Influence of Oxygen Content on Dy Grain Boundary Diffusion in Nd-Fe-B Sintered Magnets

Influence of the oxygen content in sintered NdFeB magnets on the increase of Dy and coercivity after the grain boundary diffusion process (GBDP) was investigated. By comparing a variety of high and low oxygen magnets after GBDP, we find that the amount of Dy diffusion and the increments of coercivity of the low -oxygen magnets are significantly higher than those of the high-oxygen ones. Comparison of nine 0 wt% Dy samples with different oxygen contents proves that the decrease of oxygen content is beneficial to rise of Dy diffusion amount and coercivity. Dy content gradient analysis reveals that Dy amount gap between each layer is narrower in the low-oxygen ones. According to the EPMA figures, Dy-rich shells in the grain boundaries of the low-oxygen magnets are more continuous and brighter, and Dy almost completely surrounds each matrix grain. The optimization of the microstructure also enables the low-oxygen samples to possess more noticeable increment of anisotropic field in comparison to the high-oxygen ones. Decrease of oxygen leads to a phenomenon that Nd-rich phase surrounds the matrix phase more homogeneously and continuously, which provides a more continuous tunnel for Dy to diffuse into the magnet, thus improving Dy content and coercivity.