Macroscopically heterogeneous grain boundary diffusion process for efficient coercivity enhancement of Nd–Fe–B magnets.

Grain boundary diffusion (GBD) is an effective process to enhance coercivity for Nd–Fe–B magnets with relatively low consumption of expensive heavy rare earths (HREs). For conventional GBD, the surface of the magnet is evenly covered by diffusion source, followed by diffusion heat treatment. In this work, a macroscopically heterogeneous GBD (MHGBD) process is proposed in order to further reduce the use of HRE resource. Based on the micromagnetic simulations, magnetically strengthening the edge area of the magnet is more effective than strengthening the center area in terms of coercivity enhancement for whole magnet. Hence, the HRE-based diffusion source was used for enhancing the edge area and the light rare-earth-based source was used for the center area. In details, Tb₇₀Al₂₀Cu₁₀ and Pr₇₀Al₂₀Cu₁₀ diffusion sources were covered at the edge and center areas of the two c-planes of a sintered Nd–Fe–B magnet, respectively. After the MHGBD by using Tb₇₀Al₂₀Cu₁₀/Pr₇₀Al₂₀Cu₁₀ (1:1, at.%), the magnet exhibits the increased coercivity from 1182 to 1911 kA/m. For comparison, the homogeneous diffusion of HRE-based Tb₇₀Al₂₀Cu₁₀ source only enhances the coercivity to 1798 kA/m. The microstructure characterizations indicated that diffusion source of Tb₇₀Al₂₀Cu₁₀ can form Tb-rich shells with high anisotropy field on the surface of Nd₂Fe₁₄B grains, and Pr₇₀Al₂₀Cu₁₀ can provide more liquid grain boundary phase for GBD. The synergistic effect between these two sources improves the infiltration of Tb at the edge area of the magnet. Compared with the homogeneous Tb₇₀Al₂₀Cu₁₀ diffusion, MHGBD process can not only exhibit higher diffusion efficiency, but also enhance the performance/cost ratio of the diffused magnets, evident by the increased coercivity enhancement per unit source from 0.23 to 0.51 kA m⁻¹/($/kg). [ABSTRACT FROM AUTHOR]