Effect of various factors on the metastable to stable phase transformation behavior of a thermally grown oxide scale of Al
2O
3 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 Al
2O
3 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 Al
2O
3 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 α-Al
2O
3. This effect can be considered to be due to an initial formation of Fe
2O
3, which is an isomorphous crystal structure with α-Al
2O
3. Further contributing factor of Fe on the transformation can be a doping of Fe
3+ ion, which formed during an initial transient oxidation stage, in the Al
2O
3 scale. The oxidation atmospheres are considered to indirectly affect the phase transformation by changing in the formation behavior of Fe
2O
3 and (Al, Fe)
2O
3 solid solution during an initial transient oxidation stage.
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