Effect of Al content of Nd-Fe-B sintered magnet on grain boundary diffusion process of Tb coating

•Different Al contents in the sintered Nd-Fe-B have a significant effect on the coercivity of Tb grain boundary diffusion magnets.•Compared to magnets with different Al contents is more likely to replace Nd in the grains in the main phase.•The presence of the Nd6Fe11Al3 and Nd (Fe, Al)2 hinders the diffusion of Tb and the incremental of coercivity for magnet was decreases. The effects of different Al content in Nd-Fe-B magnets on the microstructure and magnetic properties after the diffusion of Tb at grain boundary were investigated. The magnetic properties and microstructure of magnets with low, medium and high Al content after Tb diffusion were investigated. It was shown that the average thickness of Tb shell in the near-surface region after diffusion was about 5 μm in magnets with low Al content. The tendency of Tb diffusing into the magnet along the grain boundaries in the depth direction and the main phase along the grains was stronger, resulting in more obvious core-shell structure. The magnet coercivity increased from 1046 kA/m to 1788 kA/m for low Al content after Grain Boundary Diffusion Process (GBDP). In magnets with high Al content, because there are more Nd6Fe11Al3 phase (δ-phase) and Nd (Fe, Al)2 phase (μ-phase), Nd in the δ-phase is replaced by part of Tb. Therefore, the amount of Tb diffused into the main phase (2:14:1) decreases. In addition, the μ-phase forms on the surface of the main phase, which hinders the diffusion of Tb to the main phase. Therefore, the thickness and concentration of the Tb shell in magnets with high Al content are much lower than those of the magnets with low Al content. Compared with magnets with high Al content, Nd is more likely to be replaced by Tb in the magnets with low Al content in the grains of main phase, consequently, the coercivity of low Al content magnet increases more.