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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.

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.

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.

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.

We investigated the additional (secondary) cooling effect of casted strips on the magnetic properties of Nd–Fe–B sintered magnets. The Nd–Fe–B sintered magnets were fabricated with the casted strips prepared without and with additional cooling. Additional cooling was achieved by blowing Ar gas at various pressures (0.1, 0.3, and 0.6 MPa) on the free-side surface of the strips during the strip-casting process. The higher magnetic properties of Hc, Br, and (BH)max of the final Nd–Fe–B sintered magnets were obtained for 0.1 MPa rather than for 0.0 MPa. The best microstructure of the columnar grains in the casted strips was produced with the aid of a lower pressure of gas on the free-side surface. It was found that the microstructure of the strips affects the distribution of grains grown in the sintered magnets. This report demonstrates that the improved magnetic performance of Nd–Fe–B sintered magnets was achieved via additional gas cooling.

High maximum energy product ((BH)max) Sm(CobalFe0.18Cu0.07Zr0.03)7.7 magnet (type-A) and high temperature Sm(CobalFe0.1Cu0.09Zr0.03)7.2 magnet (type-B) were prepared by a traditional powder metallurgical technology. A record (BH)max of 98.7 kJ/m3 with a coercivity (Hcj) of 501.5 kA/m at 773 K was achieved for the type-B magnet, which is much higher than that of type-A magnet (63.7 kJ/m3). The microstructures of the magnets were revealed by high-resolution transmission electron microscope. The average cell size of the type-A and B magnet are 110 nm and 90 nm, respectively. Moreover, the type-B magnet shows a wider cell boundary than the type-A magnet. Additionally, the element distribution of the cell/cell boundary interfaces was measured by energy-dispersive spectroscopy. The cell phase of the type-A magnet contains a higher Fe content as about 17 at%, comparing with that of the type-B magnet (∼8.9 at%). On the other hand, the Cu content of the cell boundary phase is 18 at% almost twice higher than the type-B magnet (8.6 at%). Theoretical Hcj temperature dependence of these two kinds of magnets indicates that the lower Cu content in the cell boundary phase and the appropriate Fe content in the cell phase are the key factors for the high Hcj for the type-B magnet at elevated temperature.

We investigated the microstructural and magnetic property changes of DyCo, Cu + DyCo, and Al + DyCo diffusion-treated NdFeB sintered magnets. The coercivity of all diffusion treated magnet was increased at 880ºC of 1stpost annealing(PA), by 6.1 kOe in Cu and 7.0 kOe in Al mixed DyCo coated magnets, whereas this increment was found to be relatively low (3.9 kOe) in the magnet coated with DyCo only. The diffusivity and diffusion depth of Dy were increased in those magnets which were treated with Cu or Al mixed DyCo, mainly due to comparatively easy diffusion path provided by Cu and Al because of their solubility with Ndrich grain boundary phase. The formation of Cu/Al-rich grain boundary phase might have enhanced the diffusivity of Dy-atoms. Moreover, relatively a large number of Dy atoms reached into the magnet and mostly segregated at the interface of Nd2Fe14B and grain boundary phases covering Nd2Fe14B grains so that the core-shell type structures were developed. The formation of highly anisotropic (Nd, Dy)2Fe14B phase layer, which acted as the shell in the core-shell type structure so as to prevent the reverse domain movement, was the cause of enhancing the coercivity of diffusion treated NdFeB magnets. Segregation of cobalt in Nd-rich TJP followed by the formation of Co-rich phase was beneficial for the coercivity enhancement, resulting in the stabilization of the metastable c-Nd2O3phase.

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.

Aiming at refining the unideal microstructure and magnetic performance of sintered Ce–Fe–B magnets, La was introduced into the alloy. The role of La addition on the microstructure and magnetic properties revolution of Ce–Fe–B magnet was investigated in this study. The results suggested that La substitution could obstacle the phase structure deterioration during sintering process and maintained high 2:14:1 phase proportion. Meanwhile, uniform grain size and continuous RE-rich grain boundary phase appeared with La addition, which could be attributed to the segregation behaviors of La element and enhanced oxidation resistance with more 2:14:1 phase. The coercivity showed noteworthy increase from 2.93 kOe to 4.78 kOe under the synergetic effect of maintained 2:14:1 phase proportion, uniform RE2Fe[Formula: see text]B grain and even RE-rich phase distribution. This work indicated that La could act as a useful additive to obtain the cost-effective magnetic materials with moderate performance.

