The grain boundary diffusion process (GBDP) has become an important technique in improving the coercivity and thermal stability of Dy-free sintered Nd-Fe-B magnets. The influence of Dy₇₀Al₁₀Ga₂₀ and (Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀ alloys by the GBDP on sintered Nd-Fe-B magnets are investigated in this paper. After diffusing Dy₇₀Al₁₀Ga₂₀ and (Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀ alloys, the coercivity (H_cj) of the magnets increased from 13.58 kOe to 20.10 kOe and 18.11 kOe, respectively. Meanwhile, the remanence of the magnets decreased slightly. The thermal stability of the diffused magnets was improved by the GBDP. The microstructure shows continuous Rare-earth-rich (RE-rich) grain boundary phases and (Dy, Pr/Nd)₂Fe₁₄B core-shell structures which contribute to improving the coercivity. Moreover, the Dy concentration on the surface of the (Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀ diffused magnets decreased with the Pr substitution for the Dy element. The openness of the recoil loops for the (Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀ diffused magnets is smaller than that of the original magnets and Dy₇₀Al₁₀Ga₂₀ diffused magnets. The results show that the (Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀ alloys can effectively optimize the microstructure and improve the magnetic properties and thermal stability of the sintered Nd-Fe-B magnets.

This article investigates the relationship between the magnetic properties of magnets and the percentage and distribution of the CeFe₂ phase at different sintering temperatures. At the lower sintering temperature, the grain boundary phase flow of the magnet is poor, more hole defects are generated in the magnet, and the comprehensive magnetic properties of the magnet are poor. An increase in sintering temperature increases the ratio of CeFe₂ phase, improves the fluidity of grain boundary liquid phase, fills the hole defects and causes an increase in remanence. However, an increase in grain size also inhibits the coercivity of the magnet at this temperature. When the sintering temperature reaches 1080 °C, the CeFe₂ phase ratio continues to increase, providing more liquid phase. The phase Ce₂Fe₁₇ was also decomposed into liquid phase, the continuity and wettability of grain boundary phase were optimized, and the coercivity reached a maximum of 13.18 kOe. However, the orientation of the magnet changed and the proportion of the main phase decreased, resulting in a slight decrease in the remanence (B_r = 13.17 kGs).

Magnetocrystalline anisotropy has many advantages over shape anisotropy regarding coercivity in permanent magnets, making it a promising approach to enhance the coercivity of AlNiCo magnets. In this work, AlNiCo magnets with NdFeB-nanocrystalline phase were prepared by spark plasma sintering (SPS), and the effect of the NdFeB phase on coercivity was uncovered. AlNiCo powder with a spinodal structure and NdFeB powder with a nanocrystalline structure, which exhibit shape anisotropy and magnetocrystalline anisotropy, respectively, were sintered by SPS. With the advantages of low-temperature densification achieved by the SPS process, the spinodal and nanocrystalline structures were mostly retained. The microstructure analysis indicated that the SPS-ed magnet primarily consisted of AlNiCo regions with a spinodal structure, NdFeB regions with a nanocrystalline structure, and a transition region approximately 1~7 µm wide between them. A significant effect of the magnetic anisotropy of the NdFeB phase on magnetization behavior was found. The hysteresis loop of the SPS-ed magnets became single-phase magnetization, in contrast with the double-phase magnetization observed in the simple mixed powder. As the magnetocrystalline anisotropy of the NdFeB phase possesses higher coercivity, the coercivity of the SPS-ed magnet increased from 1250 Oe (of the AlNiCo raw powder) to 2490 Oe. This work provides valuable information for the coercivity enhancement of AlNiCo magnets.

In order to obtain high magnetic properties in Ce-Pr-Nd-Fe-B spark plasma sintered (SPSed) magnets, the alloys were introduced by dual alloy method. The relationship of microstructure, magnetic properties and thermal stability was investigated. The remanent magnetization, coercivity and maximum energy product were sharply enhanced from 0.68 T, 653 kA/m and 75.62 kJ/m3 to 0.80 T, 865 kA/m and 104 kJ/m3 for the magnets that with 0 wt.% and 75 wt.% Pr-Fe-B addition. The Curie temperature also increased from 542 K to 563 K after adding Pr-Fe-B alloys. The scanning electron microscopy (SEM) analysis showed that, during the sintering process, the rare earth elements migrated into the flake boundary and diffused in 2:14:1 main phases, thus (Ce, Pr, Nd)2Fe14B hard magnetic phases were formed, which is helpful to optimize the microstructure and magnetic properties. An enhancement of exchange coupling among the grains of SPSed magnets was obtained as a result of grain refinement by adding Pr-Fe-B alloys.