Comparative study of Dy and Tb diffusion behavior in grain boundary diffusion processed Nd-Fe-B sintered magnets

The grain boundary diffusion process (GBDP) is a highly effective method for enhancing the coercivity of Nd-Fe-B magnets while minimizing heavy rare earth elements (HREs) consumption. Understanding the diffusion behavior of HREs during GBDP is essential for optimizing their efficient utilization. In this study, GBDP was performed on sintered Nd-Fe-B magnets using DyH x and TbH x as diffusion sources to investigate the diffusion behavior of Dy and Tb. The coercivity of the magnets increased significantly, from an initial value of 14.65 kOe to 18.90 kOe and 25.62 kOe for Dy- and Tb-diffused magnets, respectively, corresponding to increases of 31.80% and 78.66%. The pronounced coercivity enhancement in Tb-diffused magnets is attributed to the formation of a Tb-enriched shell structure with a higher anisotropy field compared to Dy-diffused magnets. However, Dy demonstrated higher diffusion efficiency than Tb, as evidenced by its greater diffusion depth and the formation of a notably thicker shell structure. First-principles calculations revealed that the substitution energy ( E sub ) for Nd atoms replaced by Dy in the Nd 2 Fe 14 B phase is more negative than for Tb, providing a theoretical explanation for the higher diffusion efficiency of Dy. Micromagnetic simulations further confirmed the critical role of a deeper diffusion layer and a thicker HRE-enriched shell structure in achieving enhanced coercivity.