On Dy–M (M = Al, Ni, and Cu) diffusion sources for enhancing coercivity and corrosion resistance of Nd–Fe–B magnets

Abstract

Dy based alloys have been widely employed as the grain boundary diffusion (GBD) sources for Nd–Fe–B magnets. To reveal the design principle of these alloys, we carried out a systematic investigation on the magnetic properties, microstructure evolution, and corrosion behavior of grain boundary diffused (Nd,Pr)–Fe–B based sintered magnets using Dy_100−xAl_x, Dy_100−xNi_x, and Dy_100−xCu_x (x = 0, 10, 20, 30, 40 at%) alloys as the diffusion sources. It is found that Dy₉₀Al₁₀, Dy₈₀Ni₂₀, and Dy₈₀Cu₂₀ alloys exhibit the highest coercivity enhancements of 429 kA m⁻¹, 334 kA m⁻¹, and 183 kA m⁻¹, respectively, in each series of alloys due to their relatively high Dy content and low melting points. The superior performance of the Al-containing diffusion source originates from the facilitated formation of continuous grain boundary phases (GBPs) during diffusion and the high Dy diffusion efficiency. All diffused magnets exhibited improved corrosion resistance, for example reduced corrosion current density, compared to the initial magnet. The results indicate that the diffusion of non-rare earth elements modifies the chemical composition and distribution of the intergranular phases. This microstructural optimization alters the local electrode potentials of both the main phase and grain boundaries, thereby improving the overall corrosion resistance. Notably, Dy–Al and Dy–Ni diffused magnets show better corrosion resistance performance than Dy–Cu in the GBD process diffused magnets. This work provides critical insights for designing high-performance diffusion sources to achieve high comprehensive performance in GBD treated Nd–Fe–B magnets.