Oxidation mechanisms in relation to high temperature crack propagation properties of alloy 718 in H2/H2O inert gas environment

High temperature crack propagation testing was performed under different oxygen partial pressures, ranging from 10(-37) MPa to 10(-3) MPa, on specimens taken from an Ahoy 718 forging. Oxygen partial pressure was varied by manipulating the hydrogen and water vapor contents of the test atmosphere while maintaining a constant total pressure of 15.7 psia. In contrast to previous results obtained in partial vacuum by other authors, crack growth rate was not a straightforward function of oxygen partial pressure. However, in agreement with the previous work, slow crack growth rates were associated with the formation of a Cr-rich crack tip oxide scale while fast crack growth rates were associated with the formation of a Ni-and Fe-rich crack tip oxide scale. These conclusions are drawn through analytical electron microscopy of thin foils containing the crack tip. These results are used to discuss thermodynamic and kinetic aspects of environmental embrittlement of Alloy 718.