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Usually the improved coercivity of rare earth (RE) based 2:14:1-type permanent magnets via RE-rich intergranular additives is achieved at the cost of more corrosion channels and deteriorated corrosion resistance, which remains a challenging hurdle in the RE-Fe-B community. Distinctly, here we report the concurrent improvements of corrosion resistance and coercivity in 40 wt.% Ce-substituted Nd-Ce-Fe-B sintered magnets through engineering the intergranular phase using simple (Nd, Pr)Hx additive. The dehydrogenated Nd/Pr changes the RE concentration gradients between 2:14:1 matrix and intergranular phases during sintering and enlarges the fraction of corrosion-resistant REFe2 phase, rather than the conventionally assumed Nd/Pr-rich intergranular phase with high chemical vulnerability. The spontaneous formation of REFe2 intergranular phase after (Nd, Pr)Hx addition generates the uniquely enhanced corrosion resistance against the hot/humid and acidic environments, and counts as one peculiar feature of Nd-Ce-Fe-B magnets at high Ce substitution level, being distinct from previously reported Ce-free/lean RE-Fe-B. Simultaneously, the formation of continuous grain boundaries enhances the coercivity from 8.7 to 12.5 kOe with trace addition of (Nd, Pr)Hx. Above findings may spur progress towards developing a high-performance Nd-Ce-Fe-B permanent magnet.

M.Uenohara, H.Nishio, K.Toyoki, et al. Magnetic properties and magnetic domain observation of ndfeb sintered magnets treated by grain boundary diffusion process with dyal co-sorption. Materials Transactions 62, 1216 (2021); https://doi.org/10.2320/matertrans.MT-M2020389., Effective grain boundary diffusion (GBD) process with DyAl co-sorption is applied to enhance the coercivity of NdFeB sintered magnets. The coercivity of the magnet (HcJ = 1789 kA m⁻¹) subjected to the present GBD treatment was observed to be superior to that of the untreated magnet (HcJ = 1003 kA m⁻¹) and the conventional GBD magnet (HcJ = 1661 kA m⁻¹) treated with DyAl alloy. In the present GBD magnet, the DyAl co-sorption process facilitated Dy diffusion into the center region of the magnet (thickness: 3.5 mm), resulting in high coercivity. Further, magnetic domain observations were made using magnetic force microscopy (MFM) to observe the thermal demagnetization behavior of the present GBD magnet. The present GBD magnet suppressed the continuous domain reversal of adjacent grains; thus, the partially persistent single-domain structure remained, even at 453 K.

The alignment and angular dependences of the coercivity of (Sm, Ce)2(Co, Fe, Cu, Zr)17 magnets were investigated. The coercivity of an aligned magnet is slightly larger than that of the isotropically aligned magnet. This result differs from those of Nd–Fe–B sintered magnets and ferrite magnets. In regard to the angular dependence, the coercivity of (Sm, Ce)2(Co, Fe, Cu, Zr)17 magnets decreases from 0° to 40° and increases thereafter. The trend is similar to that observed in Ga-doped Nd–Fe–B sintered magnets and ferrite magnets and reproduces those expected from a Stoner–Wohlfarth model or a coherent rotation of magnetization, even though the magnetization reversals for these magnets proceed through the motion of magnetic domain walls. The angular dependence of the coercivity of isotropically aligned magnets agrees well with the calculation results for angles up to 50° obtained under the assumption that the magnetization of every grain reverses independently through the motion of magnetic domain walls. These results support strongly the conclusion that the coercivity of Ga-doped Nd–Fe–B sintered magnets and ferrite magnets, both which have an angular dependence similar to (Sm, Ce)2(Co, Fe, Cu, Zr)17 magnets, is determined by the motion of magnetic domain walls.

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.

The effect of doping Cu on the mechanical properties and magnetic properties of Sm(Co0.71Fe0.15Cu0.07Zr0.03)7.6 magnets was studied systematically. The results show that the Cu-lean grain boundaries and bending strength of the magnet can be improved by increasing of doping Cu content. The optimal magnetic properties of magnet with 1.0 wt% Cu doping are $B_{\mathrm {r}} =9.9$ kG, $H_{\mathrm {cj}} =23.0$ kOe, and ( BH ) $_{\mathrm {max}} = 22.2$ MGOe. With the doping content of Cu powders, the bending strength first reaches a peak value of 143.0 MPa at 1.0 wt% and then drops. The bending strength of the magnet with 1.0 wt% Cu doping is about 51.8% higher than that of the magnet with 0 wt% Cu doping. The coercivity and bending strength of the magnet are improved.

