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In this study, the influence of the content of Al and Co in the diffusion source on the magnetic performance and microstructure of the diffused magnet was discussed by grain boundary diffusion treatment with Pr70Al30–xCox (x=0 at%, 10 at%, 15 at%, 20 at%, 30 at%) alloys. When the Co content in the diffusion source increases from 0 at% to 10 at%, the coercivity enhancement in the Pr70Al20Co10 diffused magnet is the highest, increased from 1.62 to 2.24 T, higher than 2.01 T of the Pr70Al30 diffused magnet. With the further increase of Co content in the diffused source, the coercivity of the diffused magnet decreases gradually, the coercivity of Pr70Al15Co15, Pr70Al20Co10 and Pr70Co30 diffused magnet is 2.15, 1.99 and 1.81 T, respectively. Microstructural analysis shows that plenty of continuous grain boundary phases (CGBPs) can be formed in the Pr70Al20Co10 diffused magnet under the synergistic effect of Al and Co, which leads to the enhancement of magnetic isolation between more adjacent grains. However, the amount of CGBP in the diffused magnets gradually decreases with the further increase of Co content in the diffusion source.

Low cost and high abundance rare earth elements Y and Ce have attracted tremendous interests of the industrial and scientific societies for fabricating the highly cost-performance efficient rare earth permanent magnets. However, the effect of separate replacement of Nd by Y or Ce on the performances of NdFeB-type magnet under the same atomic ratio and preparation conditions is still unclear. In this work, we systematically investigated the magnetic properties, thermal stabilities and service performances of (Nd0.8Y0.2)13.80FebalAl0.24Cu0.1B6.04 (at%, 20Y) and (Nd0.8Ce0.2)13.80FebalAl0.24Cu0.1B6.04 (at%, 20Ce) magnets. The results demonstrate that the μ0Mr, μ0Hc and (BH)max of 20Y magnet are respectively 1.325 T, 1.173 T and 343.467 kJ/m3, which are comprehensively higher than those of 20Ce magnet (μ0Mr = 1.310 T, μ0Hc = 0.948 T, (BH)max = 321.105 kJ/m3). Moreover, the 20Y magnet has higher thermal stability compared with 20Ce magnet which is favorable for the magnetic performances at elevated temperatures. The investigation of microstructure and elemental distribution indicates that the excellent magnetic performances of NdY-Fe-B magnet can be attributed not only to the preferable intrinsic properties 4πMs, Ha and Tc of Y2Fe14B, but also to the in-situ core-shell structure of the 2:14:1 matrix phase grain with Y-rich core and Nd-rich shell, along with the thicker grain boundary layer between the adjacent matrix phase grains in NdY-Fe-B magnet. Furthermore, the 20Y magnet exhibits better mechanical property and higher corrosion resistance than 20Ce magnet, which are helpful for prolonging the service life of the magnet in practical application.

By intergranular addition of Pr-Cu-Ti alloy powders in the Nd-Fe-B sintered magnets with the normal B component, we propose an approach to the optimization of grain boundary and local Nd-Fe-B composition system. The coercivity is enhanced from 1.42 to 1.86 T, while further addition leads to a reduction in remanence and coercivity. The analyses of phase composition reveal that Ti mainly exists in the form of metallic Ti alloy, and part of Ti combines with B to form the TiB2 phase after the liquid phase sintering process. This process results in a consumption of B in the local Nd-Fe-B composition system and a change of the grain boundary component, which contributes to the formation process of the RE6(Fe,M)14 phase after the annealing process. Therefore, with the modification of grain boundary and composition system, the intergranular addition of Pr-Cu-Ti induces the generation of continuous thin grain boundary phases. It promotes the intergrain exchange decoupling, increasing the coercivity in the annealed magnet. While the excess addition results in the segregation of TiB2, as well as the precipitation of TiB2 into the Nd-Fe-B phase, which leads to structural defects. Thus, the further effort for the addition alloy with Ti to reduce the deterioration of the microstructure will lead to a further improvement in magnetic properties.

LaCe-based sintered magnets with different microstructural features and distinct rare earth elemental distribution were designed by dual-alloy method. The sample prepared by fine LaCe-containing powder and coarse LaCe-free powder possesses higher remanence (∼13.41 kGs), whereas another sample prepared by fine LaCe-free powder and coarse LaCe-containing powder possesses higher coercivity (∼5.67 kOe). Additionally, these samples are with the same nominal compositions and their elemental distribution features are obviously different in matrix grains respectively. Their remanence difference is mainly affected by the saturation magnetization difference caused by the distribution variation of the rare earth elements at the matrix phase. The coercivity difference is affected by the component of the grain boundary phase between the adjacent grains and the distribution variation of the rare earth elements at the matrix phase. These findings may provide a new prospect for the utilization of LaCe misch-metal in 2:14:1-type permanent magnets.

