OXYGEN SOLUBILITY AND A CRITERION FOR THE TRANSITION FROM INTERNAL TOEXTERNAL OXIDATION OF TERNARY ALLOYS

Smith's model is expanded in order to derive expressions to quantitati vely describe the oxygen-solubility behavior in ternary alloys as a fu nction of alloy composition. Multicomponent-diffusion theory is used t o establish a criterion for the onset of internal oxidation beneath th e external scale when oxidizing conditions favor formation of the oxid e of the least-noble metal in a ternary alloy. The oxygen-solubility m odel and the criterion are applied to the oxidation of Ni-Cr-Al alloys in 76 torr of oxygen at 1100 and 1200 degrees C, predicting the minim um Al concentrations required to form a protective Al2O3 scale. It sho ws that sufficient Cr additions would significantly reduce the oxygent solubility and also alter the oxygen distribution in the ternary allo ys, avoiding the oxygen supersaturation necessary for the onset of int ernal oxidation. These two factors make it easier to establish the pro tective Al2O3 scale.