The effect of the chemical composition of the alloys on the magnetic properties of sintered Sm – Co – Fe – Cu – Zr magnets produced with mixing of alloy powders is studied. Elevation of (BH )max of Sm – Co – Fe – Cu – Zr magnets due to increase of the concentration of Fe to 20 wt.% requires observation of a strict proportion of the components and precision choice of the modes of heat treatment. Optimization of the chemical composition and use of step annealing with reduction of the temperature from 700 to 400°C has given sintered magnets with the following properties: Br = 11.7 – 11.9 kGs, HcB = 9 – 10 kOe, HcJ = 15 – 20 kOe, and (BH )max = 29 – 31 MGs · Oe.

The origin of low coercivity of high La–Ce-containing Nd–Fe–B sintered magnets was analyzed based on characterization of the microstructure, magnetic domain structure, and magnetization reversal process. It can be found that the coercivity of magnet decreases sharply and the ratio value of coercivity (Hcj) to anisotropic field (HA) is below the theoretical value when the amount of La–Ce substitution for Nd is more than 40 wt%. The grain boundaries with poor magnetic isolation in the high La–Ce-containing Nd–Fe–B sintered magnets play an important role in reducing the coercivity of the magnet. In addition, a larger proportion of the reversible portions can be found for the magnet with a large content of La–Ce, which will lead to the low performance of the magnet.

Massive rare earth oxides inevitably form at high La–Ce substitution, causing inadequate liquid-phase-sintering, direct exchange coupling of ferromagnetic grains, obscure core–shell morphology and severe coercivity degradation of multi-main-phase (MMP) Nd–La–Ce–Fe–B sintered magnets. To fully exploit the abundant La–Ce for application, here we designed a facile approach, (Pr, Nd)Hx grain boundary diffusion that combines the joint benefits of forming thick non-ferromagnetic grain boundary layer and PrNd-rich hardening shell, yielding a nearly threefold coercivity of 14.2 kOe with enhanced energy product of 38.5 MGOe for LaCe-40 magnet. These findings suggest a reliable recipe for relieving the main constraints of MMP Nd–La–Ce–Fe–B magnets.

To reduce the irreversible demagnetization of Nd–Fe–B sintered magnets at elevated temperatures, it is required to discover the method of improving the squareness ratio ( ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ ), where ${H} _{\mathrm{ k}}$ and ${H} _{\mathrm{ cJ}}$ are the knee field and coercivity, respectively. It is pointed out that the intrinsic nucleation field ( ${H} _{\mathrm{ N}}$ ) and instability field of uniaxial single-domain particles are determined the effects of the second anisotropy constant ( ${K} _{2}$ ) and oblique magnetic fields. ${H} _{\mathrm{ N}}$ is influenced by the misaligned grains which deteriorate ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ . Based on the results, we studied mainly the effects of ${K} _{2}$ to the first anisotropy constant ( ${K} _{1}$ ) on ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ for Nd–Fe–B sintered magnets at a temperature range from 298 to 473 K. It has also been examined the values of limiting condition ( $4{K} _{2}/{K}_{1} ) for preventing from the deterioration of ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ . The samples used were Nd–Fe–B sintered magnets with ${H} _{\mathrm{ cJ}}$ of 0.99 and 2.73 MA/m. For a magnet with lower ${H} _{\mathrm{ cJ}}$ above approximately 400 K, the value of ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ decreases rapidly, whereas ${K} _{2}/{K} _{1}$ increases. For a magnet with higher ${H} _{\mathrm{ cJ}}$ , ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ slightly decreases and ${K} _{2}/{K} _{1}$ gradually increases as the temperature increases. However, the temperature changes in the values of ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ and ${K} _{2}/{K} _{1}$ for a magnet with the higher ${H} _{\mathrm{ cJ}}$ are considerably smaller than those of a magnet with the lower ${H} _{\mathrm{ cJ}}$ . The temperature dependence of ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ and ${K} _{2}/{K} _{1}$ shows the opposite trends regardless of ${H} _{\mathrm{ cJ}}$ . The value of $4~{K} _{2}/{K} _{1}$ shows the opposite trend of ${H} _{\mathrm{ k}}/{H} _{\mathrm{ cJ}}$ also. According to a simulation of the torque curve, the direction of the easy axis was widely distributed in the case of a very large value for ${K} _{2}/{K} _{1}$ . The excessive ${K} _{2}/{K} _{1}$ contributes to spread out the region of easy axis, that is, it will hinder the uniaxial anisotropy which is essential for anisotropic permanent magnets. Most of these may be attributed to the effect of large ${K} _{1}$ of indispensable Dy for a very high ${H} _{\mathrm{ cJ}}$ magnet.

