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The coercivity of the sintered Nd-Fe-B permanent magnets prepared by the dual-main-phased (DMP) process can be significantly enhanced. This paper provides a comprehensive review of this method. The methodology and process of the DMP method are outlined, along with its mechanisms and advantages in improving magnet performance. Furthermore, this study highlights the outstanding performance of DMP magnets in terms of mechanical properties and corrosion resistance. Lastly, the future development trends and research directions for the DMP method are explored, such as optimizing process parameters, exploring novel alloy designs, and developing higher-performance magnet materials. [ABSTRACT FROM AUTHOR]

To respond to the gradually increasing demand for thermal stability of sintered NdFeB magnets and grasp the evolution rules of their phase structure, research on sintered Nd-Dy-Fe-Co-B magnets was conducted. The magnets used in this study were prepared by the powder metallurgy method (Strip Casting + Hydrogen Decrepitation + Jet Milling). The room temperature magnetic properties of the magnet sample were B r = 11.45 kGs and H cj = 22.42 kOe, with a low-temperature coefficient of B r (α Br (20 °C∼100 °C) = −0.059 %/°C) and a high Curie temperature (T c = 460 °C). Based on the microstructure observation, there were planar anisotropic soft magnetic phase Nd 2 Co 17 within grain boundaries and the "Cu-Co Opposite Distribution". Low-melting-point binary alloy Pr 80 Cu 20 was introduced into the magnet by grain boundary addition to eliminate Nd 2 Co 17 phases and improve the coercivity. After the grain boundary addition, the "Cu-Co Opposite Distribution" was used to eliminate the Nd 2 Co 17 phase, the grain boundary structure was optimized, and the coercivity was increased by approximately 2.6 kOe, which was approximately 10 % of the total coercivity. Therefore, Cu-containing low-melting-point binary alloy grain boundary addition is an effective method for optimizing the grain boundary structure of Nd-Dy-Fe-Co-B magnets, thereby improving the high-temperature stability and coercivity. • The "Cu-Co Opposite Distribution" was found in the Nd-Dy-Fe-Co-B magnets. • The planar anisotropic soft magnetic phase Nd 2 Co 17 was found in grain boundaries. • Pr 80 Cu 20 binary alloy grain boundary addition is an effective method for optimizing the grain boundary structure. • The orientation behavior of sintered NdFeB magnets cannot be changed by the substitution of Co. [ABSTRACT FROM AUTHOR]

Fe-rich Sm 2 Co 17 -type magnets have many essential applications prospects in high-temperature operation environments. However, the formation of complete cellular microstructure becomes increasingly difficult when the Fe content of Sm 2 Co 17 -type magnets exceeds 20 wt%. Herein, the phase stability of 1:7 H phase and its effect on the precipitation of the 1:5 H precipitates in Fe-rich Sm 2 Co 17 -type magnets are investigated. The results confirm that the stability of the 1:7 H phase decreases with the increasing of Fe content for the solution-treated magnets. Not only 2:17 R phase but also a lot of 2:17 R′ phase is found in solution-treated magnets when the Fe content exceeds 22 wt%. As the Fe content increases, the density of initial defects decreases for the growth of 2:17 R nanotwins in the solution-treated magnets. Furthermore, the segregation of Cu in the defects-aggregated cell boundaries weakened gradually. Consequently, the nucleation and precipitation of cell boundary precipitates is difficult, and the cellular microstructure becomes incomplete and coarse when the Fe content is above 22 wt%. The present work may provide a further insight for understanding the evolution of cellular microstructure depending on the phase decomposition behavior in Fe-rich Sm 2 Co 17 -type magnets. • Various 2:17 R′ phases were found in solution-treated Sm 2 Co 17 -type magnet when the Fe content is above 22 wt%. • The ordering of 2:17 R phase during the solution treatment is not conducive to the segregation of elements. • According to XRD Rietveld analysis, the stability of 1:7 H phase decreases with the increase of Fe contents for the solution-treated magnet. [ABSTRACT FROM AUTHOR]

The Pr 80 Al 20 (at%) alloy was diffused into the dual-main-phase Ce magnets (Ce/TRE = 40 wt.%, TRE: total rare earth) with different diffusion time by grain boundary diffusion process (GBDP). The magnetic property and the microstructure of a series of magnets with different diffusion time were investigated. The coercivity gradually increased as the diffusion time increased, while the remanence remained basically constant. The improvement of coercivity originated from the synergistic contribution of modified grain boundaries with decoupling effect and Ce-rich main phase containing Pr with improved magnetocrystalline anisotropy. The percentage of the irreversible portion and the nucleation field of the reverse domain calculated from recoil loops curves further confirmed that the diffusion of Pr 80 Al 20 alloy enhanced the coercivity. The enhanced anisotropy of the Ce-rich grains compensates for the decay of the remanence carried by the diffusion-induced increase in the nonmagnetic grain boundary phase, which ultimately results in the remanence remaining essentially constant. The above phenomena further broaden the room-temperature applications of dual-main-phase magnets with high Ce content without heavy rare-earth grain boundary diffusion. • Dual-main-phase magnets with high Ce content were diffused with light rare earth Pr-Al alloy. • Pr element mapping contrast clearly indicated sufficient diffusion inside the magnet. • Small amounts of Pr entered into the Ce-rich main phase grains to improve magnetocrystalline anisotropy. • Synchronous enhancement mechanism in coercivity and maximum energy product of the magnets was revealed. [ABSTRACT FROM AUTHOR]

