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In order to study the effect of microstructure changes on the selective oxidation of the most reactive component in two-phase alloys, the oxidation behaviors of an as-cast Fe-50Cu alloy and its sputtered coating with quite different grain sizes were examined at 800 degreesC. For the as-cast Fe-50Cu alloy, a complex scale formed after air oxidation, containing an outermost layer of copper oxide (CuO + Cu2O) plus an inner region composed of a mixture of the oxides of Fe and Cu as well as some Fe-Cu double oxides. A region of internal oxidation of Fe was also observed. Under oxygen pressure low enough that only Fe can be oxidized, a simultaneous external and internal oxidation of Fe was present. However, a single external Fe oxide scale formed in Fe-50Cu sputtered coating when the grain size was largely reduced. The transition of the oxidation modes above indicates that the grain size reduction can promote the selective oxidation of Fe effectively. The mechanism is mainly attributed to the significantly extended solubility of iron in copper, and also to the presence of a large number of grain boundaries in the coating, as a direct consequence of the grain size reduction, which may act as short-circuit diffusion paths for the fast outward diffusion of iron during oxidation process. (C) 2001 Elsevier Science B.V. All rights reserved.