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Hydrogen–disproportionation–desorption–recombination (HDDR) Nd–Fe–B magnetic powders are promising to prepare bulk anisotropic magnets, but high magnetic performance has not been achieved due to the absence of an Nd-rich phase in the powder and the low degree of orientation of the bulk magnets. In this study, an alternative process of pre-orientation sintering via magnetic alignment followed by spark plasma sintering was proposed to prepare the precursor of hot-deformation (HD) magnets, and a high maximum energy product of 294 kJ m−3 was achieved in the HD magnet with a relatively low height reduction of 35%, then an improved coercivity of 1107 kA m−1 could be obtained followed by a grain boundary diffusion of Pr40Tb30Al20Cu10. Microstructure analysis indicates that pre-orientation of HDDR powders facilitates grain rotation and alignment during the HD process, thereby reducing the minimum deformation ratio. It helps to obtain the deformed grains with lower shape anisotropy and smaller grain size, enabling a good compatibility of magnetic and mechanical behaviors. In-situ Lorentzian transmission electron microscopy results show that the magnetic domains have been strongly fixed by the thick intergranular RE-rich phase and the fully Tb-diffused grains, which contributes to the improved coercivity after grain boundary diffusion. This study provides a guiding significance for the preparation of the anisotropic Nd–Fe–B HDDR magnets with optimized performance. [ABSTRACT FROM AUTHOR]

The microstructure, magnetic properties, corrosion and aging resistance, and mechanical properties of Nd–Fe–B, (Nd, La, Ce)–Fe–B, and (Ce, La, Y)–Fe–B rubber magnets have been investigated. For Nd–Fe–B magnets, increasing particle size and filling ratio of magnetic particles not only improves the overall magnetic properties but also increases the tensile strength. Using the Nd26.4Fe67.6Co5B powder with particle size of 75–106 μm and filling ratio of 90%, the optimum magnetic properties with the coercivity Hcj = 738.2 kA m−1, remanence Jr = 0.51 T, and maximum energy product (BH)max = 43.5 kJ m−3 are obtained. The magnet also exhibits an enhanced tensile strength of 4.52 MPa owing to its compact and uniform microstructure. In addition, the magnetic properties of Hcj = 518 kA m−1, Jr = 0.34 T, and (BH)max = 20.6 kJ m−3 are obtained in the partial high‐abundance (Nd, La, Ce)–Fe–B rubber magnet, which is superior to some of commercial Nd–Fe–B rubber magnets. For full high‐abundance (Ce, La, Y)–Fe–B magnet, these values are 320 kA m−1, 0.27 T, and 10.5 kJ m−3, respectively. Despite its lower magnetic properties than Nd‐based rubber magnets, (Ce, La, Y)–Fe–B rubber magnet exhibits a competitive advantage over the Nd‐containing magnets in terms of corrosion resistance and aging resistance performance. Moreover, (Ce, La, Y)–Fe–B magnet shows a tensile strength of 7.86 MPa, which is nearly 74.2% and 56.8% higher than the Nd–Fe–B and (Nd, La, Ce)–Fe–B rubber magnets. The present results indicate that the Ce, La, and Y elements can be reasonably selected to manufacture flexible rare earths–Fe–B rubber magnets with a high performance/cost ratio and good performance stability. [ABSTRACT FROM AUTHOR]

Back extrusion is an important process to prepare radially oriented NdFeB ring magnets. In this work, we fabricate the ring magnets using amorphous magnetic powders as the raw material. The microstructure, magnetic properties, corrosion resistance, and mechanical properties of the backward extruded magnet at different positions along the axial direction have been investigated, and the inhomogeneity of the magnet is clarified. The results showed that the grains in the middle region of the ring magnet exhibit a strong c-axis orientation, whereas the grains at the bottom and top regions are disordered with random orientation. The microstructure variation is related to the distribution of the grain boundary phase and the degree of grain deformation. Due to the microstructure difference, the magnetic properties, temperature stability, corrosion resistance, and mechanical properties in the middle region of the magnet are higher than those in the top and bottom regions. The exchange coupling between grains also varies in different regions, which is related to the grain size and grain boundary thickness. In addition, different Co element segregations were observed in different regions, which has a crucial effect on the Curie temperature and thermal stability of the magnet. The microstructure difference also leads to the variation of corrosion resistance and mechanical properties for the samples from different regions of the magnet. This work suggests that the amorphous powder can be used to directly prepare radially oriented ring magnets, and the inhomogeneity of the magnet should be fully understood. [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]

Hot pressed and hot deformed nanocrystalline Nd-Ce-Fe-B magnets were prepared by dual-alloy method using melt-spun Nd-Fe-B and Ce-Fe-B powders with various weight ratios. For both hot pressed and hot deformed magnets, increasing Ce-Fe-B addition reduces the magnetic properties, but 10 wt. % Ce-Fe-B leads to an abnormal increase of coercivity. As a result, the remanence J r = 1.21 T, intrinsic coercivity H cj = 1215 kA/m, and maximum energy product (BH) max = 266 kJ/m3 are obtained for the magnets with 10 wt. % Ce-Fe-B. To modify their properties, the dual-main phase magnets are treated by grain boundary diffusion of Nd 50 Tb 20 Cu 30 alloy. The coercivity of the deformed magnet with 10 wt. % Ce-Fe-B addition increased from 1215 kA/m to 1402 kA/m after diffusion, which is comparable to that of the as-diffused Nd-Fe-B magnets (1495 kA/m). For the magnet with 30 wt. % Ce-Fe-B addition, a 50% increase of coercivity, about 376 kA/m, was obtained by diffusion. The microstructure characterizations demonstrated that the modification of grain boundary phase by the elimination of RE 5 Fe 17 phase and the formation more significant core-shell structure of the Nd-rich hard magnetic 2:14:1 phase by Tb diffusion are responsible for the improved coercivity. • Coercivity abnormal increased by adding 10 wt% Ce-Fe-B in Nd-Ce-Fe-B magnet. • For diffusion, the Nd-Ce core-shell structure of Nd-rich main phase was found. • Tb tends to form (Nd,Ce)-Tb core-shell structure, further enhancing the coercivity. [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]