We developed the NdY-Fe-B sintered magnet by effective microstructure modification, which exhibits an excellent thermal stability of coercivity with 15 at.% Nd substituted by Y. The feature of microstructure and its inherent link with the magnetic configurations were comprehensively investigated by experimental and micromagnetic simulation approaches. Microstructure analysis confirmed that the novel inhomogeneous distribution of Y in the core and shell regions of the grains was obtained in the whole magnets, contributing to the magnetic hardening of the grain surface areas. TEM observation indicates the tendency of Y depletion in grain boundary (GB) phases, which permits the significantly formation of Nd6Fe13(Ga,Cu)1 phase in the post-annealing magnet, accompanying with the optimization of intergranular GB phase distribution. The thick and Nd-riched intergranular GB phases play a significant role in magnetic isolating the adjacent grains and thus gives rise to a remarkable coercivity increase from 1.16 to 1.72 T. In addition, since the weakened temperature dependence of the magnetocrystalline anisotropy field, the substitution of Nd by Y effectively relieves the deterioration of the coercivity with temperature increasing. Relying on the combined effect of Y substitution on microstructure and intrinsic properties, a low absolute value (0.548%/ °C) of the temperature coefficient (20–120 °C) of the coercivity is achieved.

The high-coercivity Nd-Fe-B magnets with “core-shell” structure grains were prepared by doping TbCu powders and hot pressing after pre-sintering in the traditional powder metallurgy process. The coercivity for the hot-pressed pretreated Nd-Fe-B magnet is obviously enhanced to 28.6 kOe from 24.3 kOe by TbCu alloy doping. The Tb contents at the shell structure of grain in the hot-pressed pretreated magnet is 5.7 wt% while that in the dual-alloy magnet is 3.0 wt%. The enrichment of Tb elements in the shell layer further enhances the anisotropy of the grain surface and effectively improves the utilization efficiency of heavy rare earths. The existence of the hard (Tb, Nd)2Fe14B shell, which can effectively inhibit the reverse nucleation of magnetic domains, contribute to enhance the coercivity with few reduction of the remanence. Compared with the grain boundary diffusion process, this technology can be used to prepare large pieces of high performance Nd-Fe-B bulk magnets without diffusion depth limit.

Al aided TbH2 grain boundary diffusion (GBD) was applied to sintered Nd-Fe-B magnet in comparison with the TbH2 GBD magnet. The coercivity was enhanced from 13.73 kOe to 23.29 kOe, higher than the 22.26 kOe of TbH2 GBD magnet. The Tb content in the bulk magnet was reduced to 0.47 wt%, much lower than the 1.85 wt% of the TbH2 GBD magnet. Microstructure analysis showed that continuous grain boundary phases were formed and completely enveloped the grains with thin Tb-rich shells, which can be attributed to the reduced Tb content in diffusion source and the promoted GBD aided by Al.

Balanced usages of the high-abundance rare-earth elements, such as La, Ce and Y, have attracted tremendous interests of the industrial and scientific societies because of the economic and environmental merits. One of the key issues is of understanding the whole process metallurgical behavior of the high-abundance rare-earth elements in order to optimize the microstructures for a high magnetic performance of the RE-Fe-B sintered magnets, which contain a high concentration of the high-abundance rare-earth elements. In this work, we systematically characterized the metallurgical behaviors during the whole process of strip casting, sintering and annealing for the magnets (Nd0.75LRE0.25)30.5FebalAl0.1Cu0.1B1 (LRE = La, Ce and Y, wt. %). During the strip casting (SC), La and Ce mainly distribute in the grain boundary phase while Y distributes uniformly in the 2:14:1 matrix phase grains. Under the sintering and annealing, La is squeezed out from the 2:14:1 matrix phase to segregate in the grain boundaries, while Ce and Y migrate from the grain boundary to the 2:14:1 matrix phase. The crystalline structures of matrix phase and grain boundary phase of Nd-La/Ce/Y-Fe-B are tetragonal phase (space group P42/mnm) and hcp-Re2O3. The investigations of the grain boundary of Nd-La/Ce/Y-Fe-B magnets demonstrate that the grain boundary of the magnet with Ce is thicker than that of magnets with La and Y. The different metallurgical behaviors of elements La, Ce and Y are attributed to the different solidification temperature and substitution energy of Re2Fe14B phases. Core-shell grain structure was formed in the case of Y-containing magnets. Elemental analysis demonstrates that Y prefers to locate in the core of the grain boundary and Nd distributes in the shell of the grain boundary.

