Effect of diffusing TbF3 powder on magnetic properties and microstructure transformation of sintered Nd-Fe-Cu-B magnets

The coercivity of sintered Nd-Fe-Cu-B magnets is markedly enhanced from 12.57 to 21.70 kOe while the remanence decreases from 13.80 to 13.49 kGs by grain boundary diffusion of TbF3 powder for 2 h. Microstructure analysis suggests that, during the diffusion process, F diffuses into the magnets easily and forms a new F-rich phase. The enrichment of F in grain boundary near the surface leads to the Cu movement into the interior and the Cu reduction in the surface of magnets. Diffusion of Tb leads to an increase of local total rare earth elements content. Under the combined effect of Cu reduction and increase of local total rare earth elements content, grain growth area is formed and further diffusion is suppressed. That excessive Tb diffuses into matrix phase leads to a decrease in remanence. When the grain growth area is removed, the deterioration of remanence recovers to 13.80 kGs without any reduction of coercivity.