High-temperature scale formation of Fe-36% Ni bicrystals in air

Fe-36% Ni bicrystals were oxidized at temperatures from 1100 to 1250 K in air, both under and without tensile stress. The scales were divided into three categories: external scales, intragranular subscales, and intergranular subscales. A linear relationship exists beween the thickness of scales and subscales and the square root of oxidation time. Tensile stress significantly accelerates intergranular oxidation, but has essentially no influence on either the formation of intragranular subscales or that of external scales. The external scales are composed of α-Fe₂O₃ and Fe₃O₄ after a longer oxidation time. The intergranular and intragranular subscales consist of Fe₃O₄ and FeO particles and an Ni-enriched matrix. Cellular or dendritic oxide nodules sometimes exist in the intragranular subscales, causing their frontier interfaces to significantly undulate.