Grain Boundary Diffusion of Dy-LRE-Zn-Cu Sources for Improved Magnetic Properties of Nd-Fe-B Magnets

Abstract

Nd-Fe-B sintered magnets with high magnetic properties are indispensable for a variety of advanced applications, including electric vehicle drive motors, wind turbine generators, and other high-efficiency energy systems. Despite their outstanding remanence and maximum energy product, these magnets suffer from a pronounced reduction in coercivity and thermal stability at elevated operating temperatures. This degradation severely limits their applicability in demanding environments where high thermal resistance is essential. Conventional grain boundary diffusion processing (GBDP) employing heavy rare earth (HRE) elements such as Dy has been introduced as an effective strategy to mitigate this limitation. However, the practical use of pure Dy diffusion still faces several challenges, including inefficient infiltration due to its high melting point, restricted availability of HREs, and unstable price, underscoring the urgent need for more resource-efficient alternatives. In this study, diffusion sources were developed by partially substituting Dy with light rare-earth elements (Pr, Nd) and transition metals (Zn, Cu), thereby producing low-melting compositions suitable for efficient GBDP. These sources were fabricated by induction melting and subsequently processed into fine powders through melt spinning, hydrogen decrepitation, and sieving in an inert atmosphere. The GBDP was carried out under various heat treatment conditions, enabling systematic evaluation of the relationship between heat-treatment temperature and magnetic properties. After heat treatment at 850 °C, both Dy2oPr6oZnioCuio- and Dy20NdeoZn 1 oCu 1 o-treated magnets exhibited room temperature coercivity values comparable to that of the Dyioo-treated magnet, while achieving superior magnetic properties at elevated temperatures (200 °C). Importantly, these results were obtained with only 20 wt.% Dy in the diffusion source, demonstrating a significant reduction in heavy rare-earth consumption. EPMA elemental mapping further revealed that, unlike the Dyioo-treated magnet which showed a nonunifbrm Dy distribution, the Dy2oPr6oZn 1 oCui 0- and Dy20Nd6oZn 1 oCu 1 o-treated magnets developed a distinct core-shell microstructure with a Dy-enriched shell. The formation of this core-shell structure is the key factor enabling comparable room-temperature coercivity and enhanced high-temperature properties while improving Dy utilization efficiency.