Diffusion behavior along Nd–Fe–B grain boundaries in different directions and mechanism of coercivity strengthening

Abstract

In this study, DyF3 powder was sprayed onto the polar and side surfaces of the magnets to determine the anisotropic diffusion mechanism of Dy in the sintered Nd–Fe–B magnet. The coercivity and squareness of the magnet in which the diffusion of Dy is perpendicular to the c-axis (a-magnet) are lower than those of the magnet with the diffusion of Dy parallel to the c-axis (c-magnet). Compared with the c-magnet, the a-magnet has a longer Dy-enrichment region from the diffusion surface, where Dy is enriched in the 2:14:1 grain. By contrast, the Dy concentration in the grain boundaries beyond the Dy enrichment region is lower in the a-magnet. Moreover, the Dy shells beyond the Dy enrichment region in the a-magnet are distributed on the side surfaces of the 2:14:1 grains but not on the polar surfaces. Based on the micromagnetic simulation, the Dy shells on the polar surfaces of the grains are more effective in enhancing coercivity. According to first-principle calculations, Dy migrating through [001] into the Nd vacancy in the Nd2Fe14B crystal has a higher diffusion barrier, thus indicating that the lattice diffusion of Dy parallel to the c-axis is more difficult.