Oxidation behaviour of NiAl—II. Cavity formation beneath the oxide scale on NiAl of different stoichiometries

The formation of voids at the metal/oxide interface was observed after the oxidation of NiAl in the temperature range 1173–1473 K. The number of voids and extent of the undermining of the scale increased with the Ni content of the NiAl. The growth of the voids to large cavities led to loss of scale adherence which was fastest for the Ni-rich materials. Calculations of Al diffusion profiles were performed to determine the steady state Al surface concentration at the metal surface of a cavity and the maximum evaporation rate of Al. At high temperatures the Al transport by evaporation is fast enough to supply the amount necessary for scale growth. At lower temperatures, e.g. 1173 K, the supply is insufficient during the initial oxidation and surface diffusion in the void may play a role.