Abstract: Selected area grain boundary diffusion (SAGBD) has shown potential in enhancing the coercivity of thick Nd-Fe-B magnets while minimizing the consumption of heavy rare earths. However, the relationships among the SAGBD process, diffusion efficiency, and performance enhancement of the magnets have not yet been well established. Here, different grain boundary diffusion (GBD) approaches were proposed to treat the magnets with different thicknesses using a Tb-Cu alloy diffusion source. The SAGBD was approached with the source diffused from the twelve edge areas of the magnet and from eight vertices of the magnet were called SA-edge and Vertex GBD, respectively. Those diffusing from the edge areas of the four side planes and two c-planes were named SA-ab and SA-c diffusions, respectively. The GBDs from two easy magnetization planes (c-plane) and two parallel planes perpendicular to the c-plane were named c-plane and a-plane diffusions, respectively. It was found that the optimal GBD approaches for the magnets with thicknesses of 5, 7.5, 10, and 12.5 mm were c-plane, c-plane, SA-edge, and SA-ab diffusions, respectively. Specifically, the SA-ab diffusion shows the highest utilization efficiency of Tb in 12.5 mm-thick magnets, reaching a value of 405.5 kA m^-1/wt.%, which is 2.9 and 3.8 times higher than c-plane and SA-c diffusions, respectively. In addition, similar magnetic properties were obtained by c-plane and a-plane diffusions, suggesting that Tb-Cu diffusion does not exhibit significant anisotropic behavior. The microstructure observations and micromagnetic simulations suggest that the reversed domains tend to nucleate at the edges and corners of the magnet. The Vertex diffusion can directly hinder the nucleation and propagation of reversed domains at the corners, but the SA-edge diffusion can effectively postpone their rapid propagation over the entire magnet. This study provides the guidelines for the future development of SAGBD for sintered Nd-Fe-B magnets.

Key words: Nd-Fe-B magnets, Selected area grain boundary diffusion, Coercivity, Heavy rare earth, Demagnetization behavior