Annealing of a (Pr0.53Dy0.47)12(Fe0.77Co0.23)80B8 (at %) alloy is found to affect the magnetic properties of the sintered magnets fabricated from the alloy. Annealing of the alloy is shown to lead to a 8% increase in the remanence and to nonmonotonic variations of the coercive force by 10–12%. The detected 4% decrease in the dysprosium content in the main magnetic phase and 1% increase in the phase content in the sintered magnet can favor an increase in the residual magnetization. Annealing of the alloy does not lead to changes in the temperature coefficient of induction of the sintered magnets. The diffusion of dysprosium is the most probable cause for the detected changes.

Magnetic and structural characteristics of the main phases contained in the Nd−Dy−Fe−Co−B magnetically hard materials (in particular, (Nd0.33Dy0.67)1(Fe0.60Co0.40)4.74B1.12 and (Nd0.90Dy0.10)(Fe0.61Co0.39)3.13 intermetallic compounds) are studied. Parameters of unit cells, saturation magnetizations at 80 and 350 K, and the temperature dependence of the magnetization in the presence of a field of 800 kA/m at temperatures ranging from 320 to 600 K are analyzed. It is shown that the above compounds provide 10–15% contribution to the magnetization of the Nd–Dy–Fe–Co–B sintered magnets even at relatively low concentrations of up to 20%. The results can be used to separate contributions of the main and secondary phases in sintered magnets with different compositions.

The time changes in the magnetization of the magnets (Pr0.51Dy0.49)13(Fe0.64Co0.36)80B7 and (Pr0.51Dy0.49)13(Fe0.64Co0.36)79Cu1B7 (at %) sintered at 160°C are studied. Alloying of the Pr–Dy–Fe–Co–B materials with copper is shown to cause a decrease in the magnetization losses in time.

The alignment dependence and angular dependence of coercivity (ALDC and ANDC, respectively) of aligned and isotropically aligned Ga-doped Nd14.79Ga0.57B5,24Co1.11Febal sintered magnets were analyzed, and the results were compared with those of ferrite magnets, which were reported in our previous paper. The coercivity of Ga-doped Nd-Fe-B magnets decreases as alignment improves. The ALDC of this magnet is similar to that of ferrite magnets. The angle of the magnetization-reversal area (AMRA) of the aligned Ga-doped magnets has at coercivity an angular tilt of 41.4°, which is wider than that of the ferrite magnets. The ANDC of Ga-doped Nd-Fe-B sintered magnets displays similar behavior with that of ferrite magnets but is slightly lower than that of ferrite magnets, which is expected from AMRA of Ga-doped Nd-Fe-B sintered magnets. These results support our proposed coercivity mechanism; specifically, the magnetic domain walls are pinned at grains with tilted easy magnetization directions or $c$ -axis tilted from the easy magnetization of the magnet. When the magnetic domain walls are de-pinned from the pinning sites, they jump through several grains, thereby determining the coercivity.

REFe2 (RE = rare earth) phase has been considered to form spontaneously in Nd-Ce-Fe-B cast strips and inherit into sintered magnets. However, here we report that REFe2 phase, which is absent in starting strip, occurs in as-sintered magnet with an identical composition of 25 wt% Ce. More interestingly, the formation of REFe2 induces apparent Ce-enriched shell surrounding RE2Fe14B grain, which is further exacerbated in annealed magnet containing a higher fraction of REFe2 phase. Influences of increasing REFe2 fraction on magnetic performance are revealed. Above findings on REFe2 phase evolution and resultant Ce segregation may shed lights on fabricating high-performance Nd-Ce-Fe-B.

