Breaking through the coercivity enhancement limitation of heavy rare earth-diffused Ce-rich Nd-Ce-Fe-B magnets via tailored grain boundary engineering.

The grain boundary diffusion (GBD) process shows advantages in enhancing coercivity and reducing the usage of heavy rare earth (HRE) elements in Nd-Fe-B magnets but faces challenges in Ce-rich Nd-Ce-Fe-B magnets. Here, we investigated the effects of the intergranular CeFe2 phase on Dy diffusion in sintered Nd-Ce-Fe-B magnets with 13 wt% Ce. The results revealed that the intrinsically higher diffusion efficiency of Dy in the liquid phase triggers preferential Dy segregation in the RE-rich phases, resulting in non-uniform Dy-rich grain shell formation. The dissolution of Dy in the CeFe2 phase reduces the available Dy atoms for the formation of an effective Dy-rich shell, limiting coercivity enhancement to only 217.7 kA m-1via DyHx GBD. A tailored grain boundary engineering (GBE) strategy was proposed to overcome this limitation. Pr-Al-Ga alloy was initially employed to substantially reduce the CeFe2 phase content and form a RE-rich phase-dominated grain boundary structure in the magnet, enabling subsequent highly efficient DyHx diffusion. Specifically, the Pr-Al-Ga GBE process slightly increased the coercivity by 205.6 kA m-1, while the subsequent DyHx diffusion led to a further substantial increase of 500.7 kA m-1. Finally, this grain boundary engineering-assisted diffusion led to a remarkable coercivity enhancement of 706.3 kA m-1, which is comparable or even higher than those of the DyHx-diffused Ce-free Nd-Fe-B magnets. Importantly, validation with TbHx diffusion suggests that the proposed strategy is potentially applicable to different HRE diffusion sources. This work not only provides a feasible approach for breaking through the coercivity enhancement limitation of Ce-rich Nd-Ce-Fe-B magnets but also elucidates the critical role of the intergranular phase structure in GBD.