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Grain boundary diffusion (GBD) process is an effective method to fabricate high-coercive Nd-Fe-B magnets with less consumption of heavy rare earth (HRE). By this approach, HRE-rich shell forms around the Nd2Fe14B grain, which can hinder the magnetic reversal starting at the edge of the grains and enhance the coercivity of whole magnet. Recently, an anti-core–shell structure was observed in the HRE diffused magnets after over-saturated diffusion, where the HRE concentration in the core is even higher than that in the shell. In this work, the effects of the anti-core–shell structure on the magnetization reversal and magnetic properties of diffused magnet have been clarified by micromagnetic simulations. Three-dimension models were established to analyze the demagnetization process. The results indicate that the anti-core–shell structure leads to a large stray field, which will accelerate the magnetization reversal of the whole magnet. As a result, the beneficial effect of HRE GBD on the coercivity has been reduced. Combined with the existing experimental results, the formation of anti-core–shell structure should be avoided during diffusion. Hence, appropriate diffusion time and diffusion source dosage should be selected for GBD process in order to obtain high-performance products and efficiently use the HRE resources. [ABSTRACT FROM AUTHOR]

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]

Grain boundary diffusion (GBD) is an effective approach for improving the coercivity of Nd-Fe-B magnets with less heavy rare earth consumption. However, it generally only works well for thin magnets since the infiltration depth of diffusion source is limited. Here, a GBD approach that mainly diffuses the source from the edge and corner areas of the magnet, named selected area GBD (SAGBD), is proposed for treating thick magnets. After diffusing Pr-Tb-Al-Cu alloy by SAGBD, the coercivity of a 12-mm thick magnet was increased from 1070 to 1675 kA/m. By this approach, the coercivity enhancement with 1 wt. % Tb was 465 kA·m−1, 1.6 times as much as that by diffusion from two easy magnetization planes. For achieving a coercivity enhancement of ∼600 kA/m, the thickness of treated magnet was increased by 4 mm. Although the diffusion from six planes can further increase the coercivity, SAGBD leads to high efficient use of Tb and less reduction of remanence. The microstructure characterizations and micromagnetic simulation demonstrated that the edges and corners are magnetic "weak" areas of the magnet, and SAGBD can effectively hinder the nucleation of reversed domains in these places and the rapid reversal propagation in whole magnet. The present approach is important for the high-efficient use of the diffusion source and the fabrication of large-size Nd-Fe-B products with high stability. [ABSTRACT FROM AUTHOR]