Rare Earth-rich Phase Research Articles

High‐Cerium‐Content Fe–Ce–Nd–B Sintered Magnets with High Coercivity

Ce substitution of Nd in FeNdB sintered magnets is very interesting for reasons of resource efficiency, sustainability and costs. However, the magnetic properties of high Ce‐content magnets are poor. This is mainly due to the lower values of intrinsic magnetic properties of Fe14Ce2B compared to Fe14Nd2B as well as the Laves phase Fe2Ce, which is formed in the grain boundaries and forms flat aggregates for higher Ce‐content. In this article, sintered magnets with 75 at% degree of substitution of the composition Fe70.9–[(Ce1−xLax)0.75Nd0.25]18.8–B5.8–M4.5 (M = Co, Ti, Al, Ga, Cu, 0 ≤ x ≤ 0.3) are produced and analyzed. It is shown that a new rare earth rich grain boundary phase is formed adding La and that the Laves phase fraction can be reduced by up to 85%. With increasing La content, the remanence and Curie temperature increase and the temperature coefficient of Hc improves. A coercivity, remanence, and maximum energy density of up to μ0Hc = 0.74 T, Jr = 0.98 T, and (BH)max = 139.9 kJ m−3 have been achieved.

Deformation mechanism and hardening behavior of gradient heterostructured magnesium alloys prepared by severe shear deformation

Lightweight equipment in aerospace and other fields urgently requires magnesium (Mg) alloys components with high-strength plasticity. For this reason, in this study, a gradient heterogeneous structure containing nanocrystallines was constructed successfully using a severe shear deformation process that combines severe plastic deformation techniques and torsional deformation characteristics. The evolution of the microstructure and hardness of Mg-12.12Gd-4.2Y-2.28Zn-0.36Zr (wt%) alloys prepared by severe shear deformation with changes in deformation temperature were investigated. The gradient heterogeneous structure of the alloys formed naturally under high hydrostatic pressure and severe shear stress, and its thickness increased with the deformation temperature. With the application of shear stress, a large amount of rotational dynamic recrystallization (RDRX) behavior occurs in the alloys, refining the coarse initial grains (∼126.3 µm) into fine grains (∼2.03 µm) or even ultrafine grains (∼1 µm). During the RDRX process, low angle grain boundaries (LAGBs) began to form at high distorted grain boundaries and unique lamellar structures, followed by a rapid expansion into the interior of the grains, which ultimately results in the grains being completely covered by subgrains. Meanwhile, the continuously broken long-period stacking-ordered (LPSO) phase and the precipitated bright rare earth rich (RE-rich) phase provide a large number of nucleation sites, further promoting the refinement process of the grains. Subsequently, the grains were further refined to nanocrystallines through the formation and rotation of subgrains under continuous loading of stress. The massive formation of nano-precipitated phases (β phases) along grain boundaries were accompanied by the complete disappearance of other types of second phases. The sources of β phases formation include the layer-by-layer decomposition of the cellular phases and the dissolution/re-precipitation of the LPSO phases. In addition, the introduction of shear stress promotes the random orientation of the DRXed grains through the extensive activation of pyramidal <a> and pyramidal <c+a> slip. The hardness of the alloys gradually decreased as the cumulative strain decreased at different deformation temperature conditions. When the deformation temperature was 440°C, the alloys reached a maximum hardness of 127.67 HV, which was nearly 43.5% higher than homogenized alloys. The ultra-high hardness of the alloys prepared by severe shear deformation is mainly attributed to grain boundary hardening, diffusion hardening and hetero-deformation induced (HDI) hardening.

