The effect of oxygen partial pressure on crack growth rates in Alloy 617 has been studied using both static and fatigue loadings at $650°C$ over the oxygen partial pressure range $10−19−10−3 atm$. Tests were conducted at either the constant stress intensity factor $K$ for static conditions or the constant $ΔK$ in fatigue. Oxygen concentration was measured on both the inlet and outlet of the test retort as well as in situ with a probe located directly at the specimen surface. For fatigue loading the crack path was observed to be transgranular but crystallographic with a decreasing growth rate as the oxygen concentration decreased. However, for static loading the crack path shifted to intergranular at the same $Kmax$ (fatigue) and exhibited what appears to be an increasing crack growth rate with decreasing oxygen concentration.

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