Internal oxidation of Fe-Al alloys in the ?-phase region

Abstract

The internal oxidation behavior of Fe-0.069, 0.158, and 0.274 wt% Al alloys was investigated in the α-phase region. The internal oxidation experiments have been made over the temperature range from 1023 to 1123 K using a mixture of iron and its oxide powders. A parabolic rate law holds in the present alloys, where the rate constant, Kp, depends upon the oxidation temperature as well as the aluminum content. The internal oxidation of Fe-Al alloys is, therefore, controlled by a diffusion process of oxygen in the alloy. The oxide formed in the oxidation layer is the stoichiometric FeAl2O4 (hercynite). The aluminum concentration, N Al Io , in the oxidation layer was calculated by taking account of counterdiffusion of aluminum. Furthermore, the oxygen concentration, N O S , at the specimen surface was evaluated on the basis of thermodynamics. Using these estimated values of Kp, N Al IO , and N O S , the diffusion coefficient of oxygen, D O IO , in the oxidation layer, where the oxide particles were dispersed, was also calculated. D O IO increases as the volume fraction of the oxide, fIO, increases. The diffusion coefficient of oxygen, DO, in α-iron was determined by extrapolating D O IO to fIO=0.