Substantial Improvement of Tb-Utilization in Diffused Nd-Fe-B Magnets with Restricted Tb-Rich Shell and Defective Grain Boundaries

Abstract

Grain boundary diffusion (GBD) is the most effective method to enhance the coercivity of Nd-Fe-B sintered magnets with limited heavy rare earths (HRE) usage. However, a significant amount of HRE tends to aggregate at surface, leading to insufficient enhancement of coercivity and serious waste of HRE. In this paper, Tb-utilization efficiency in GBD processed Nd-Fe-B sintered magnets was significantly improved with restricted Tb-rich shell and defective grain boundary (GB) phases. The microstructure and magnetic properties of different Tb-diffused Nd-Fe-B magnets were investigated. Tb elements aggregation near surface of the magnets can be markedly suppressed in diffused Nd-Fe-B magnets with restricted Tb-rich shell and defective GBs. TiB2 precipitates at the GBs not only suppressed the grain growth during the sintering process, but also inhibited the thickness of Tb-rich shell in the GBD process. Additionally, presence of TiB2 precipitates has a significant impact on the composition and microstructure of Nd-rich GB phases. Defective GB phases significantly accelerate the diffusion process of Tb. The coercivity increased significantly to 27.83 kOe (11.3 kOe higher than that of the post-sintered annealed state) in the magnet with high Tb-utilization. This work provides new insights into the manufacture of high-performance diffused magnets with extreme utilization of precious HRE resources.

Keywords: Nd-Fe-B sintered magnets, grain boundary diffusion, coercivity enhancement, microstructure optimization