Oxidation-induced cavity formation in binary β-NiAl alloys

Previously, it has been established that oxidation-induced cavitation of binary NiAl alloys occurs primarily when Ni-rich compositions are oxidized. In this work it is shown that at 1100degreesC cavities form principally during the very early stages of oxidation for these compositions. During this time, the growing oxide is a transient, theta-alumina whereas on the Al-rich compositions the growing oxide rapidly transforms to alpha alumina. It is concluded that cavity formation is associated with tensile stresses generated by the flux divergence created by the outward diffusion of Al during the growth of theta-alumina and the inward diffusion of nickel. Once the transient oxide transforms to alpha alumina, the outward flux of aluminum is decreased and with it the flux divergence. Values for the inter-diffusion coefficients for the alloys investigated, derived from the diffusion profiles, are also reported.