OXIDATION OF GAMMA'-NI3AL AND GAMMA'-NI3AL(SI) INTERMETALLIC COMPOUNDS AT LOW-OXYGEN PRESSURES

The oxidation behavior of gamma'-Ni3Al and gamma'-Ni3Al(Si) (Ni75Al20S i5) inter metallic compounds was studied at 1073 K and oxygen partial pressures of 5 similar to 9 x 10(-6) atm, 1.2 x 10(-14) atm, and 1.2 x 10(-19) atm by means of a manometric apparatus, and Rhines packs of N iO/Ni and FeO/Fe, respectively. Oxidation kinetics were determined eit her by recording weight gains or by measuring the internal-oxidation-z one depths. The structures and morphologies of oxides were also studie d. Relatively low oxidation rates occurred for both compounds when oxi dized in the manometric apparatus, while fast internal oxidation was o bserved for both compounds in the NiO/Ni pack, The fastest oxidation o ccurred in the gamma'-Ni3Al compound in the FeO/Fe pack. A healing Al2 O3 layer was formed on the surface of gamma'-Ni3Al(Si) compound in the FeO/Fe pack, indicating a synergistic effect between the solute eleme nts in the compound at the oxygen pressures corresponding to the disso ciation of FeO. The oxidation rate was found to depend on the volume e xpansion associated with solute-atom oxidation.