The effects of Ho substitution for Nd on the microstructure, corrosion resistance and thermal stability of the Nd-Fe-B magnets were investigated. The (Nd,Ho)-O phase was formed with increasing Ho substitution. The results of potentiodynamic polarization and highly accelerated stress test show improved corrosion resistance with increasing Ho substitution. The optimum mass loss 0.29 mg/cm2 is achieved. Moreover, the average temperature coefficients for remanence and coercivity in the range of 25–150 °C are both closer to zero, indicating improved thermal stability. The mechanisms for the improved corrosion resistance and thermal stability are discussed in relation to the microstructure featuring the (Nd,Ho)-O phase.

Thermal stability of coercivity and domain structure in sintered Nd–Fe–B magnets with various grain sizes has been studied. Microstructure analysis shows that the improved thermal stability arises from the grain morphology beneficial to a small effective demagnetization factor Neff. In-situ magnetic domain evolution during thermal demagnetization indicates that lower surface stray field in fine-grained magnets suppresses the reversion of domain in high temperature condition, which results from temperature-dependent competition between the local demagnetizing field and effective anisotropy field of surface grains.

In this study, we presented an effective approach for the substantial coercivity enhancement of Dy-free sintered Nd-Fe-B magnets with Ga-doping. The coercivity was substantially enhanced from 10.1 kOe to 19.8 kOe by effective microstructure optimization and phase constitution regulation in antecedent alloys. A good isolation of 2:14:1 grains by Nd-rich intergranular phase is obtained in final magnets accompanying with the formation of intermetallic Nd6Fe13Ga phase. However, when dehydrogenating above 500 °C, masses of preformed Nd6Fe13Ga phases are detected in the initial strip casting alloys and the homologous magnet suffers an unanticipated degradation of coercivity. Magnetic domain observation and thermal magnetic analysis imply a pronounced difference in the magnetism of grain boundary (GB) phases, which closely associated with the composition homogeneity of Nd-rich phases in antecedent alloys. The mechanism of the coercivity variation in the sintered magnets with different GBs magnetism is discussed based on magnetic behavior analysis.

The coercivity of sintered Nd-Fe-Cu-B magnets is markedly enhanced from 12.57 to 21.70 kOe while the remanence decreases from 13.80 to 13.49 kGs by grain boundary diffusion of TbF3 powder for 2 h. Microstructure analysis suggests that, during the diffusion process, F diffuses into the magnets easily and forms a new F-rich phase. The enrichment of F in grain boundary near the surface leads to the Cu movement into the interior and the Cu reduction in the surface of magnets. Diffusion of Tb leads to an increase of local total rare earth elements content. Under the combined effect of Cu reduction and increase of local total rare earth elements content, grain growth area is formed and further diffusion is suppressed. That excessive Tb diffuses into matrix phase leads to a decrease in remanence. When the grain growth area is removed, the deterioration of remanence recovers to 13.80 kGs without any reduction of coercivity.

Commercial sintered Nd-Fe-B magnet was grain boundary diffusion (GBD) processed by Pr60Tb13Al27 and Tb73Al27 alloys. Although the Tb content is only 13 at% in the Pr60Tb13Al27 alloy, the coercivity was significantly enhanced from 15.18 kOe to 26.30 kOe in the magnet diffused by Pr60Tb13Al27, higher than 24.08 kOe of the Tb73Al27 GBD magnet. This suggests that the substitution of Tb(60 at%) by Pr in the Tb73Al27 alloy can further enhance coercivity. Detailed microstructure analysis showed that, in the Pr60Tb13Al27 GBD magnet, the Tb-rich shell thickness was reduced and the two-layer Tb-rich shell was formed. Furthermore, a large number of continuous grain boundary phases was observed, which significantly enhances the exchange-decoupling effect between adjacent grains. The effect of Pr for the Tb-rich shell and grain boundary structure was discussed based on the experimental results.

