Anisotropic diffusion mechanism in grain boundary diffusion processed Nd–Fe–B sintered magnet

Abstract

We investigated the anisotropic diffusion mechanism of Dy in a DyH2 dip-coated magnet in terms of both the crystal orientation of the Nd2Fe14B phase and the aligned direction of the magnet. A Dy-rich shell was formed preferentially at the interface, which is parallel to the [001] axis (c-axis) of the Nd2Fe14B crystal, during the grain boundary diffusion process (GBDP) because lattice diffusion of Dy perpendicular to the c-axis of the Nd2Fe14B crystal is much easier than that parallel to the c-axis. In contrast, the grain boundary diffusion depth in the direction perpendicular to the aligned direction (c-axis) of the magnet was much shorter (∼100 μm) than that in the direction parallel to the aligned direction (∼250 μm). Anisotropic grain boundary diffusion depending on the aligned direction of the magnet was not the result of the anisotropic Nd-rich grain boundary phase distribution but of anisotropic lattice diffusion depending on the Nd2Fe14B crystal orientation. Increasing the ratio of the surface areas perpendicular and parallel to the aligned direction (c-axis) of the magnet is expected to further improve the coercivity of the GBDP magnet.