Abstract
The Rinetics of A12O3 growth and spalling were observed for Ni-42 at. pct Al oxidized in air at 1100°C up to 1500 h. While crystallographic voids formed at the oxide-metal interface due to the oxidation process, the oxidation resistance was good in 1 h cycle tests. Spalling to bare metal was the predominant mode of oxide loss. The A12O3 grain size at the metal interface varied with a time-exponent of 0.2. In isothermal tests the oxide thicRness was nearly parabolic with time, having a time-exponent of 0.40, while the volume per void varied directly with residence time underneath intact oxide. The total void volume accounted for ~1/2 the aluminum needed to form the AI2O3 scale. In 1 h cycle tests the oxide thicRness and volume per void reached a plateau at ~150 h due to the spalling process. The correlation between total void volume and oxide volume was eventually obliterated by extensive cycling. A cyclic step-process spall model was used to predict the parabolic rate constant, Rp, and the oxide spall fraction, Rs, from gravimetric curves. Predicted values of Rp agreed well with experimental values, while predicted Rs values were often less than measured values. According to this model the severity of a long time test can be rated according to the factor fMe√ Rs · Rp · Δt mg/cm2 ·cycle, where fMe is the ratio of metal-to-oxygen in the oxide and Δt is the cycle time. Measured values of Rs in isothermal tests varied linearly with exposure time or approximately with (oxide thicRness).2 Cyclic tests showed more scatter and less dependence of Rs on oxide thicRness, presumably due to the complex oxide topography and relaxed stress states.
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Smialek, J.L. Oxide morphology and spalling model for NiAl. Metall Trans A 9, 309–320 (1978). https://doi.org/10.1007/BF02646380
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DOI: https://doi.org/10.1007/BF02646380