Transition from internal to surface crack initiation is controlled by oxidation assisted fatigue-crack process in the very-high-cycle fatigue regime. Between 760 and 1000 °C, single crack initiation site is associated with internal casting defect, followed by a crystallographic Stage I propagation. By contrast, multiple surface crack initiation sites appear at 1100 °C, as the consequence of internal oxide penetration. The fatal crack follows a Mode I propagation and oxygen can diffuse into the material along the crack path. γ′-phase depletion appears surrounding the oxidised and cracked regions, while localised rafting can occur close to the crack tip.

在超高周疲劳状态下，从内部裂纹萌生到表面裂纹萌生的转变由氧化辅助疲劳裂纹过程控制。在 760 到 1000 °C 之间，单个裂纹萌生点与内部铸造缺陷有关，然后是晶体学阶段 I 扩展。相比之下，由于内部氧化物渗透，在 1100 °C 时会出现多个表面裂纹起始点。致命裂纹遵循模式 I 传播，氧气可以沿着裂纹路径扩散到材料中。γ'相耗尽出现在氧化和裂纹区域周围，而局部漂流可能发生在裂纹尖端附近。