Strategy to improve magnetic property of sintered (Nd, Ce)–Fe–B magnets: Moderate replacement of cerium with lanthanum

In this work, we proposed a method to enhance the magnetic properties of (Nd, Ce)–Fe–B magnets with Ce/TRE ratios below 25 wt% by introducing a moderate amount of La elements. The segregation behavior of La elements towards grain boundaries (GBs) was utilized to optimize the GB phase structure. Incorporation of La atoms into the main phase induces lattice expansion, leading to an increased formation of Ce3+ ions with enhanced magnetic moments. Comparative analysis with the original magnet (La/Ce = 0 wt%) demonstrates that the magnet with a La/Ce ratio of 10 wt% exhibits improvements of 0.3% in remanence, 12.6% in coercivity, and 0.6% in maximum energy product. These results underscore that the moderate addition of La elements enhances the fluidity of the rare earth-rich phase and optimizes the distribution of lamellar GB, consequently reinforcing the magnetic isolation effect. Furthermore, the promotion of the transformation from Ce4+ to Ce3+ ions contributes to the comprehensive enhancement of the magnetic properties. This research offers a novel strategy for fabricating high-performance and resource-efficient sintered magnets based on LaCe alloys.

Comparison of microstructure and thermal-fluid coupling heat transfer behavior of strip-cast Nd-Fe-B flakes at different cooling roller speeds

Strip-casting is a key step among processes for manufacturing the Nd-Fe-B permanent magnets. Current research on the process parameters of the strip casting process mostly relies on the trial-and-error method. In this paper, the influence of the cooling roller speed on the strip-cast Nd-Fe-B flakes is investigated by a combination of simulation and experiment. The microstructures of the strip-cast flakes prepared at different roller speeds are observed and analyzed. The simulation is further performed by using the thermal-fluid coupling method. The temperature field and cooling water flow field of the cooling rollers at different roller speeds are obtained to explain the principle of the microstructural evolution. The temperature field distribution affects the uniformity of the columnar grains on both sides of the strip-cast flake. Higher or lower temperature can also lead to the formation of α-Fe, cluster rare earth (RE) rich phases, or ultra-fine equiaxed grains. An increase in cooling water flow rate is helpful to obtain the columnar grain with even thickness. The simulation results are consistent with the microstructural results. Furthermore, the simulated results and microstructural analyses are combined to indirectly evaluate the trend of magnetic properties of final sintered magnets from the tested strip-cast flakes. By verifying the performance of magnets prepared under the condition of only changing the roller speed during the entire process flow, the measurement results are consistent with the evaluation results, proving that the combined analysis proposed in this paper can reasonably predict the trend of magnetic properties of sintered Nd-Fe-B permanent magnet materials from the strip-cast flakes prepared at different roller speeds. This paper can provide a transferable idea and method for using numerical simulation to assist in the design and production of Nd-Fe-B flakes and sintered magnets.

The effect of Nd content on the elemental interdiffusion mechanism of Nd–Dy–Fe–B magnets

In this work, the effect of Nd content on the interdiffusion mechanism of Nd/Dy elements in Nd-Dy-Fe-B magnets was elucidated by studying the differences in the magnetic properties, microstructure, and demagnetization process. As the Nd content increased, a uniform, continuous, and thicker layer of rare earth-rich (RE-rich) phase formed inside the magnets, which had a good magnetic isolation effect. During the demagnetization process, the strong ferromagnetic coupling effect between the main phase grains could be effectively weakened. At the same time, the thicker RE-rich phase layer could lead to a more obvious interdiffusion phenomenon of Nd/Dy elements, forming more (Nd, Dy)2Fe14B phases with high magnetocrystalline anisotropy fields, resulting in higher coercivity of the magnets. This paper elucidated the influence of Nd content on the interdiffusion mechanism of dual alloy Nd-Dy-Fe-B magnets, which had important significance for the preparation of high-performance Nd-Dy-Fe-B magnets.