In this study, the effects of Dy2O3/WS2 co-doping on magnetic properties and microstructure of Nd-Fe-B–sintered magnets were investigated. With WS2 addition, microstructure regulation and coercivity improvement were achieved in the Dy-containing magnets. The optimal magnetic performance was obtained in the 2 wt% Dy2O3/0.4 wt% WS2 co-doped magnets. The coercivity, remanence, and maximum energy product were 1727.1 kA/m, 1.211 T, and 286.5 kJ/m3, respectively. The coercivity was 10.5% higher than the WS2-free magnets, while there was no obvious change in the remanence and the maximum energy product. Relatively clear grain boundary phases were observed with smaller grain size. The average grain size of the co-doped magnets was 6.4 μm, which was about 1.6 μm smaller than the WS2-free magnets. More uniform Dy distribution was also obtained with the Nd2O2S phase formation. Besides, the W element likely precipitated at the grain boundaries.

In this paper, dependence of magnetic properties on microstructure and composition of Ce-Fe-B sintered magnets with Cu-doped Ce-rich alloy addition was investigated. It shows that the maximum energy product (BH)max and coercivity Hcj of Ce-Fe-B sintered magnet are improved from 6.76 to 9.13 MGOe by 35.1%, and from 1.44 to 1.67 kOe by 16.0%, respectively, via adding 5 wt% liquid phase alloy of Ce35.58Fe57.47Cu6B0.95 (at%). Compared with the magnet without Ce-rich alloy addition, the volume fraction of the grain-boundary phase with low melting point increases in the magnet with Ce-rich alloy addition, which is beneficial to improving the microstructure and promoting the coercivity enhancement of the magnet. In the Ce-Fe-B magnet with Ce-rich alloy addition, Cu and Ce enrich in the grain boundaries of the magnet after annealing, therefore the as-annealed magnet has a higher coercivity than the as-sintered magnet. A distinct Fe-rich layer with the average thickness of 60 nm is found in the grain boundaries in the magnet without Ce-rich alloy addition, but it seems that Fe-rich phase disappears in the magnet with Ce-rich alloy addition. The present work suggests that the further improvement of coercivity in the Ce-Fe-B sintered magnets is expectable by designing the composition and structure of added liquid phase alloys.

—Changes in the magnetic properties of the sintered (Pr0.76Dy0.24)12.9(Fe0.53Co0.47)81.5B5.6 material, which are caused by solid-phase alloying with the (Pr0.63Dy0.37)(Fe0.31Co0.69)2B2 alloy are studied. The dependences of the phase composition and the coercive force of the sintered material on the amount of the added alloy is obtained. As the boron content in the sintered material increases, the content of the main magnetic (Pr, Dy)2(Fe, Co)14B phase decreases from 89.2 to 51.4 vol % and the content of the (Pr, Dy)(Fe, Co)3 phase increases from 7.1 to 43.9 vol %. It was found that, in the course of 190-h storage of sintered magnets in air, their coercive force and density are first unchanged and subsequently decrease; the coercive force decreases to zero.

Ce-based magnets have attracted extensive attention in both academia and industry due to their excellent property–price ratio and distinctive phase structures. Characteristically, Ce-based sintered magnets have widely tunable magnetic properties with changing Ce contents. Therefore, they can be used to meet many different application requirements from packaging market to driving motors, etc. The intrinsic magnetic properties, phase composition, and microstructures of the Ce-based sintered magnets with different Ce contents have been summarized. The service performances such as temperature stability, corrosion resistance, mechanical properties and thermal expansion of commercial Ce-based sintered magnets are introduced. The research and development trends of the Ce-based magnets in the future are pointed out.

Dy distributions in Dy-free and Dy-containing Nd Fe B sintered magnets in the course of the Dy-vapor grain boundary diffusion (GBD) process have been investigated in order to understand the origin of the high coercivity of 3.0 T that is reachable only when the initial magnet is alloyed with Dy. We have discovered the formation of a secondary Dy-rich shell within the well-known primary Dy-rich shell, which is a key contributor to the 3.0 T coercivity. The coercivity increment of the Dy-containing magnet after the GBD treatment was only 0.08 T, much lower than 0.87 T for the Dy-free magnet; however, it was substantially enhanced by a post-diffusion annealing. Compared to the Dy-free magnet, a larger amount of Dy atoms were diffused from the Nd-rich grain boundary (GB) phase to the primary Dy-rich shell in the Dy-containing magnet after the annealing, resulting in the formation of a secondary Dy-rich shell with a higher Dy-concentration at the GB phase/secondary shell interfaces.

