DIFFUSIONAL TRANSPORT AND PREDICTING OXIDATIVE FAILURE DURING CYCLIC OXIDATION OF BETA-NIAL ALLOYS

Nickel aluminides (NiAl) containing 40-50 at.% Al and up to 0.1 at.% Zr have been studied following cyclic oxidation at 1200, 1300, 1350 and 1400-degrees-C. The selective oxidation of aluminum resulted in the formation of protective Al2O3 scales on each alloy composition at each temperature. However, repeated cycling eventually resulted in the gradual formation of less protective NiAl2O4. The appearance of the NiAl2O4, signaling the end of the protective scale-forming capability of the alloy, was related to the presence of gamma'-(Ni3Al) which formed as a result of the loss of aluminum from the sample. A simple methodology is presented to predict the protective life of beta-NiAl alloys. This method predicts the oxidative lifetime due to aluminum depletion when the aluminum concentration decreases to a critical concentration. The time interval preceding NiAl2O4 formation (i.e. the lifetime based on protective Al2O3 formation) and predicted lifetimes are compared and discussed. Use of the method to predict the maximum use temperature for NiAl-Zr alloys is also discussed.