Metastable alumina （including γ- 、δ-and θ- Al2O3） generally forms during oxidation of alumina forming alloys at 800℃ - 1100℃, then transforms to stable α- Al2O3 with oxidation process. This phase transformation has distinct effects on the oxidation of the alloys. Influences of oxidation temperature, time, alloying element, rnicrostructure on the θ - α transformation have been reviewed. As the properties of alumina scales are very important for oxidation resistance of superalloys or protective coatings, further studies are needed in order to understand the mechanism of the phase transformation.
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