We investigate the effects of post-sinter annealing on the microstructure and magnetic properties in B-lean Nd–Fe–B sintered magnets with different quantities of Nd–Ga intergranular additions. The magnet with fewer Nd–Ga additions can enhance 0.2 T in coercivity, with its remanences nearly unchanged after annealing. With the further increase of the Nd–Ga addition, the annealing process leads coercivity to increase 0.4 T, accompanied by a slight decrease of remanence. With the Nd–Ga addition further increasing and after annealing, however, the increase of coercivity is basically constant and the change of remanence is reduced. Microstructure observation indicates that the matrix grains are covered by continuous thin grain boundary phase in the magnets with an appropriate Nd–Ga concentration after the annealing process. However, the exceeding Nd–Ga addition brings out notable segregation of grain boundary phase, and prior formation of part RE 6Fe13Ga phase in the sintered magnet. This prior formation results in a weaker change of remanence after the annealing process. Therefore, the diverse changes of magnetic properties with different Nd–Ga concentrations are based on the respective evolution of grain boundary after the annealing process.

Ce-Fe-B sintered magnets with enhanced coercivity were prepared by the powder metallurgy method. The mechanism of the coercivity enhancement in Ce-Fe-B sintered magnets with the low-melting point intergranular additive was discussed in details. It was speculated that the low coercivity of Ce-Fe-B sintered magnet was related to the irregular sharps and relatively low magneto-anisotropy field of the matrix phase. After introducing a 20 wt.% Nd-based intergranular additive, the coercivity markedly increased from 108 Oe to 2560 Oe due to the formation of thin and continuous grain boundary layers and the surface modification of the matrix phase grains. Additionally, the formation of the high anisotropy field (Nd,Ce) 2 Fe 14 B shell was beneficial to the increase of the coercivity as well. This work suggested that adding low-melting point intergranular additives was effective to fabricate the practical Ce-Fe-B sintered magnets.

The commercial 42M Nd–Fe–B magnet was treated by grain boundary diffusion (GBD) with Pr70Co30 (PC), Pr70Al30 (PA) and Pr70Co15Al15 (PCA) alloys, respectively. The mechanism of coercivity enhancement in the GBD magnets was investigated. The coercivity was enhanced from 1.63 T to 2.15 T in the PCA GBD magnet, higher than the 1.81 T of the PC GBD magnet and the 2.01 T of the PA GBD magnet. This indicates that the joint addition of Co and Al in the diffusion source can further improve the coercivity. Microstructural investigations show that the coercivity enhancement is mainly attributed to the exchange-decoupling of the GB phases. In the PCA GBD magnet, the wider thin GB phases can be formed and the thin GB phases can still be observed at the diffusion depth of 1500 μm due to the combined action of Co and Al. At the same time, the formation of the Pr-rich shell can also be observed, which is helpful for the coercivity enhancement.

In this work, the effects of rare earth(RE) elements distribution on the magnetic properties and the microstructure of the (PrNd)–(Y10-xCex)–Fe–B sintered magnets have been studied. A remarkable increase has been achieved in coercivity from 8.85 kOe to 10.39 kOe for x = 2. Microstructure studies show that the recognizable core-shell distribution characteristics are maintained in the matrix phase with x increasing. In addition, high abundance rare earths(HARE) content in the matrix phase is restrained because of amount of Ce-rich intergranular phase forming in the triple junction areas. Combining the merit of the core-shell distribution of Y and reduced HARE content in 2:14:1 phase, a higher magnetocrystalline anisotropy field is obtained. The formation of Ce-rich intergranular phase is also beneficial for the (PrNd)–(YCe)–Fe–B sintered magnets to maintain high Curie temperature. This work suggests that Y-Ce co-substitution can tune the distribution of HARE elements to effectively improve the magnetic properties of the (PrNd)-(YCe)-Fe-B sintered magnets and promote the efficient utilization of RE elements.

