Oxidation ofγ′-Ni3Al andγ′-Ni3Al(Si) intermetallic compounds at low-oxygen pressures

The oxidation behavior of γ′-Ni3Al and γ′-Ni3Al(Si) (Ni75Al20Si5) intermetallic compounds was studied at 1073 K and oxygen partial pressures of 5∼9×10−6 atm, 1.2×10−14 atm, and 1.2×10−19 atm by means of a manometric apparatus, and Rhines packs of NiO/Ni and FeO/Fe, respectively. Oxidation kinetics were determined either by recording weight gains or by measuring the internal-oxidation-zone depths. The structures and morphologies of oxides were also studied. Relatively low oxidation rates occurred for both compounds when oxidized in the manometric apparatus, while fast internal oxidation was observed for both compounds in the NiO/Ni pack. The fastest oxidation occurred in the γ′-Ni3Al compound in the FeO/Fe pack. A healing Al2O3 layer was formed on the surface of γ′-Ni3Al(Si) compound in the FeO/Fe pack, indicating a synergistic effect between the solute elements in the compound at the oxygen pressures corresponding to the dissociation of FeO. The oxidation rate was found to depend on the volume expansion associated with solute-atom oxidation.