Observation of 3-dimensional morphology of the subscale formed in Fe-Ni alloys oxidized at high temperature by an organic solvent system dissolution technique

Fe-Ni base alloys form a subscale in the alloy interior by selective oxidat ion of the elements hating a large affinity of oxygen together with the ext ernal scale formation on the alloy surface by high-temperature oxidation. A lmost of all the previous papers concerning with subscale which consists of both intergranular and intragralular oxides have been on the basis of the two-dimensional observation on the cross-sections of oxidized specimens. Ho wever, it is difficult to estimate exactly the three-dimensional morphology development of the subscale because its morphology is peculiar. In order t o examine directly the three-dimensional morphology we have developed a met al matrix dissolution technique for the oxidized Fe-Ni alloy without dissol ving the metallic oxide phases constituting the subscale by using an organi c solvent system solution. In the present study, we report here on the appr opriate dissolution of the Fe-36%Ni alloy oxidized in air at high temperatu res under the applied tensile stress conditions in the organic solvent syst em solution which consists of bromine, tetraethyl ammonium and acetonitrile . The three-dimensional morphology of the intergranular and intragranular o xides formed in the Fe-36%Ni alloy suggests the difference of each growth m echanism.