Effect of the grain boundary Tb/Dy diffused microstructure on the magnetic properties of sintered Nd-Fe-B magnets

The effect of heavy rare earth element (HREE, Tb and Dy) coating thickness on the grain boundary diffusion process (GBDP) of sintered Nd-Fe-B magnets is investigated. The distribution of HREE concentration fits well with the distribution depicted by the dual-trend diffusion model for the magnets with thin HREE coatings. However, when the HREE coating is thicker than a critical value, two new types of structures of HREE distribution, namely, the anti-shell/core structure and the transitional structure, besides the well-reported shell/core structure, appear sequentially around the Nd-Fe-B grains near the close surface of the diffused magnets. The evolution of the microstructures and the magnetic properties of the diffused magnets are further studied on the basis of the dual-trend diffusion model. The coercivity of the diffused magnets with thick HREE coatings slowly improves when anti-shell/core and transitional structures form on the outer layer of the diffused magnets. The ultimate coercivity of the HREE-diffused magnets is the result of the combined action of the above three structures.