Grain boundary self-diffusion in directionally solidified equiatomic Fe–Co alloy

Grain boundary diffusivities of 59Fe and 57Co in a binary Fe-50 at.%Co alloy have been determined at a temperature range between 883 and 1123 K by serial sputter-microsectioning technique using specimen unidirectionally solidified at three different solidification rates of 12000, 60 and 32 mm/h. Above the order–disorder transformation temperature of 1003 K, the Arrhenius plots of the grain boundary diffusivities in the alloy solidified at three rates show straight lines. The diffusivities in the specimen with 32 mm/h are higher than those with 60 and 12000 mm/h. Below 1003 K, all the Arrhenius plots of the diffusivities show downward curvature remarkably owing to the atomic ordering. From the analysis of depth profiles, the atomic ordering gives large influence on the diffusion along subgrain boundaries rather than the diffusion along random grain boundaries.