Effect of Oxygen Potential on Crack Growth in Alloys for Advanced Energy Systems

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 $10−19−10−3atm$⁠. 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.