THE OXIDATION OF NIAL .3. INTERNAL AND INTERGRANULAR OXIDATION

In ampoule exposures with Cu-Cu2O, Fe3O4-Fe2O3, Ni-NiO and other buffer mixtures establishing oxygen pressures between 10(-20) and 10(-2) bar O2, beta-NiAl shows intergranular and internal oxidation at temperatures between 500-degrees and 1000-degrees-C. The internal oxidation starts locally, most probably initiated by impurities (such as sulphur) and firstly leads to inward and outward growth of cones, composed of Ni and Al2O3. Later, a layered structure of an internally oxidized zone and an Al-depleted zone (Ni3Al) develops. Intergranular oxidation starts by inward diffusion of oxygen at grain boundaries, followed by precipitation of Al2O3. Both phenomena are not observed in flowing atmospheres at similar oxygen pressures, but only in the ampoule exposures, where restricted oxygen supply and transfer of impurities plays a role.