ACCELERATED OXIDATION OF ALUMINIDES AT INTERMEDIATE TEMPERATURES

The aluminides NbAl3 and NiAl may show accelerated oxidation in a range of intermediate temperatures 600-1000-degrees-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 after 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.