Predicting Minimum Al Concentrations for Protective Scale Formation on Ni‐Base Alloys: I . Isothermal Oxidation

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Abstract
Various criteria to predict the minimum bulk solute concentration necessary to form a continuous, external solute oxide scale during isothermal oxidation were applied to Ni‐Al and Ni‐Cr‐Al alloys. The first criterion, proposed by Wagner, predicts this minimum concentration by equating the maximum possible solute flux in the alloy with the rate of solute consumption at the oxide‐metal interface. This criterion underpredicted by a factor of three the experimentally observed minimum Al concentrations for the formation of an external scale on Ni‐Al alloys at 1200°C. A second criterion, also proposed by Wagner, predicts a transition from internal to external oxidation when a critical volume fraction of oxide precipitates is deposited in the subscale region. This criterion predicted a transition from internal oxidation to continuous scale formation in good agreement with experimentally observed concentrations for Ni‐Al alloys at 1200°C. The former criterion was extended to include isothermal oxidation of ternary alloys by including ternary interactions in determining the maximum solute flux in the alloy. This criterion was applied to formation on Ni‐Cr‐Al alloys oxidized at 1200°C and predicted minimum Al concentrations in good agreement with that experimentally observed only for intermediate Cr concentrations between 5 and 20 atomic percent. It was shown that the beneficial effect of Cr additions in aiding the formation of scales on Ni‐Al alloys cannot be accounted for by this criterion.

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