Heterogeneous deformation-induced strengthening of extruded Tip/WE43 composites

Titanium (Ti) particles reinforced WE43 (Mg-4Y-2Nd-1Gd-0.5Zr) matrix composites (Tip/WE43) were prepared via ultrasonic-assisted stirring casting and following hot extrusion. Ti particles can refine the grains and promote the precipitation of the second phases in magnesium (Mg) matrix. Meanwhile, bimodal structures combining coarse deformed grains and recrystallization equiaxed grains were formed. The generation of heterogeneous interfaces promotes heterogeneous deformation-induced (HDI) strengthening. Extruded 3Tip/WE43 with a yield strength (YS) of 241 MPa, an ultimate tensile strength (UTS) of 283 MPa, and an elongation (EL) of 17.3%, respectively, possesses an excellent strength–ductility combination. The interfacial product of Ti/Mg was mainly composed Ti3Al layer with about 200 nm, which was conducive to interfacial bonding and stress transfer. The dislocation entanglements and stacking faults inside the Ti particles can promote the coordinated deformation between the Mg matrix and Ti particles. The rare-earth rich phases Ti2ZrAl, Mg41Nd5, and Mg24Y5 precipitated in the matrix can hinder dislocation movement by activating Orowan strengthening. The key factors of YS increment of the composites were considered to be HDI strengthening, fine grain strengthening, and thermal mismatch strengthening.

Clarifying the dominated coercivity enhancement mechanism of grain boundary diffused Nd-Fe-B magnets by Pr-based alloys

Low-melting point Pr-based alloys have been confirmed to be cost-effective grain boundary diffusion sources for enhancing the coercivity of Nd-Fe-B magnets. Various coercivity enhancement mechanisms have been reported for these alloys so far, but the dominated one has not been clarified. Here, the sintered Nd-Fe-B magnet was treated by Pr75Al25 alloy grain boundary diffusion at different temperatures. A detailed investigation on the diffusion kinetics of the elements and the microstructure evaluation have been carried out. After 800 °C diffusion, the intrinsic coercivity of the magnet increased from 1070 kA/m to 1348 kA/m without significant reduction of remanence. The increased rare earth-rich phase continuously distributed along the grain boundaries has great contribution to the magnetic decoupling, leading to high coercivity. In comparison, after 900 °C diffusion, although Pr and Al diffused more sufficiently in the magnet, a drastic lattice diffusion occurred and less grain boundary layer was formed, resulting in insufficient coercivity enhancement. The results thus indicate that, for Pr-Al diffusion, the formation of RE-rich layer is more important than that of (Nd,Pr)2Fe14B shell in order to enhance the coercivity. Therefore, different from that for heavy rare earth diffusion, the diffusion treatment for Pr-based sources should be carefully optimized to form continuous grain boundary phase and suppress the excessive lattice diffusion.

Simultaneously enhancing coercivity and remanence of hot-deformed Nd-Fe-B magnets by flake copper powder assisted DyF3 interflake addition

It is still a challenge to simultaneously enhance coercivity (Hcj) and remanence (Jr) of hot-deformed Nd-Fe-B magnet due to the coercivity-remanence trade-off dilemma. Here, we achieved this balance between Hcj and Jr by flake Cu powder assisted DyF3 interflake addition. The Hcj increases from 1218 to 1496 kA/m and Jr increases from 1.32 to 1.34 T compared with the original magnet. Results show that the width of coarse grain layers reduces because of the introduction of flake Cu, which increases the contact areas of the adjacent grains at ribbon interfaces and suppresses the excessive growth of grains. The stronger degree of texture and higher density compared with the original magnet should take the responsibility for the increase of Jr. Additionally, the aggregation regions of rare earth rich (RE-rich) phase reduce and the betterment of the microstructure is another reason for the enhancement of Jr in the flake Cu aided DyF3 hot-deformed magnet. This strategy of using flake powder additives provides a promising method for optimizing microstructure and enhancing magnetic properties of hot-deformed Nd-Fe-B magnets.

