Grain size effects on the oxidation of two-phase Cu–Fe alloys

The oxidation behavior of thin layers of two Cu-Fe alloys containing 25 and 50 wt.% Fe, respectively, prepared by magnetron sputtering deposition on cast alloys of the same composition (Cu-Fe coatings) and presenting grain sizes in the nanometer range, was studied at 600-800degreesC in air to examine the influence of the reduction in the grain size on the selective oxidation of the most reactive component in two-phase binary systems. A continuous Fe3O4 layer formed beneath an external region of copper oxide on the Cu-25Fe coating, whereas an external iron oxide scale mostly composed of Fe3O4 free from copper oxides formed on the Fe-50Cu coating. In both cases, an iron-depleted region was present in a subsurface alloy layer. These results differ remarkably from the oxidation behavior of cast Cu-Fe alloys of similar composition but with a large grain size, which formed mixed external scales of iron and copper oxides in air and simultaneous internal and external oxidation of Fe under both high and low oxygen pressures. Therefore, a grain size reduction can effectively promote the selective external oxidation of the more reactive component in binary two-phase alloys due to an increase in the mutual solubility of the two components associated with the method of alloy preparation as well as to the presence of a large density of grain boundaries in the coatings which may act as short-circuit diffusion paths, allowing a faster outward diffusion of iron during oxidation. (C) 2002 Elsevier Science Ltd. All rights reserved.