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Microstructure phenomena resulting from high temperature oxidation of a simple nickel alloy and two cobalt-base superalloys were studied by microstructure examinations and thermodynamic calculations. The latter allowed to better quantify what happened to carbon during oxidation, which was not possible with microstructure examination and microanalysis alone, and to explain the microstructure changes due to oxidation. At 1000°C, due to the Cr oxidation, the grain boundary carbides dissolve and the carbon diffuses inwards, where new carbides form. However, at 1100 and 1200°C, the carbon diffuses outwards to the surface. Thermodynamic calculations were used to explain the seemingly different mechanisms of carbon diffusion.

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