Architecture of new {Ca, Eu, Yb}4CuMg complex intermetallics based on polyicosahedral clusters

The crystal structures of R4CuMg (RCa, Eu, Yb) complex intermetallics, solved by single crystal X-ray diffraction, were found to represent a new trigonal prototype (hR144-h7c2ba, space group R3‾m), being the first examples of 4:1:1 rare earth-rich phases not crystallizing as cF96-Gd4RhIn, adopted by more than 200 compounds known so far. The divalent character of the R component, together with Cu atomic size could contribute to explain the observed structural peculiarity.Indeed, the occurrence of fragments typical of compositionally similar compounds, namely Cu-centered trigonal prisms and Mg-centered core-shell polyicosahedral clusters with R at vertices, prompted a search for significant structural relationships. In this work, a description of the rhombohedral R4CuMg crystal structure is proposed as a linear intergrowth along the c-direction of the two types of slabs R9.5CuMg3.5 (parent type: hP28-kh2ca, SG 194) and R13Cu6Mg (parent type: hR60-b6a2, SG 160). The ratio of these slabs in the studied structure is 2:1, corresponding to the simple equation 6×R9.5CuMg3.5+3×R13Cu6Mg = 24×R4CuMg, normalized to the unit cell content.According to the proposed description, symmetry properties of the slabs (depicted by the p3m1 layer group) were used for screening of possible space groups for other potential members of the same/similar intergrowth families.

Intermetallic compounds RE 2Ga2Mg (RE = Tb–Tm, Lu) with Mo2B2Fe-type structure

Abstract The rare earth intermetallic compounds RE 2Ga2Mg with RE = Tb–Tm and Lu were synthesized from the elements in sealed tantalum ampoules in a high-frequency furnace. These rare earth-rich phases crystallize with the tetragonal Mo2B2Fe-type structure, space group P4/mbm and Z = 2. The polycrystalline samples were characterized through their Guinier powder patterns. The structures of Er2Ga2.092(1)Mg0.908(1), Tm2Ga2.037(1)Mg0.963(1) and Lu2Ga2.176(1)Mg0.824(1) have been refined from single crystal X-ray diffractometer data. The refinements revealed small homogeneity ranges (small degrees of Mg/Ga mixing on the 2a sites). The magnesium atoms show square planar coordination by Ga2 dumbbells (282 pm Mg–Ga and 257 pm Ga–Ga in the lutetium compound). Geometrically one can describe the RE 2Ga2Mg phases as 1:1 intergrowth structures of CsCl and AlB2-related slabs of compositions REMg and REGa2. From DFT based calculations, charge transfer from the rare earth and magnesium atoms towards gallium can be illustrated in electron localization function ELF slice planes showing strong localization around gallium in the basal plane as well as along the tetragonal c axis signaling Ga–Ga pair interactions. The site-projected density of states DOS and COOP data further quantify this observation. Temperature dependent magnetic susceptibility measurements show Pauli paramagnetism for Sc2Ga2Mg and Lu2Ga2Mg with low room temperature susceptibility values of 2.1(1) × 10−4 and 1.1(1) × 10−4 emu mol−1, respectively. Ho2Ga2Mg, Er2Ga2Mg and Tm2Ga2Mg are Curie-Weiss paramagnets with stable trivalent rare earth ground states. Antiferromagnetic ordering was detected below the Néel temperatures of T N = 18.6(1) (RE = Ho), 11.9(1) (RE = Er) and 6.4(1) K (RE = Tm). The three compounds show metamagnetic transitions in their 3 K magnetization isotherms. Tm2Ga2Mg exhibits a square loop behavior with small hysteresis.