Herein, we probed the microstructure of Tb-diffused Nd-Fe-B magnets to investigate the relationship between Tb-diffused area and coercivity enhancement, employing prolonged stepwise heat treatment to ensure sufficient diffusion of Tb in relatively large-size magnets and revealing that this stepwise annealing generated core-shell structures. Quantitative compositional changes pertaining to individual phases of the multiphase system in each heat treatment process were analyzed by constructing ternary diagrams based on electron probe microanalysis compositional maps. During the grain boundary diffusion process, coercivity increased from 15.28 to 24.86 kOe, while only negligible remanence and energy product decreases were concomitantly observed. Microstructure analysis suggested that coercivity was closely related to the concentration and distribution of Tb; more precisely, the abovementioned core-shell structures successfully suppressed the nucleation of reverse domains at Nd-rich phase/main phase interfaces and therefore enhanced magnet coercivity without decreasing remanence and energy product.

In this paper, the Ce-Fe-B and (La, Ce)-Fe-B strips were prepared by strip-casting technique and the multi-phases RE-Fe-B (RE = La, Ce, Pr and Nd) magnets were fabricated by dual-alloy method with (Pr, Nd)-Fe-B alloy. The effects of La substitution for Ce on the microstructure of Ce-Fe-B strip-casting alloy were investigated, especially for the formability of 2:14:1 phase. The results showed that the proportion of RE2Fe14B phase significantly increased from 0.0% to 48.0% and the CeFe2 phase drastically decreased from 67.1% to 38.1% with La substitution for 35 wt% Ce in Ce-Fe-B strip-casting alloy. It could be attributed to the enlarged temperature gap between the formation of 2:14:1 phase and Ce2Fe17 phase, which was beneficial for the formability of 2:14:1 phase. Owing to the rise of the proportion of the 2:14:1 phase, the remanence (Br) and maximum energy product ((BH)max) of dual-alloy magnets increased from 11.49 kGs, 28.68 MGOe to 12.38 kGs, 34.76 MGOe with respect to 32.0 wt% Ce or La-Ce substitution content for Pr-Nd, respectively. Meanwhile, the coercivity (Hcj) slightly decreased from 6.49 kOe to 6.16 kOe, which was caused by the decrease of HA (54.09 kOe → 42.37 kOe) and the deterioration of microstructure of the magnet. This work indicated that the cost-effective La-contained RE-Fe-B permanent magnets could be prepared by dual-alloy method.

Hydrogen treatment technology is developed to recycle the waste Nd-Fe-B sintered magnets in the way of short route, low cost, and high efficiency. In present study, effect of hydrogen pressure on hydrogen decrepitation (HD) process of sintered Nd-Fe-B strip casting flakes (SC) and waste sintered magnets (SM) was systematically studied. Both SC and SM show the approximate HD process, which accelerates with increasing pressure. The surface activation process accelerates with increasing pressure, and almost disappears when the initial hydrogen pressure reaches to 6 MPa. The hydrogen content of SM is lower than SC, and increases with increasing pressure. For the SM, blasting power of HD process decreases with the increase of the initial hydrogen pressure. It provides beneficial reference for recycling waste Nd-Fe-B sintered magnets.

Wetting experiments of Tb70Cu30, Pr52.5Tb17.5Cu30, Pr60Tb20Al20 and Pr60Tb10Cu15Al15 (at.%) ingots over Nd-Fe-B sintered magnet were carried out and suggested that alloys containing Al possessed better wettability and mobility. Interfacial analysis suggested that at interface between Pr60Tb20Al20 and magnet, Tb mainly diffused along grain boundaries without abnormal Tb-rich grains, which were observed in the systems of Cu-added ingots. On the basis of these, grain boundary diffusion with alloy ribbons was applied and coercivity increment of over 11 kOe was achieved by consuming below 0.5 wt.% Tb in diffused magnet by Pr60Tb10Cu15Al15, which was mainly attributed to homogeneous and thin Tb-rich shells.