Formation of anti-shell/core structure of heavy rare earth elements (Tb, Dy) in sintered Nd-Fe-B magnet after grain boundary diffusion process

The evolution of microstructure close to the surface of sintered Nd-Fe-B magnet after grain boundary diffusion process (GBDP) of heavy rare earth elements (HREE, Tb, Dy) was investigated. It was found that two new types of microstructure, anti-shell/core and no-shell structures, were formed in the magnet with the diffusion time longer than 0.5 h. The anti-shell/core structure was formed due to an abnormal diffusion of HREE from grain to grain boundary based on the dual-trend diffusion model, it contributes a negative effect on the coercivity of diffused magnets.