Textured (Ce,La,Y)–Fe–B permanent magnets by hot deformation

Nanocrystalline 2:14:1-type Ce–Fe–B alloy powders show potential application for low-cost hard magnets, but it is still a big challenge to prepare bulk anisotropic Ce–Fe–B magnet. In this study, we investigated the fabrication of bulk Ce–Fe–B, (Ce,La)–Fe–B and (Ce,La,Y)–Fe–B magnets by hot-deformation. It is found that no texture can be obtained for Ce–Fe–B ternary alloy after deformation due to the lack of rare-earth rich phase. However, a clear preferred orientation is obtained in the magnets with La substitution for Ce. The formation of texture is attributed to the reduced amount of CeFe2 Laves phase in the (Ce,La)–Fe–B alloy, allowing the formation of a rare-earth rich intergranular phase, which is necessary to induce texture during hot deformation. Adding Y improves the thermal stability of the magnets while maintaining the texture. The bulk [(Ce0.9La0.1)0.8Y0.2]16.83Fe77.23B5.94 magnet exhibits an intrinsic coercivity of 124 kA/m, a remanence of 0.63 T and a maximum energy product of 29.5 kJ/m3. This study provides a feasible approach for fabricating anisotropic nanocrystalline Ce-based 2:14:1-type permanent magnets with considerable hard magnetic properties.

Influence of heat treatment on the tensile properties and fatigue properties of Mg-8.8Gd-3.5Y-1.5Zn-0.5Zr alloy

The microstructure, tensile properties and fatigue properties of Mg- 8.8Gd- 3.5Y- 1.5Zn- 0.5Zr alloy under T4 and T6 heat treatment have been studied. The as-cast alloy was homogenized to eliminate the inhomogeneous structure and the strength of the alloy was increased by aging treatment. The microstructure evolution and mechanical properties of the alloy were investigated by optical microscope, scanning electron microscope, energy dispersive spectrometer and universal mechanical testing machine. The as-cast alloy is composed of dendritic α-Mg matrix, metastable lamellar stacking fault, eutectic Mg24(Gd, Y, Zn)5, massive long-period stacking ordered (LPSO) phase Mg12(Gd, Y)Zn and a few rare earth rich phases. In the process of homogenization, the lamellar phase and Mg24(Gd, Y, Zn)5 eutectic phase dissolved in the α-Mg matrix gradually, and the massive LPSO phase disappeared gradually, while the lamellar LPSO phase steadily grew into grains and formed some precipitate particles near the grain boundary. As the tensile temperature increases, the ductility of solid solution alloy increases, but decreases at 250 °C, which is related to the melting reaction of γ phase at high temperature and the cracking caused by grain boundary sliding. Intergranular fracture occurs at 250 °C. When the tensile temperature rises to 150 °C, the ultimate tensile strength of the alloy increases first before 150 °C and decreases after 150 °C, which is related to two strengthening mechanisms, namely, dissolution of Gd and Y elements into α-Mg matrix results in solid solution strengthening and the co-lattice strengthening between petal β′ and α-Mg matrix. Compared with the fatigue properties of the alloy after solution treatment and aging treatment, the fatigue life of the alloy after aging treatment is longer.

Analysis on deformation and texture formation mechanism of hot-deformed Nd-Fe-B magnets based on heterogeneous structure evolution

In this paper, microstructure, micromagnetic structure, texture, together with magnetic properties of the hot-deformed (HD) Nd-Fe-B magnets were systematically studied to understand the deformation process and the formation mechanism of c-axis texture. The results show that the platelet grains are formed in the fine-grain regions at the initial stage of the deformation. As the amount of deformation increases, the proportion of platelet grains increases and arranges gradually, causing the formation of c-axis texture, till the grain merging occurres when the deformation is excessive. It should be noted that the rare earth-rich phase in the fine-grained region slowly diffuses to the coarse-grained region where only grain growth can be observed during deformation. The deformation mechanism and formation of c-axis texture in HD Nd-Fe-B magnets can be deduced to be accomplished by the processes of dissolution-precipitation diffusion, grain rotation and grain arrangement, based on the characterization of microstructure and texture evolution. Also, approaches to optimize the preparation process and magnetic properties of the hot-deformed Nd-Fe-B magnets were discussed.