In this work, the dependence of microstructure and magnetic properties on the iron content for Sm(CobalFexCu0.073Zr0.024)7.6 (x = 0.226, 0.233 and 0.24) magnets has been systematically studied. The magnet with x of 0.226 (relatively low Fe content) shows a homogeneous microstructure and optimal magnetic properties after it has undergone a solid solution treatment for only 2 h at 1443 K. However, under the same solid solution treatment, a largely uneven composition distribution is observed in magnets with an increased Fe content (x = 0.24). As a result, the peak and gradient of Cu concentration in the cellular boundaries are lower, giving rise to poor magnetic properties. It is impressive that there Cu precipitates are found in the as-solutionized magnet though the cellular structure has not yet occurred. Our results show that a prolonged solid solution treatment (ts) is necessary to improve the microstructure and magnetic properties of the magnets with higher Fe content. Moreover, the optimal ts can be shortened by increasing the solid solution temperature (Ts). However, when the Ts is too high, the composition is not uniform but segregated, and the magnet shows poor magnetic properties as well. Furthermore, it is also found that the homogeneous microstructure and optimal magnetic properties are deteriorated when the magnets undergo a solution treatment at much higher Ts. However, it can be recovered if the magnet is further solution treated at the proper conditions. Our results suggest that the transformation of microstructure and magnetic properties can be controlled reversibly by adjusting the solution treated conditions.

Misch-metal (MM) containing four kinds of rare earth elements of La, Ce, Pr and Nd, leads to the complex multi-main-phase (MMP) structure of MM-based sintered magnets. In order to prepare high-performance MM-based sintered magnets, it is necessary to explore the correlation between the microstructure and properties. Here, the MMP [(Pr, Nd)0.65MM0.35]-Fe-B sintered magnets were prepared by the mixtures of MM-free Pr-Fe-B powders and MM-containing (Nd0.5MM0.5)-Fe-B powders, and the microstructures of the magnets were controlled by tuning annealing conditions to optimize the magnetic properties. The results show that the continuous RE-rich grain boundary (GB) phases are formed between the adjacent main phase grains by annealing, which is beneficial to weaken the short-range ferromagnetic coupling and improve the coercivity. On the other hand, the inter-diffusion in the main phase grains will take place during the annealing, which makes the composition of the main phase grains to homogenize gradually. Thus, the magnetic impedance effect of the MM-rich grains by the Pr-rich grains is weakened, leading to the reduction of coercivity. Combining these two factors, the MMP [(Pr, Nd)0.65MM0.35]-Fe-B magnet annealed at 420 °C possessing optimal microstructure could obtain excellent magnetic properties of Hcj = 11.03 kOe and (BH)max = 38.21 MGOe.

The annealing effects of NdFeB magnets were usually related to the eutectic liquid segregation from junction to grain boundary regions, atoms solid diffusion at grain boundary regions or structure transition of junction phases. In this work, the microstructure and annealing effects of sintered NdFeB magnets with eutectic Pr-Cu boundary additions (0, 5 and 10 wt%) have been systematically investigated. There were two kinds of boundary phases (Cu-lower phase: fcc REO2; Cu-higher phase: h-RE2O3) in Pr-Cu processed magnets. Pr-Cu showed no positive influence on the coercivity of as-sintered magnets and the first-step annealing at 900 °C followed by quenching was also useless. The best annealing temperature (480 °C) for Pr-Cu processed magnets was slightly higher than the lowest eutectic temperature (476 °C). Microstructural investigations demonstrated that the 480 °C annealing showed no influence on the structure transition of boundary phases but changed their distribution patterns. The dominant annealing effect was the formation of strong exchange-decoupling grain boundary phases (GBPs) which were originated from the eutectic liquid segregation. The repeated heating to 900 °C followed by quenching/furnace cooling for optimal annealed (900 + 480 °C) magnet demonstrated reversible eutectic liquid segregation. Annealing under the lowest eutectic temperature (460 °C) was harmful which indicated that the atoms solid diffusion mechanism was not accessible in Pr-Cu processed magnets.

