Elemental Distribution near the Grain Boundary in a Nd–Fe–B Sintered Magnet Subjected to Grain-Boundary Diffusion with Dy<sub>2</sub>O<sub>3</sub>

We have investigated the microstructure in a Nd–Fe–B sintered magnet subjected to the grain-boundary diffusion (GBD) process with Dy2O3. It was found that thin (Nd,Dy)2Fe14B shells with a thickness of at least 10 nm and an average Dy concentration of approximately 0.7 at% were formed even at the center of the specimen, at a depth of approximately 1 mm from the specimen surface. However, the area fraction of the fcc NdO phase remained almost constant at approximately 3% throughout the magnet irrespective of depth, and the grain-boundary wetting phase hardly changed except in the vicinity of the surface. Furthermore, the thickness (ca. 3 nm) and structure (amorphous) of the thin grain boundary phase existing between Nd2Fe14B grains remained almost unchanged by the Dy-GBD process, despite the formation of the core–shell structure. Therefore, the Dy concentration in the thin Dy-substituted shell is important for improving the coercivity via the GBD process.