Influence of rare earth element terbium doping on microstructure and magnetostrictive properties of Fe81Al19 alloy

Fe81Al19Tbx (x=0, 0.05, 0.1, 0.2, 0.3, 0.4) alloys were prepared by a non-consumable vacuum arc melting technique under an inert argon gas atmosphere. The microstructures of the alloys were studied by X-ray diffraction (XRD) and scanning electron microscopy combined with an energy dispersive spectroscopy (SEM/EDS). The magnetic properties and magnetostriction coefficients of the alloys were measured by a vibrating sample magnetometer (VSM) and adifferential resistive strain sensor, respectively. The results show that the Fe81Al19 alloy consists of a single A2 phase with bcc structure, whereas the Tb doped Fe81Al19 alloys are composed of the A2 phase and a small amount of rare earth-rich phase. The doping of the rare earth element Tb makes the Fe81Al19 alloy preferentially oriented along with the <100> crystal direction. With the increase of Tb content, the magnetostriction coefficient of the alloy first increases and then decreases. When x=0.1, the magnetostriction coefficient reaches the maximum, which is 146 × 10−6. Compared with the as-cast Fe81Al19 alloy (27 × 10−6), it increases by 441%. The enhanced magnetostrictive properties are mainly attributed to the preferred orientation along <100> of A2 phase of the Tb doped Fe81Al19 alloys and lattice distortion caused by the small amount of rare earth atoms entering the Fe–Al alloy lattice.

Investigation on the grain boundary diffusion of Dy2O3 film prepared by electrophoretic deposition for sintered Nd-Fe-B magnets

Based on the diffusion source of Dy2O3 film prepared by electrophoretic deposition (EPD), grain boundary diffusion (GBD) is performed to improve properties of sintered Nd-Fe-B magnets. Effects of EPD and GBD processes on microstructure and magnetic properties are investigated in details. The studied results show that 30 s of EPD time is the most suitable for the preparation of diffusion source, and the optimal magnetic properties with Hci of 934 kA/m, Br of 1.41 T and (BH)max of 349 kJ/m3 can be achieved under GBD condition of 800 °C/6 h followed by 500 °C/1 h, enhancing by 27.6%, 0.7% and 13.7%, respectively, comparing to those of as-prepared magnet. The formation of (Nd, Dy)2Fe14B shell surrounding Nd2Fe14B core and improved distribution and structure of rare earth-rich (RE-rich) phase jointly contribute to the improvement of coercivity. Moreover, Thermal stability (temperature coefficient of remanence and coercivity) is improved considerably. Dense Dy2O3 film and formed filamentous substance covering the surface together promote the improvement of corrosion resistance.

Corrosion resistance and mechanical properties of (Ho,Nd)FeB magnets

(Ho,Nd)FeB magnets with different Ho contents were prepared by Ho substitution for part of Nd during the casting process. Effects of Ho contents on the corrosion resistance and mechanical properties of (Ho,Nd)FeB magnets were analyzed by a highly accelerated aging tester, an electrochemical workstation, a microhardness tester, a bending tester, a scanning electron microscope and an X-ray diffractometer. Results show that the addition of Ho can change the main phase structure, optimize the distribution of rare-earth rich (RE-rich) phases in grain boundary, and improve the corrosion resistance and mechanical properties of NdFeB magnets. When the content of Ho increases from 0 to 21.0 wt%, the weight loss of magnets decreases from 2.672 to 0.933 mg/cm2, and the microhardness and bending strength increase from 528.74 HV and 374.92 MPa to 633.84 HV and 459.80 MPa, respectively.

Coercivity enhancement mechanism of grain boundary diffused Nd-Fe-B sintered magnets by magnetic domain evolution observation

The grain boundary diffusion (GBD) technology was used to prepare high performance Nd-Fe-B sintered magnets by NdH3 and TbH3 nanoparticle diffusion. The factors affecting the coercivity of GBD magnets include distribution of rare earth rich grain boundary phase and substitution of the heavy rare earth. In order to distinguish the influence of various factors on the coercivity, the microstructure and magnetic domain evolution of the original, reference, Nd-diffused, and Tb-diffused magnets were analyzed. The core-shell structure formed by heavy rare earth substitution is the main factor of coercivity enhancement, and it can transform the magnetic domain reversal mode from easy-nucleation (EN) to difficult-nucleation (DN). With increasing the diffusion depth, the shell of the core-shell structure gradually becomes thinner, DN grains gradually decrease while the EN grains gradually increase, indicating that the magnetic domain reversal mode is directly related to the core-shell structure.

