The role of oxidation-induced cavities on the failure of the thermally grown oxide on binary β-NiAl alloys

The thermally grown aluminum oxide formed on binary beta-NiAl alloys has a propensity to spall from oxidation-induced cavities. Microstructural observations of the spalling failures reveal that the oxide fails as a result of a sequential process. Initially, cavities nucleate and grow in the alloy at its interface with the growing oxide. On cooling, the oxide buckles above the cavities and separates from the remaining alloy by propagation along the intact oxide/alloy interface, allowing adjacent buckles to link together until the mechanical condition for macroscopic spalling occurs and the oxide spalls away from the alloy. The buckling and spalling is motivated by the compressive stress in the oxide on cooling as a result of the thermal expansion mismatch with the alloy. Since failure requires fracture along the oxide/alloy interface, it is susceptible to segregation of impurities and, possibly, sub-critical crack growth.