Diffusional transport during the cyclic oxidation of γ+β, Ni-Cr-Al(Y, Zr) alloys

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The cyclic oxidation behavior of several cast γ+β, Ni-Cr-Al(Y, Zr) alloys and one LPPS γ+β, Ni-Co-Cr-Al(Y) alloy was examined (γ, fcc; β, NiAl structure). Cyclic oxidation was performed by cycling between 1200°C and approximately 70°C. Oxide morphologies and microstructural changes during cyclic oxidation were noted. Recession of the high-Al β phase was nonparabolic with time. Kirkendall porosity resulting from diffusional transport within the alloy was observed in the near-surface γ-phase layer of one alloy. Concentration profiles for Ni, Cr, and Al were measured in the γ-phase layer after various cyclic oxidation exposures. It was observed that cyclic oxidation results in a decreasing Al concentration at the oxide-metal interface due to a high demand for Al (a high rate of Al consumption) associated with oxide scale cracking and spalling. In addition, diffusion paths plotted on the ternary phase diagram shifted to higher Ni concentrations with increasing cyclic oxidation exposures. The alloy with the highest rate of Al consumption, and highest Al content, underwent breakaway oxidation after 500 1-hr cycles at 1200°C. Breakaway oxidation occurred when the Al concentration at the oxide-metal interface approached zero. The relationship between the Al transport in the alloy and breakaway oxidation is discussed. © 1988 Plenum Publishing Corporation.

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Oxidation of Metals