In this study, the relationship between Dy2O3 and MoS2 on magnetic performance and microstructure in Nd-Fe-B sintered magnets was investigated. Compared with the Dy2O3 doped magnets, the coercivity of the Dy2O3/MoS2 co-doped magnets increased with the increasing of MoS2 content. A comprehensive magnetic performance was achieved in the 2 wt% Dy2O3/0.5 wt% MoS2 co-doped magnets. The coercivity increased from 1563.2 kA/m to 1710 kA/m while there was no obvious decrease in the remanence and maximum energy product. Furthermore, the average grain size decreased from 8 μm to 6.7 μm. In the co-doped magnets, the distribution of Dy element was relatively uniform because Dy atoms were found to avoid the Nd2O2S phase. Besides, the Mo atoms were likely diffused into the Nd2Fe14B phase instead of the Nd-rich phase.

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.

Optimization of the microstructure of plates from alloys of the R – Fe – B (R: Nd, Dy) system obtained by pouring the melt onto a rotating copper drum by the method of strip casting is studied. The strip-cast Nd – Fe – B and Nd – Dy – Fe – B alloys with optimized microstructure are used to produce high-energy and high-coercivity sintered magnets.

It is known that the coercivity in Nd-Fe-B magnets is correlated to the grain size as showed by Ramesh et al. [1], where he concluded that coercivity decreases as grain size increases, varying inversely as the logarithm of the square of the average grain size of the magnet.

Sn powders as grain boundary modifiers were applied to improve the corrosion resistance of Nd–Fe–B sintered magnets. Effects of additions of Sn on the corrosion behavior in sulphuric acid and sodium chloride solutions were investigated by potentiodynamic polarization and electrochemical impedance spectroscopy experiments as well as SEM observation. It was found that additions of Sn ⩽ 0.6 wt% can remarkably improve the corrosion resistance of Nd31.8FebalAl0.3B1.0 magnets and optimum addition amount is 0.3 wt%. The improved corrosion resistance is attributed to the increase of chemical stabilization and the microstructural modification of grain boundaries. Also the effect mechanism of additions of Sn on the corrosion resistance of magnets was discussed based on experimental results.

The starting alloy has a strong influence on the processing and the magnetic properties of the sintered magnets. Aiming at the preparation of the high-performance low-cost (Nd, Ce)–Fe–B magnets, the phase constitution and microstructure of Ce–Fe–B alloy are systematically investigated in this work. The results showed that Ce2Fe14B phase coexists with CeFe2 phase in the Ce–Fe–B alloy. A fairly high proportion of 2:14:1 phase (～93.1%) and column crystals structure which is similar to that of Nd-Fe-B alloy could be obtained by regulating the composition of Ce–Fe–B alloy. Owing to the structure optimization of Ce–Fe–B alloy, the sintered magnet which possessed a microstructure of fine grains together with smooth and continuous grain boundaries could maintain the superior magnetic properties of Br = 13.1 kGs, Hcj = 7.79 kOe and (BH)max = 40.49 MGOe by the mixture of (Pr, Nd)12.75Fe80.25M1.0B6.0 and Ce17FebalM2.0B6.0 (M = Al, Cu, Ga, Nb in at.%). This work thus provides useful guideline for the preparation of Ce-doped hard magnets.

