Compositionally tailored Dy-LRE-Zn-Cu diffusion sources for efficient grain boundary diffusion and reduced Dy consumption in Nd-Fe-B magnets

Abstract

Grain boundary diffusion (GBD) utilizing heavy rare-earth elements is an industrially important strategy for fabricating high-coercivity Nd-Fe-B magnets. However, conventional Dy-based sources often result in abnormal microstructures or excessively thick Dy-rich shells in near-surface region, limiting the GBD depth and leading to inefficient utilization of critical Dy resources. To overcome this challenge, we developed low-melting Dy-LRE-Zn-Cu [LRE (light rare-earth) = none, Pr, Nd] diffusion sources based on the LRE contained in the initial (Nd, Pr)-Fe-B magnet and benchmarked them to a Dy100 reference source. The incorporation of Zn and Cu to the Dy80Zn10Cu10 source increased Dy diffusion depth but also caused excessive intragranular diffusion. In contrast, Dy20Pr60Zn10Cu10 and Dy20Nd60Zn10Cu10 controlled Dy distribution within the grain boundary phase, preventing abnormal structure formation and enabling the formation of thin, continuous Dy-rich shells. This led to a significant improvement in Dy utilization efficiency, resulting in a coercivity gain of 14.25 kOe/wt% Dy for Dy20LRE60Zn10Cu10 compared with 3.08 kOe/wt% for Dy100. Consequently, magnets treated with Dy20LRE60Zn10Cu10 sources achieved coercivity values comparable to Dy100-treated magnets, despite utilizing five times less of Dy. Additionally, magnets treated with Dy20LRE60Zn10Cu10 diffusion sources exhibited enhanced thermal stability, demonstrating improved magnetic performance at 150 degrees C compared with Dy100-treated magnets. These findings emphasize the importance of designing diffusion sources tailored to the initial magnet composition to minimize Dy consumption and establish a resource-efficient approach for producing highcoercivity Nd-Fe-B magnets.