Various Factors Affecting Al2O3 Scale Transformation

Effect of various factors on the metastable to stable phase transformation behavior of a thermally grown oxide scale of Al2O3 was reviewed and discussed on the basis of the results obtained from our recent studies. In this paper, among the factors affecting this phase transformation, particular attention has focused on the effect of Fe in order to understand the different phase transformation behavior of Al2O3 scale formed on different alloy systems in different oxidation environments. In order to explore the effect of Fe on the phase transformation, the oxidation behavior of various Al2O3 forming Fe-based model alloys was systematically investigated. The development of different oxide scales on different alloys during a high-temperature oxidation, including the initial transient stage of oxidation followed by isothermal oxidation in different oxidation atmospheres, was examined by in-situ X-ray diffraction by means of the Synchrotron radiation. Fe was found to accelerate the phase transformation to alpha-Al2O3. This effect can be considered to be due to an initial formation of Fe2O3, which is an isomorphous crystal structure with alpha-Al2O3. Further contributing factor of Fe on the transformation can be a doping of Fe3+ ion, which formed during an initial transient oxidation stage, in the Al2O3 scale. The oxidation atmospheres are considered to indirectly affect the phase transformation by changing in the formation behavior of Fe2O3 and (Al, Fe)(2)O-3 solid solution during an initial transient oxidation stage.