Crack nucleation/initiation at high temperatures has been studied in CMSX4 in both air and vacuum environments, to elucidate the effect of oxidation on the notch fatigue initiation process. In air, crack nucleation/initiation occurred at subsurface interdendritic pores in all cases. The subsurface crack grows initially under vacuum conditions, before breaking out to the top surface. Lifetime is then dependent on initiating pore size and distance from the notch root surface. In vacuum conditions, crack nucleation/initiation has been observed more consistently from surface or close-to-surface pores, indicating that surface oxidation is in-filling/‘healing’ surface pores or providing significant local stress transfer to shift initiation to subsurface pores
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