This paper reviews recent progresses in coercivity improvement of hot-deformed anisotropic Nd-Fe-B magnets by the low temperature grain boundary diffusion process using RE70Cu30 (RE = Pr, Nd) eutectic alloys. Using Pr70Cu30 alloy, the coercivity was enhanced from 1.60 T to 2.56 T. The hot-deformed samples are expanded along only c-axis direction after the diffusion process by the formation of thick Nd-rich grain boundary phase parallel to the c-planes of platelet shaped Nd2Fe14B grains. To suppress the reduction in remanence caused by the excess expansion during the diffusion process, we developed an expansion constraint eutectic diffusion process. We demonstrated high performance Dy-free anisotropy magnet with coercivity, μ0Hc = 1.94 T, and remanence, Br = 1.35 T, by the expansion constraint diffusion process using a Nd-Cu alloy as a diffusion source.
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