Effect of sulfur on the cyclic oxidation behavior of a single crystalline, nickel-base superalloy

A commercial, single crystalline, nickel-base superalloy, René N6, was cyclically oxidized in 1200 °C air to evaluate the impact of indigenous sulfur impurities on the oxidation behavior. The sulfur concentrations of the samples were adjusted to levels in the sub-ppm range by heat treatment in hydrogen. Correlation between the residual sulfur levels and the degree of scale spallation are shown. Reduction of sulfur content to <0.1 ppm is shown to prevent any spalling of scale from the metal surface. Higher sulfur levels may be tolerated depending on oxidation time and temperature. microscopy is used to demonstrate that extensive segregation (up to 27 at.%) of sulfur occurs at the metal/scale interface. In vacuum, debonding of the scale is shown to be difficult even when high concentrations of sulfur are present. The possible importance of humidity in the spallation process is discussed.