One effective approach for obtaining high coercivity in sintered Nd-Fe-B magnets is to refine the grain size.[1] Recently, Sagawa and Une have developed He jet milling and pressless sintering technology for producing fine-grained Nd-Fe-B magnets with high coercivity of 2 T.[2] However, it is hard to refine the grain size further because that the size of lamellar structure consisting of Nd 2 Fe 14 B in conventional strip casting (SC) Nd-Fe-B alloys is as wide as 3 μm, which limits the refinement during jet milling process. Thus precursor alloy with fine grain size surrounded by homogeneous Nd-rich phase is a promising option. Nd-Fe-B alloys with a homogeneous micsturcture and an average grain size of about 0.3 μm can be produced by Hydrogenation-disproportionation-desorption-recombination (HDDR) process.[3] Therefore, more attention has been paid to HDDR powders as precursor alloys for jet milling.[4]

The intergranular addition process using Y72Co28 eutectic alloy powders has been applied to (Pr, Nd)27Dy4Fe67.65Cu0.15Al0.2B1(wt.%) sintered magnets, resulting in an enhanced thermal stability. It was found that the coercivity of magnets at 100 °C increased while the addition of Y72Co28 is 1 wt.%, whereas the magnetic properties decreased slightly at room temperature due to lower magnetocrystalline anisotropy field ( $ {H}\!_{ {A}}$ ) of Y2Fe14B. The temperature coefficient of remanence ( $ \alpha $ ) and coercivity ( $ \beta $ ) for the magnets improved significantly. Microstructure observation indicated that Y atoms prefer to enter into the 2-14-1 phase and form a core-shell structure with an outer layer of (Nd, Dy, Y)2(Fe, Co)14B. Magnetic domain and intrinsic properties analysis suggested that the improved thermal stability resulted from the modified grain boundaries and the introduction of Y in matrix phases.

A novel practice of surface nanocrystallization (SNC) to protect sintered Nd-Fe-B magnet from corrosion was achieved by a controlled hydrogenation-disproportionation-desorption-recombination (HDDR). The SNC magnet showed much better corrosion resistance compared with the originally unprocessed. The nanocrystallized surface of 50 μm thick layer with dispersed Nd-rich intergranular phases played crucial role in blocking intergranular corrosion. For SNC magnet, corrosion test showed a highly oxidized and chemically homogeneous film had formed preventing corrosion deep inwards. Hardness testing revealed the surface was strengthened showing a totally different cracking pattern. To verify its applicable ability, the influences of hydrogen reaction with bulk magnet on microstructures and magnetic properties in different depth were studied. For a large-sized magnet, the detrimental effects were almost negligible. This technique could be a possible new method of surface processing utilized in large bulk Nd-Fe-B magnet for good corrosion resistance and surface strength. Keywords: Sintered Nd-Fe-B magnet, Corrosion resistance, Surface nanocrystallization

Using conventional N2 gas jet milling, a coercivity of 19.01 kOe was obtained in Dy-free sintered Nd–Fe–B magnets with the grain size refined to 2.6 $\mu$ m. Through the high-frequency induction heat treatment (HIHT) at 600°, the coercivity was enhanced to 20.56 kOe, while the remanence remained unchanged. The transmission electron microscope observation showed that the grain boundaries were broadened through the HIHT process. The increasement of coercivity may be related to the strong stirring effect of induction vortex, which facilitates the continuous distribution of the Nd-rich grain boundary phase and thus enhances the magnetic insulation between the matrix grains.

Using electrophoretic deposition (EPD) method, the impact of TbF3 diffusion on the coercivity, microstructure and thermal stability of sintered Nd–Fe–B magnets with different rare earth (RE) content was investigated. In the diffused magnets with the RE content of 34 wt.%, the maximum coercivity about 28.12 kOe with less than 1.44 wt.% Tb was achieved, the coercivity temperature coefficient (β) was improved to -0.50 %/°C from -0.58 %/°C within the temperature interval 25-160 °C, and the maximum operating temperature further increased to about 160 °C. It suggested that TbF3 diffused magnets had much superior thermal stability than the annealed samples. This was attributed to the formation of the Tb-rich (Nd, Tb)2Fe14B phase in the outer region of the matrix grains and the improved Nd-rich grain boundary phase after TbF3 diffusion.

The effect of Nd-Fe additives on magnetic properties and microstructure in (Nd,Ce)-Fe-B sintered magnet has been investigated. By doping 3wt% Nd-Fe additives, the coercivity of the magnet increases from 10.56kOe to 12.69kOe with slight remanence decrease. Microstructure observation reveals that the volume fraction of the grain boundary phase increases accompanying with the thickening of the thin grain boundary between the adjacent grains. The RE6Fe13Cu phase which has low melting temperature develops at the triple junction position and the out shell of the matrix grains get magnetically hardened with 3wt% Nd-Fe additives. The results of the dynamic magnetic domains between the original magnet and processed magnet elucidate that the formation of reversed magnetic domains are more difficult for the processed magnet which is the direct evidence to clarify the coercivity enhancement.