SmFe 12 -based magnets have attracted considerable attention because of their high intrinsic magnetic properties. The metastable ThMn 12 -type SmFe phase can be easily decomposed into the α-Fe phase during heat treatment process. The nanocomposite theory of soft and hard magnetic phases is used to improve the magnetic properties of SmFe 12 -based magnets. At present, it is difficult to obtain strong exchange coupling in SmFe 12 /α-Fe nanocomposite magnets prepared by melt spinning or melt spinning followed by isothermal heat treatment. This study proposes a SmFe 12 /α-Fe nanocomposite magnet with a maximum energy product of 84.5 kJ/m3 by melt spinning followed by rapid thermal processing (RTP). The effects of RTP time on the phase composition, microstructure, and exchange coupling of the magnets were systematically studied. The results showed that a suitable RTP time benefitted the formation of fine and uniform 1:12 phase grains in the magnet. Henkel curves, magnetic domains and quantitative analysis based on the exchange coupling volume were employed to study the exchange coupling of the magnets, which indicated that the enhanced hard/hard-exchange coupling strength originating from the uniform and fine hard magnetic 1:12 phase grains is the main reason for the improvement in the overall exchange coupling strength. We believe this study provides a new technology for preparing high-performance SmFe 12 /α-Fe nanocomposite magnets with a high exchange coupling strength. [Display omitted] • By rapid thermal processing, a SmFe 12 /α-Fe nanocomposite magnet with the (BH) max of 84.5 kJ/m3 was prepared. • Rapid thermal processing improved exchange coupling within SmFe 12 /α-Fe nanocomposite magnets. • The enhanced interaction between the grains of hard magnetic phase is the main reason for the enhanced exchange coupling. [ABSTRACT FROM AUTHOR]

In this study, we investigated the effects of magnetic interactions within Nd-Fe-B magnets on first-order reversal curve (FORC) and the origin of negative regions in FORC diagram. Magnetostatic interaction causes the FORC diagram pattern to expand along the H u axis, especially in the direction of positive half-axis of H u. And the negative regions will tend to be embedded in the positive regions when magnetostatic interaction is not negligible. The origin of the negative peaks is the difference of the onset of magnetization and the subsequent magnetization process of adjacent FORCs caused by the different and complex stray fields within magnets at the beginning of the FORCs. Besides, the origin of the positive peaks is analyzed in detail as well. • The detailed effects of magnetic interactions within Nd-Fe-B magnets on their FORC are studied. • The origin of the negative region in the FORC diagram from a microscopic perspective is revealed. • Magnetostatic interaction has a significant impact on the relative position distribution of positive and negative regions. [ABSTRACT FROM AUTHOR]

• A short-range grain boundary diffusion process is proposed to improve the uniformity of coercivity. • The disappearance of α-Fe phase and the formation of intergranular phases contribute to huge improvement of coercivity. • Anisotropic hot deformed Nd-Fe-B magnets with high performance were fabricated. • The metallurgical behavior of elements and intergranular phase evolution are investigated. [Display omitted] For usual grain boundary diffusion, the diffused source infiltrates into the magnet from the surface, and a long diffusion distance leads to non-uniform distribution of elements, which seriously restricts the improvement of properties and reduces the uniformity of coercivity. In this work, based on nanocomposite Nd-Fe-B ribbon, a short-range grain boundary diffusion (SRGBD) process is employed to improve coercivity, obtaining a high coercivity of 24.27 kOe in PrCuAl-diffused nanocomposite powder, increasing by 233.0% comparing to the initial ribbon. By spark plasma sintering (SPS) technique, an isotropic magnet with high coercivity of 24.13 kOe is prepared, and meanwhile, good uniformity of coercivity is also achieved. The disappearance of the α-Fe phase and the formation of intergranular phases contribute to a huge improvement of coercivity. At last, with the help of the intergranular phase, established by SRGBD, anisotropic magnets are successfully prepared through hot deformation, which the optimum magnetic properties with H ci = 18.99 kOe, J r = 1.08 T, and (BH) max = 215 kJ/m3 can be achieved. [ABSTRACT FROM AUTHOR]