Microstructural and magnetic properties of SPSed Nd-Fe-B magnets: effect of TbHx content in internal grain boundary diffusion

Abstract

Nd-Fe-B sintered magnets commonly utilize grain boundary diffusion (GBD) with heavy rare earth elements such as Dy, Tb to enhance coercivity. However, conventional GBD ( c-GBD) has limitations including excessive consumption of diffusion sources near the surface and limited diffusion depth and homogeneity. In this study, spark plasma sintering (SPS) was combined with internal grain boundary diffusion (i-GBD) to overcome these issues. Nd-Fe-B powder was mixed with 1 wt.% of TbHx, followed by SPS to fabricate a high-density sintered magnet, and post-sintering heat treatment was applied to induce internal grain boundary diffusion.EPMA analysis revealed that Tb in the c-GBD magnet was mainly concentrated near the surface. In contrast, the i-GBD magnet exhibited a uniform Tb distribution throughout the entire magnet, forming homogeneous core-shell structures. As a result, the i-GBD magnet maintained consistent coercivity regardless of increasing thickness, while the c-GBD magnet showed a gradual decrease in coercivity toward the center.Subsequently, the effect of TbHx content was investigated by varying the amount from 0 to 5 wt.% (0, 0.5, 1, 3, and 5 wt.%) in both i-GBD and c-GBD magnets. Coercivity, thermal stability, and microstructure were comparatively analyzed, and the influence of TbHx content on diffusion behavior and magnetic properties was quantitatively evaluated. The results demonstrate that i-GBD is a promising process for achieving stable performance even in large-scale magnets.