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The effect of yttrium and thorium on the oxidation behavior of Ni-Cr-Al alloys

1974; Springer Science+Business Media; Volume: 8; Issue: 4 Linguagem: Inglês

10.1007/bf00604042

1573-4889

Arun Kumar, M. M. Nasrallah, D. L. Douglass,

Catalytic Processes in Materials Science

The effect of quaternary additions of 0.5% Y and 0.5 and 1.0% Th to a base alloy of Ni-10Cr-5Al on the oxidation behavior and mechanism was studied during oxidation in air over the range of 1000–1200°C. The presence of yttrium decreased the oxidation kinetics slightly, whereas the addition of thorium caused a slight increase. Oxide scale adherence was markedly improved by the addition of the quaternary elements. Although a number of oxides formed on yttrium-containing alloys, quantitative x-ray diffraction clearly showed that the rate-controlling step was the diffusion of oxygen through short-circuit paths in a thin layer of alumina that formed parabolically with time. Mixed oxides containing both aluminum and yttrium formed by the reaction of Y2O3 to form YAlOP3 initially, and Y3Al5O12 (YAG) after longer times. Although the scale adherence of the yttrium-containing alloy was considerably better than the base alloys, spalling did occur that was attributed to the formation of the voluminous YAG particles that grew in a "mushroom"-like manner, lifting the protective scale off the substrate locally. The YAG particles formed primarily at grain boundaries in the substrate in which the yttrium originally existed as YNi9. This intermetallic compound reacted to form Y2O3, liberating metallic nickel that subsequently reacted to form NiO or NiAl2O4 spinel or both. The Y2O3 reacted with aluminum to ultimately form the YAG "mushrooms." Thorium did not form any mixed oxides; the only oxide involving thorium was ThO2, which existed as small particles at the oxide-metal interface. A highly beneficial effect of the thoria particles in reducing film spalling was observed. Scale spalling in the base alloy was attributed to void formation at the oxide-metal interface, the voids forming by condensation of excess vacancies from the Kirkendall effect associated with fast back-diffusion, of nickel into the substrate as aluminum was preferentially oxidized and diffused slowly outward. The mechanism of improved scale adherence in the quaternary alloys was the elimination of voids by annihilation of the Kirkendall vacancies at vacancy sinks introduced by the noncoherent interfaces between yttrium and thorium-containing intermetallics or oxides or both.