Long-Term Oxidation of an As-Cast Ni3Al Alloy at 900°C and 1100°C

The oxidation behavior of a cast nickel aluminide alloy, IC221M, was examined after long-term aging in air for up to 16,600 hours at 900 degrees C and 5000 hours at 1100 degrees C. The oxidation products were identified using X-ray diffraction and energy-dispersive X-ray (EDX) spectroscopy with multivariate statistical analysis. At 900 degrees C, NiO dominates the oxidation products initially, but at longer times, NiAl2O4 spinel and Al2O3 predominate and remain stable for times up to 16,600 hours. Cross-sectional observation confirmed that a continuous surface oxide that is mostly a mixture of Al2O3 and NiAl2O4 protects the base metal. In its initial stages, the oxidation process at 1100 degrees C is qualitatively similar to that at 900 degrees C but with faster kinetics. However, as aging proceeds, NiO spalls freely from the surface, and a protective continuous oxide scale does not form. The oxidation mechanism can be qualitatively understood by the selective oxidation mechanism maps developed by Giggins and Pettit.