Accelerated oxidation of aluminides at intermediate temperatures

Abstract
The aluminides NbAl₃ and NiAl may show accelerated oxidation in a range of intermediate temperatures 600-1000 °C, especially at reduced oxygen pressures. Such pressures in the range 10^-2 to 10^-30 bar O₂ have been established for exposures in ampoules with the specimens, by enclosing also buffer mixtures such as Cu-Cu₂O, Fe₃O₄-Fe₂O₃, Ni-NiO etc. NbAl₃ disintegrates into small particles (pesting) and NiAl shows internal oxidation, general or localized, and oxidation along grain boundaries. Auger analyses afier oxidation confirmed that oxygen penetrates into grain boundaries of NbAl₃ and NiAl. The transport of oxygen through the outer oxide layer appears to be rate controlling, in the case of NbAl₃ such transport is possible through Nb₂O₅ in the scale and in the case of NiAl through other defects caused by insufficient supply of oxygen in the exposures.