Magnetic properties and microstructure evolution of in-situ Tb-Cu diffusion treated hot-deformed Nd-Fe-B magnets

In this work, Tb70Cu30 alloy is employed as the addition for enhancing the hard magnetic properties of hot-deformed Nd-Fe-B magnets through in-situ diffusion. The magnets with and without Tb-Cu addition were prepared by hot pressing at 700 °C followed by hot deformation at different temperatures using melt spun Nd-Fe-B based powder as the raw materials. Tb and Cu elements were found to diffuse into the magnet after preparation. By adding 2 wt% Tb70Cu30, the coercivity and thermal stability of hot-deformed magnet were significantly improved due to the formation of local high-anisotropy (Nd,Tb)2Fe14B phase and the reduction in grain size. The intrinsic coercivity Hcj increased from 1281 kA/m to 1681 kA/m and the absolute value of temperature coefficient of coercivity |β| decreased from 0.479%/°C to 0.431%/°C for the magnets hot-deformed at 750 °C. It was confirmed that the diffused Tb atom prefers to substituting Nd in Nd2Fe14B type main phase rather than entering into RE-rich phase, and Cu distributes mainly at the grain boundaries. The reduction in crystal orientation was observed for diffused magnets, which is related to the increased rare earth-rich phase, resulting in the decreased remanence. The evolutions of the grain orientation, microstructure and magnetic properties of the magnets prepared at different hot-deformation temperatures were discussed as well.

Microstructure evolution and coercivity enhancement in Pr50Dy20Cu15Ga15-doped hot-deformed Nd-Fe-B magnets

To improve the coercivity (Hcj) and thermal stability of hot-deformed (HD) Nd-Fe-B magnets, Pr50Dy20Cu15Ga15 (at.%) alloy powders with minor heavy rare earth (HRE) are introduced. The magnetic properties, thermal stability and microstructure of the HD magnets with different amounts of Pr50Dy20Cu15Ga15 alloy powders are investigated. The Hcj of the HD magnets is enhanced from 1242 kA/m to 1595 kA/m by doping 3 wt.% Pr50Dy20Cu15Ga15 alloy powders. The thermal stability of the doped HD magnet is also improved at high temperatures. The better isolation of Nd2Fe14B grains by rare earth-rich (RE-rich) phases and the formation of local high-anisotropy (Nd, Pr, Dy)2Fe14B hard magnetic phases are the main reasons for the increase of Hcj. Notably, the non-uniform aggregation of RE-rich phases results in the inhomogeneity of magnetic anisotropy, which eventually leads to the large openness of the recoil loops. This study gives an intuitive view on the relationship between microstructure evolution and magnetic properties of the HD magnets.

High thermal stability of hot-deformed Nd–Fe–B magnets by spraying Pr70Cu30 powders on melt-spun powders

The coercivity of hot-deformed magnets is enhanced with slight reduction of remanence by spraying Pr70Cu30 powders on magnets’ precursor melt-spun Nd–Fe–B powders. The distribution of rare earth rich phases in diffused sample is superior to original magnet. The thermal stability of the diffused magnets is comparable to the sintered sample with 1 wt% Dy content and much better than the sintered grade 42SH HRE-free sample. The reversed magnetic domains of the pre-saturated samples appear when the temperature reaches 125 °C in coarse grains near ribbon surface and propagated like avalanche by further increasing the temperature. The thermal stability of hot-deformed magnet can be largely optimized from the magnetic domain variation dependent on temperature enhancement.