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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]

Grain boundary diffusion of heavy rare earth-based alloys can effectively enhance the coercivity of Nd-Fe-B magnets. Although the light rare earth elements can further improve the diffusion efficiency of Dy and Tb, they generally lead to reduced anti-corrosion properties of the magnets. Here, we successfully enhanced the coercivity and corrosion resistance of Nd-Fe-B magnets by GBD of Ni alloyed Pr-Tb-Cu diffusion source. The ternary Pr 35 Tb 35 Cu 30 alloy diffusion increased the coercivity of Nd-Fe-B magnet from 1330 to 1923 kA/m, but the coercivity was enhanced to 1990 kA/m by Pr 35 Tb 35 Ni 15 Cu 15 alloy diffusion. The formation of high-anisotropy (Nd,Tb) 2 Fe 14 B phase is the main reason for the coercivity improvement. Partially substitution of Cu by Ni can reduce the melting point of Pr 35 Tb 35 Cu 30 alloy, which is effective in promoting the infiltration of Tb. Cu is also a necessary element for improving the diffusion process, and the diffusion of Pr 35 Tb 35 Ni 30 alloy only increased the coercivity to 1747 kA/m. More interestingly, the diffusion of Pr 35 Tb 35 Ni 15 Cu 15 also enhanced the chemical stability of the magnet in NaCl solution. Scanning Kelvin probe force microscopy characterized that the modified GB phase in Pr 35 Tb 35 Ni 15 Cu 15 treated magnet exhibits a higher Volta potential than the 2:14:1 main phase, which is the main reason for the improved corrosion resistance. The present results suggest that the non-rare earth elements in the alloy diffusion sources play important roles in enhancing both coercivity and corrosion resistance of Nd-Fe-B magnets. • Both coercivity and corrosion resistance of Nd-Fe-B magnet are distinctly enhanced. • Ni alloying lowers melting point of Pr-Tb-Cu and deepens the infiltration of Tb. • Ni alloying results in higher potential of grain boundary than that of 2:14:1 phase. • Scanning Kelvin probe force microscopy was used to measure local potential. [ABSTRACT FROM AUTHOR]