Abstract
In order to clarify the origin of coercivity enhancement of Nd-Fe-B sintered magnets by the Grain BoundaryDiffusion Process (GBDP), microstructural observations were performed and the coercivities of magnets processed with various rare-earth oxides were investigated. Microstructural analysis using FE-EPMA revealed that Dy supplied from the magnet surface diffused into whole magnets with a thickness of 2 mm. At the center of a magnet, the Dy distribution width around a grain boundary is almost comparable to the width of the Nd-rich grain boundary phase.When rare-earth (R) oxides whose R provide higher anisotropy in R2Fe14B than that of Nd2Fe14B, are used for GBDP, the magnets showed higher coercivity than that of the control, and vice versa. Although the diffusion depth of Dy into the Nd2Fe14B grains was unclear at the center of the magnet, the relationship between the kinds of rare-earth elements used for GBDP and the resultant coercivities strongly suggests that the rare-earth elements supplied from the surface of the magnets and absorbed along the grain boundary phase may diffuse into very thin portions of the Nd2Fe14B grain surface.