Metallographic study of sintered rare-earth Co- 25% Sm magnets microstructure of KS-25 brand was carried out by scanning electron force microscopy (EDS-analysis). A dendritic structure with three phases was found: branches of dendrites are SmCo5; interdendritic space is a mixture of two phases SmCo5 and Sm2Co17. Crystals of 1-5 μm of Zr5Co3FeSm compound and inclusions of Sm2O3 samarium of a globular form of 2-10 μm in size were founded also in the microstructure. Study of the domain structure on surfaces perpendicular to magnetization axis by magnetic force microscopy (MFM) showed presence of strong magnetocrystalline anisotropy. Comparison of magnetic images with electron microscopic images of surface made it possible to conclude that SmCo5 dendrites correspond to large domains ∼ 30-50 μm in size and the interdendritic space consisting of a mixture of two phases SmCo5 and Sm2Co17 correspond to a domain structure in labyrinth form with a size of ∼ 3-5 μm. © 2019 Published under licence by IOP Publishing Ltd. Government Council on Grants, Russian Federation: № 02. Authors are grateful for the support of experimental works by Act 211 Government Russian Federation, contract № 02.A03.21.0006.

Micostructural change and corresponding effect on coercivity of a NdFeB sintered magnet mixed with small amount of DyMn powder was investigated. In the sintered magnet mixed with the DyMn alloy-powder Dy-rich shell was formed at outer layer of the main grains, while Mn was mostly concentrated at Nd-rich triple junction phase (TJP), lowering melting temperature of the Nd-rich phase that eventually improved the microstructural characteristics of the gain boundary phase. The coercivity of a magnet increased more than 3.5 kOe by the mixing of the DyMn alloy-powder.

Anisotropic CeCo4.325−xCu0.675Fex (x = 0.475−0.875) sintered magnets were prepared by traditional powder metallurgical method. Influence of ball-milling time and iron content on microstructure and magnetic properties of the CeCo4.325−xCu0.675Fex sintered magnets were investigated. It is shown that the properties of the magnet produced by magnetic powders ball-milled for 40 min are better than that for 30 min. With iron content increasing, remanence Br and maximum energy product (BH)m increase first and then decrease. The optimal magnetic properties are obtained for the CeCo3.65Cu0.675Fe0.675 sintered magnet: Br = 0.685 T, the intrinsic coercivity Hci = 350 kA·m−1, and (BH)m = 85.6 kJ·m−3. The increase of Br is mainly influenced by iron content of 1:5 matrix which can properly increase the saturation induction Bs; the rapid increase of the amount of Ce-rich phase and 5:19 phase gives rise to the deterioration of the magnets when x ≥ 0.775.

The properties of sintered permanent magnets produced by the traditional process (TP) and by a low-oxygen process (LOP) with the use of (Nd, Dy) – Fe – B rapidly hardened plate alloys are compared. Optimization of the LOP gives highly coercive magnets with world-level properties, i.e., B r ≥ 12.7 kGs, H c ≥ 25.0 kOe, and (BH )max ≥ 38 MGs ∙ Oe.

Peculiarities of the formation of the high-coercivity state of sintered magnets prepared from a rare-earth-metal-depleted Nd-Pr-Dy-Fe-B composition alloyed with terbium hydride were studied. Terbium hydride additions were shown to increase substantially the magnetization coercive force and to decrease slightly the residual inductance. An inhomogeneous distribution of Nd(Pr) and Tb(Dy) within hard-magnetic Nd2Fe14B phase grains was found; the size of regions of inhomogeneity was 30–150 nm.

This study introduces a novel method of electric field sintering for preparing NdFeB magnets. NdFeB alloy compacts were all sintered by electric fields for 8 min at 1000°C with different preset heating rates. The characteristics of electric field sintering and the effects of heating rate on the sintering densification of NdFeB alloys were also studied. It is found that electric field sintering is a new non-pressure rapid sintering method for preparing NdFeB magnets with fine grains at a relatively lower sintering temperature and in a shorter sintering time. Using this method, the sintering temperature and process of the compacts can be controlled accurately. When the preset heating rate increasing from 5 to 2000°C/s the densification of NdFeB sintered compacts gradually improves. As the preset heating rate is 2000°C/s, Nd-rich phases are small, dispersed and uniformly distributed in the sintered compact, and the magnet has a better microstructure than that made by conventional vacuum sintering. Also, the maximum energy product of the sintered magnet reaches 95% of conventionally vacuum sintered magnets.

