In relation to the production of rare earth-transition metal alloys by the Reduction-Diffusion process, Nd and Sm were diffused into solid Fe to investigate the growth of intermetallic compounds. The diffusing elements, Nd and Sm, were supplied to a polished surface of an Fe block from the vapor phase. As had been expected from the phase diagrams, only the Nd2 Fe 17 diffusion layer was obtained for the Nd-Fe system, and the SmFe3 and Sm2 Fe 17 diffusion layers were observed for. the Sm-Fe

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... . In the case of the latter system, however, the thickness of the Sm2Fe17 layer alone could be measured because the SmFe3 layer had grown as an outermost layer and was so brittle that most part of it was lost while the section of the Fe block was polished. In either case, it was shown that the growth rate of the compound was controlled by diffusion. Apparent activation energies were obtained for the diffusion of Nd and Sm in the respective compounds through the Arrhenius plots. The resultant activation energies differed in higher and lower temperature ranges,particularly for the diffusion of Sm in the Sm2 Fe 17 phase, though it is still ambiguous what change in the diffusion mechanism caused the difference.