Development of Non-Rare Earth Grain Boundary Diffusion Process for Nd-Fe-B Permanent Magnets

Abstract

The magnetic performance of Nd-Fe-B magnets depends on their grain boundary structure. Grain boundary diffusion process (GBDP) is an effective approach for enhancing coercivity with low material cost. This paper summarizes the development of GBDP with emphasis on our recent work using cost-effective non-rare earth (non-RE) diffusion sources. Previously, heavy rare earth (HRE) based compounds, metals and light rare earth (LRE) based alloys were successively employed as diffusion sources for coercivity enhancement. In 2015, we firstly proposed a novel GBDP based on non-RE oxide. The oxide diffused into the intergranular regions modifies the composition and structure of the grain boundary, resulting in enhanced coercivity and corrosion resistance. Subsequently, low-melting metals and alloys such as Al and Al75Cu25 can also work as the diffusion sources. Recently, we used Al₆₂Cr₃₈ coatings for both coercivity enhancement and surface protection, which shortens the production process and makes non-RE diffusion sources more competitive.

Keywords: Nd-Fe-B magnets, grain boundary diffusion process, microstructure, coercivity, corrosion resistance