An evaluation of the effect of alloying elements on the microstructure and magnetic properties of Pr15FebalCo8B7Nb0.05Mx (M = Cu, P, Gd and Ga; 0 ≤ x ≤ 0.25) sintered magnets has been carried out. A mixture of alloys and the high-energy milling technique have been used to prepare the magnets. The alloying elements have influenced the remanence, intrinsic coercivity and particularly the squareness factor (SF). Phosphorus addition improved (BH)max (254 kJm-3 ) and SF around 10% (0.89). The same improvement addition on intrinsic coercivity was observed with Gallium (1100mT) compared to the standard composition Pr15FebalCo8B7Nb0.05 (1000mT) magnet. Comparisons between the squareness factors obtained using the J×μ0H curve profile (SF), the estimated (sf) using microstructural parameters and Sf using a (BH)max and Br correlation have also been carried out.

To reduce the cost of Nd–Fe–B sintered magnets, the ultrafine powders recycled during the jet-milling process were added into intergranular regions by blending. The effects of ultrafine powder content on the magnetic properties, thermal stability and microstructure were investigated. It was found that the intrinsic coercivity H cj was significantly improved from 1048 to 1396 kA m−1 on increasing the ultrafine powder content to 10 wt-%. Meanwhile, the reversible temperature coefficient β H of H cj over the range 20–100°C was decreased in the absolute value, indicating that the thermal stability was improved. Moreover, the maximum energy product (BH)m increased slightly on adding 5 wt-% ultrafine powder, which can be attributed to the combined effects on the normal coercivity H cb and on the remanent induction B r.

Sintered Nd-Fe-B magnets have been widely used due to their excellent magnetic properties, especiallyfor driving motors of hybrid and electric vehicles. The microstructure of Nd-Fe-B magnets strongly affects their mag-netic properties, in particular the coercivity. Therefore, a post-sintering process like heat-treatment is required forimproving the magnetic properties of Nd-Fe-B sintered magnets. In this study, cyclic heat treatment was performed attemperatures between 350 o C and 450 o C up to 16 cycles in order to control microstructures such as size and shape ofthe Nd-rich phase without grain growth of the Nd 2 Fe 14 B phase. The 2 cycles specimen at this temperature rangeshowed more homogeneous microstructure which leads to higher coercivity of 35 kOe than as-sintered one. Keywords : Nd-Fe-B sintered magnets, Cyclic heat treatment, Magnetic properties, Domain pinning 1. 서론 Nd계 희토류 영구자석의 경우 1983년 M. Sagawa에 의해 최대자기에너지적 35 MGOe의 Nd계 희토류자석이 개발된[1] 이후 매우 우수한 자기적 특성으로 인해 일본, 미국, 유럽을 중심으로 활발한 연구가진행되어 왔다. 특히, 최근 수년 전부터는 에너지저감 및 환경친화형 분위기가 세계적으로 중요한 이슈로 부각 되면서 하이브리드/수소연료 자동차의 구동모터 및 발전기용으로 희토류계 영구자석에 대한 관심이 높아지고 있고, 그 수요 또한 급격히 증가하고있다[2]. 현재 개발된 Nd-Fe-B 소결자석의 경우 최대자기에너지적은 이론치인 64 MGOe에 거의 근접해져 있으나 Nd계 영구자석의 가장 큰 문제점인 큐리온도가 낮고 높은 온도에서 자기적 성능의 열화가심하게 진행된다는 단점이 있기 때문에 미래형 자동차 모터에 사용하기에는 적절하지 못하다. 따라서 새로운 합금설계 및 공정의 최적화에 의한 보자력을 향상 시킴으로써 사용온도를 200

The influence of local microstructures on the coercivity of a sintered Nd–Fe–B magnet has been investigated using a combination of magnetic measurements and cross-sectional transmission electron microscopic studies of the interfaces between surface grains of thin Nd–Fe–B magnets and sputter-deposited Nd films on the surface. The TEM study suggested that formation of an Nd oxide phase with a metastable face-centered cubic structure on the surface Nd2Fe14B grains is strongly related to the onset of coercivity in the Nd–Fe–B magnets.

