Diffusion Coefficients of Sm in Intermetallic Compounds SmCo₅ and Sm₂Co₁₇

Abstract

For the purpose of obtaining basic information on the reduction-diffusion process, diffusion experiments were carried out for the Sm-Co binary system at 1000, 1050, 1100, 1150, and 1200°C. Sm was made diffuse into Co by contacting a vapor from a molten Sm-Ca alloy with a polished surface of a Co block. The use of the Sm-containing vapor was effective to suppress the formation of a Co-containing melt on the surface of the Co block. The formation of the Co-containing melt hindered the uniform growth of diffusion layers and blurred the time of initiation of the diffusion. Though several intermetallic compounds are present in the Sm-Co binary system. only SmCo₅ and Sm₂Co₁₇ phases were found to grow systematically. Diffusion coefficients of Sm for the reactive diffusion in these phases at the respective temperatures were calculated from the relationships between the diffusion layer thickness and the diffusion time by utilizing Sm concentrations at the boundaries of the phases which were read from Sm-Co binary phase diagrams. The diffusion coefficient of Sm in SmCo₅ has been found at least three times larger than that in Sm₂Co₁₇ at each temperature of this study, and the reason for their difference was qualitatively discussed by considering the difference in their crystal structures. Also a reasonable explanation could be given for contradictory results reported by other investigators about the preferential growth of the Sm₂Co₁₇ phase.