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High temperature fatigue crack growth rate (FCGR) tests were performed at elevated temperature (-650°C) under different oxygen partial pressures, PO2, ranging from lo-* mbar to 0.2 bar on one heat of Alloy 718. A sharp transition in the FCGR was observed to take place for a pressure close to IO-3mbar. The FCGR decreases by almost three orders of magnitude when PO2 decreases from about 10-l mbar down to 10-3 mbar and when the material is tested at 650°C under AK-20MPa 6. The effect of loading frequency as well as the influence of Kmax on the transition pressure were also investigated. No influence of the mechanical loading conditions were found on the transition pressure provided that the mechanical conditions are in the range corresponding to an oxygen-assisted crack growth mechanism. It is shown that in this material the transition pressure is related to a particular oxidation behavior. Above the transition pressure the first oxide to be formed is a (Ni, Fe)0 oxide, while below the transition, Cr203 type oxide is formed preferentially. These conclusions are drawn through TEM observations and chemical analysis of the oxides formed on the surface of the specimens tested under specific conditions. These results are used to discuss the strong environmental effect observed in Alloy 7 18.

Oxidation Mechanisms in Relation to High Temperature Fatigue Crack Growth Properties of Alloy 718