The variations of intrinsic coercivity and remanence of sintered Nd-Fe-B magnets with ultra-high intrinsic coercivity were investigated. The results showed that the intrinsic coercivity and remanence declined simultaneously with increasing temperature, but the squareness of the magnets has hardly been changed. The temperature coefficients of remanence (α) and coercivity (β) for the magnets were calculated by two different methods, and the variations of the temperature coefficients and the microstructure of sintered Nd-Fe-B magnets were analyzed. The temperature coefficients of remanence (a and coercivity (β) for the sintered magnets are very small, and the existence of fine microstructure is necessary to obtain sintered Nd-Fe-B magnets with ultra-high intrinsic coercivity.

The effects of Nb on the coercivity and impact toughness of sintered Nd–Fe–B magnets have been investigated. The results show that the addition of Nb enhances the coercivity of sintered magnets, improves the microstructure and impact toughness of sintered magnets. The optimum impact toughness of sintered magnets was achieved when 1.5 at% Nb was incorporated. The reasons for improving the coercivity and impact toughness of sintered magnets are analyzed.

Due to excellent magnetic properties, sintered Nd-Fe-B magnets have been used in various fields for many years [1, 2]. However, Nd-Fe-B magnets show a relatively low temperature stability of performance which is a main disadvantage for the application in some special cases. In order to improve the temperature stability of performance, Co, Dy and Tb elements have been added to the Nd-Fe-B sintered magnets [2, 3]. It should be pointed out that the addition of heavy rare earth (Tb, Dy) leads to a drastic reduction of the saturation magnetization. In the present work, Pr 34.4 Co 65.6 alloy powder was blended with Nd-Fe-B master alloy powder using dual-alloy method. The effects of Pr 34.4 Co 65.6 powder on the thermal stability of properties and the microstructure were investigated in details.

The influences of solidification rate on the microstructures of cast strips and corresponding sintered NdFeB magnets were investigated. The experimental results show that the volume fraction and size of columnar grains vary with the wheel speed V, and the fraction is highest to more than 90% with 3.5 μm in average width at V = 2 m·s−1. The reasons for the improved magnetic performance were discussed based on the micromagnetic theory and microstructure analyses of the strips and sintered magnets. The magnetic properties of sintered NdFeB magnets made from the (Nd, Dy)13.0(Fe, Ga, Al)80.5B6.5 cast strip which was obtained at V = 2 m·s−1 are as follows: B= 1.15 T(11.25 kG), iHc = 2799 kA·m−1 (35.2 kOe) and (BH)max = 242 kj·m−3(30.35 MGOe).

The two-powder technology including the Dy–Al alloy addition in the form of a separate powder was used to prepare sintered (Nd,Dy)–Fe–B magnets. The influence of both composition and aging parameters on the intrinsic coercive force ( H ci ) was considered. The annealing behaviour of H ci was found to depend on the annealing scenario and cooling rate. The reason for non-monotonic annealing-induced changes in H ci consists in microstructural changes within intergranular Nd-rich regions. The mechanism of phase transformation-induced coercivity in Nd 2 Fe 14 B-sintered magnets is proposed.

The magnetic microstructures of two Dy–Al substituted sintered Nd–Fe–B magnets with the different nominal compositions of Nd12.2Dy0.6Fe80.4Al0.7B6 (at.%) (composition-A, C-A) and Nd13.7Dy0.6Fe78.8Al0.7B6.2(at.%) (composition-B, C-B) prepared by strip casting technique have been revealed by using a magnetic force microscope. The magnetic properties of sintered C-B magnets are worse than that of C-A sintered magnets. In particular, the value of density products (BH)max for sintered C-A magnets is about 32% higher than that of C-B magnets, which is reflected by their quite different magnetic microstructures. We believe that for the C-B samples, the inappropriate composition and thus the redundant Nd2Fe17(B) phase of the casting strips make its final magnetic microstructures worse than the C-A, and then deteriorates the performance of the C-B magnets.