Enhanced diffusion of oxygen during internal oxidation of nickel-base alloys

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Abstract
The depths of internal oxidation of Ni-Al (0.5–4 wt.%) alloys oxidized in the temperature range 800–1100°C in either Ni/NiO packs or in 1 atm oxygen are virtually independent of the alloy composition; with Ni-Cr (1–5 wt.%) alloys the depths decrease with increasing Cr content, in accord with classical theory. A model incorporating enhanced diffusion of oxygen along the incoherent interface between the internal oxide particles and the alloy matrix is proposed, since in the Ni-Al system the internal oxide particles show considerable elongation in the growth direction. In contrast, in the Ni-Cr system, the oxide particles are approximately spherical, and are not interconnected. Agreement between experiment and theory is satisfactory if the ratio of diffusion coefficients of oxygen along the interface and in the bulk lattice is of the order of 102−103. There are also differences between alloys oxidized in Ni/NiO packs, when no surface scale forms and samples oxidized in 1 atm oxygen when a surface NiO scale forms. The greater depth of penetration under these latter conditions has been associated with the injection of vacancies into the alloy by the growing surface scale which helps relieve some of the compressive stresses associated with the volume expansion upon internal oxidation, although the exact mechanism is not entirely clear.