti-6A1-4V hip prostheses fabricated using three different processes. Extensive scanning electron microscopy (SEM) was used to document this new mechanism as well as the other FCI mechanisms operating. Grain egression entails the fracture and egression of primary α grains from the surface of the sample, resulting in a sharp pit that subsequently acts as the site of crack initiation. The different sizes and morphologies of the grain-egression sites observed are very similar to the sizes and morphologies of the pri-mary α grains resulting from the three different fabrication processes, providing further evidence for grain egression as an operative FCI mechanism.