Microstructural evolution and diffusion behaviors of Dy and Tb in sintered Nd-Fe-B magnets via grain boundary diffusion of Pr Dy Tb17-M28 (M = Fe, Al, Ga) multi-component alloys

To systematically investigate the influence of Dy and Tb ratio in low-heavy rare earth (low-HRE) multi-component alloys diffusion sources on the magnetic performance of Nd-Fe-B magnets and the diffusion behaviors of Dy and Tb, grain boundary diffusion (GBD) treatments were applied to commercial N55 sintered Nd-Fe-B magnets. Diffusion sources employed were Pr 55 Dy x Tb 17- x M 28 alloys (at%, M = Fe, Al, Ga). Experimental results demonstrate that increasing the Tb atomic ratio in the diffusion source from 0 at% to 17 at% significantly enhances the coercivity ( H cj ) of the diffused magnets. Specifically, the coercivity increased from 14.90 kOe to 21.01 kOe ( x =17), 22.48 kOe ( x =13), 23.46 kOe ( x =8.5), 23.85 kOe ( x =4), and 24.22 kOe ( x =0), while the remanence ( B r ) remained nearly unchanged. Notably, the coercivity increment (Δ H cj ) decreased gradually with increasing Tb content. Microstructural characterizations revealed that the enhanced magnetic properties originate from the formation of Dy and Tb-rich shell structures extending deep into the magnet matrix. Analysis of magnets subjected to GBD treatments with Pr 55 Dy 8.5 Tb 8.5 M 28 under different heat treatment processes reveals that Dy preferentially diffuses and is consumed in the near-surface layer (resulting in a higher Dy concentration in the shell), whereas Tb diffuses to deeper regions and is primarily enriched along grain boundary (GB). This distinct element distribution pattern remains consistent across all heat treatment processes, and the concentration disparity between Dy and Tb within the shell structure gradually increases during the diffusion process. These findings provide valuable insights for the efficient utilization of HREEs and fabrication of high-performance Nd-Fe-B magnets.