Fatigue-induced oxidation assisting microcrack nucleation in Mg-RE alloy under ultrasonic fatigue

Characterizing the transition from a crack-free to a cracked state remains a challenging topic in fatigue. Mg-rare earth alloys, containing the long-period stacking ordered (LPSO) phase, exhibit superior mechanical properties. Here, microcracks are found to nucleate at the soft α-Mg nano-layers, away from the LPSO lamellae. Notably, severe oxidation is observed along the damage bands. Based on the detailed characterizations, it is suggested that dislocation motions continuously bring the new α-Mg matrix to oxidation transformation, resulting in the thickening MgO layer. However, once the fatigue-induced oxide reaches a certain thickness, it starts acting as a barrier for the further dislocation motions. As a result, dislocation accumulation and cumulative damage occur in the region ahead of the thick oxide, causing microcrack nucleation. This fatigue-induced oxidation, assisting microcrack nucleation, is distinct from the existing fatigue mechanisms.