Sulfur Segregation at Al2O3/γ-Iei plus γ'-Ni3Al Interfaces: Effects of Pt, Cr and Hf Additions

The interfacial chemistry that developed as a result Al2O3-scale growth on gamma-Iei + gamma'-Ni3Al alloys at 1150 A degrees C was studied using scanning Auger microscopy after the oxide layer was scratched to spall under ultra-high vacuum. The extent of scale spallation was used to evaluate semi-quantitatively the interfacial strength. The alloys investigated were primarily gamma' in structure, containing 22 at.% Al plus further additions of Pt, Cr and/or Hf. In the case of the binary gamma + gamma' alloy, it was found that a sub-monolayer of sulfur segregated at the alloy/scale interface. Platinum reduced and hafnium eliminated sulfur segregation, but chromium enhanced it through Cr-S co-segregation, even on Pt- and Hf-containing alloys. Platinum also segregated slightly at the alloy/scale interface. The interface strength was a strong function of the sulfur content. Beyond the effect of eliminating S segregation, Pt and Hf both showed additional beneficial effects on alumina scale adhesion.