Air oxidation of two nanophase Ni-Ag alloys at 600-700 degrees C

Two nanophase Ni-base alloys containing 50 and 25 at.% Ag prepared by mecha nical alloying, denoted Ni-50Ag and Ni-25Ag were oxidized in air at 600 and 700 degreesC for 24hr. Ni-50Ag underwent internal oxidation of nickel, ass ociated with the formation of a continuous outer most layer of silver metal with scaling rates larger than those for pure nickel. On the contrary, Ni- 25Ag formed a continuous NiO layer surmounted by a discontinuous silver lay er and internal oxidation was suppressed. The oxidation rate of Ni-25Ag dec reased with time much more rapidly than predicted by the parabolic rate law during the initial stage and eventually became parabolic, with rate consta nts much lower than those for the oxidation of pure nickel. These results a re attributed to the two-phase nature and particularly, to the very small g rain size of the two alloys.