Oxidation-induced crack initiation and propagation behaviors of Ni-based single crystal superalloy in VHCF regime

The interaction between oxidation and fatigue under lower stress in Ni-based single crystal superalloys remains insufficiently explored. In this study, a newly designed notched plate specimen was developed for VHCF testing at 1000 °C to investigate these phenomena. Quasi-in-situ observations were employed to monitor crack initiation and crack propagation processes. The results shown that all fatigue cracks initiated at the notch root and propagated in a Mode I manner. The fracture surface consists of two distinct regions, an oxidation-dominate zone (ODZ) characterized by extensive oxide coverage, and the fatigue-dominated zone (FDZ) marked by clear fatigue striations. Over 90 % of the total fatigue life was spent in the ODZ. As cracks propagated, thermally grown stress in the oxide near the crack tip altered the local stress field, promo6ting γ′ rafting and aluminum diffusion. These effects transformed the crack-tip oxide from a multilayered structure to a continuous Al₂O₃-rich scale. Within the ODZ, oxidation-induced crack closure suppressed crack growth, rendering the oxidation rate the dominant factor controlling the crack propagation rate. When the effective stress intensity factor exceeded a critical threshold, oxidation penetrated to the γ/γ′ interface within the substrate, triggering a transition from ODZ to FDZ and accelerating crack growth. Overall, these findings confirm that enhancing oxidation resistance is still critical for improving VHCF performance in Ni-based single crystal superalloys.