Air oxidation at 600-800(omicron)C of two nanophase co-50wt% Cu alloys prepared by mechanical alloying

The air oxidation of two two-phase cobalt-copper alloys prepared by mechani cal alloying, containing approximately 50 wt%Cu but presenting two differen t grain sizes, has been studied at 600-800 degreesC. The two alloys have be en obtained by sintering a mixture of nanophase Co-Cu powders by hot isosta tic pressing followed or not by an annealing at 800 degreesC for 20 hr. Bot h alloys formed composite scales containing complex mixtures of copper and cobalt oxides associated with an internal oxidation of cobalt in a copper m atrix. The kinetics of oxidation were close to parabolic at 600 and 700 deg reesC, but rather irregular at 800 degreesC. Moreover, the oxidation rates of the two alloys were similar at 600 degreesC, but the annealed alloy corr oded more slowly at 700 and 800 degreesC. On the whole, the scaling behavio r of the two nanophase alloys was rather similar to that of an ahoy of the same composition prepared by conventional casting techniques and thus prese nting a much larger grain size. Thus, the very small grain size of the pres ent alloys is not yet sufficient to produce an exclusive external oxidation of cobalt, as might be expected as a result of a possible faster diffusion of cobalt along grain boundaries in the alloy. The presence of large convo lutions in the outermost CuO layer and the extensive spallation of the unde rlying Cu2O layer are attributed to the accumulation of mechanical stresses in the scale due to the penetration of oxygen along the grain boundaries i n the scale and to a reduction of the sample size associated with the incre ase in the alloy grain size with time during oxidation.