Nd2Fe14B Grains Research Articles

Role of vacuum diffusing temperature and time on improving hysteresis characteristics of grain boundary diffused NdFeB permanent magnets by Dy-metal vapor sorption

This study investigates the impact of vacuum grain boundary diffusion (GBD) conditions on sintered NdFeB permanent magnets using Dy-vapor sorption. The study analyzed variables such as vacuum diffusing treatment temperature (T), time (t), and diffusion depth (d). In a 4.5 mm-thick cube-shaped magnet, Dy diffusion increased intrinsic coercivity from 13.13 kOe to 20.56 kOe (56.6 %) with only a 1.5 % reduction in remanence for a modified two-stage grain boundary diffused magnet (940 °C/8 h + 950 °C/0.5 h), achieving commercial G52SH grade. This enhancement exceeds previous GBD studies on non-rare earth compounds or alloys. FE-EPMA analysis attributed the increased coercivity to a continuous Dy-rich region within Nd2Fe14B grains and a thicker core-shell structure forming the (Nd,Dy)2Fe14B phase, which suppresses reversed domain formation. Glow discharge optical emission spectrometer (GDOES) analysis revealed a 2.57 times increase in peak Dy content and a 1.9 times increase in deeper Dy content at 10 μm from the magnet surface compared to the original process. These findings indicate that Dy vapor sorption, combined with a modified two-stage vacuum heat treatment, enhances both the concentration and depth of Dy grain boundary diffusion. Furthermore, the resulting magnet has a clean surface without additional cleaning, beneficial for producing low-cost, high-performance NdFeB magnets.

Identifying Optimal Processing Variables and Investigating Mechanisms of Grain Alignment in Hot-Deformed NdFeB Magnets through Design of Experiments.

This study introduces a novel approach for investigating hot-deformed NdFeB magnets by combining the minimal stress deformation process (MSDP) with the design of experiment (DoE) methodology. This study focused on enhancing the crystallographic alignment, particularly the c-axis alignment of the Nd2Fe14B grains, to optimize the magnetic properties. By utilizing the Box-Behnken design matrix and response surface regression, critical processes and variables were identified, determining that a hot-pressing temperature of 700 °C is crucial for achieving optimal grain alignment. Changing the strain rate to 0.019 mm/s under a stress of 110 MPa led to significant enhancements in the alignment, yielding magnets with a remanence of approximately 13.4 kG and a coercivity of 21 kOe. These findings highlight the effectiveness of combining the MSDP and DoE for predicting and achieving improved magnetic properties. Despite the challenges associated with understanding the complexity of crystal alignment mechanisms, this integrated approach successfully improved magnetic characteristics. The methodology represents a significant advancement in the fabrication of high-performance hot-deformed NdFeB magnets, marking a notable contribution to the field.

Novel mechanism of the grain boundary diffusion process with Tb based on the discovery of TbFe2 phase

The grain boundary diffusion process (GBDP) has proven to be an effective method for enhancing the coercivity of sintered Nd-Fe-B magnets. However, the limited diffusion depth and thicker shell structure have impeded the further development of magnetic properties. Currently, the primary debates regarding the mechanism of GBDP with Tb revolve around the dissolution-solidification mechanism and the atomic substitution mechanism. To clarify this mechanism, the microstructure evolution of sintered Nd-Fe-B magnets during the heating process of GBDP has been systematically studied by quenching at different temperatures. In this study, it was found that the formation of TbFe2 phase is related to the dissolution of Nd2Fe14B grains during GBDP with Tb. The theory of mixing heat and phase separation further confirms that the Nd2Fe14B phase dissolves to form a mixed phase of Nd and TbFe2, which then solidifies into the (Nd, Tb)2Fe14B phase. Based on the discovery of the TbFe2 phase, the dissolution-solidification mechanism is considered the primary mechanism for GBDP. This is supported by the elemental content of the two typical core-shell structures observed.

Microstructure evolution and coercivity enhancement in hot-deformed Nd-Fe-B magnet processed by Pr60Tb10Cu30 diffusion

The grain boundary diffusion process with Pr60Tb10Cu30 alloy was used to improve the microstructure and coercivity of the hot-deformed Nd-Fe-B magnet. The crystalline intergranular phases consisting of (Pr,Nd)5Ga3, (Pr,Nd)3Co, PrCu and (Nd,Pr)6Fe13Ga were observed in the Pr60Tb10Cu30 diffused magnet. Apparent (Nd,Tb)2Fe14B shell layer on the surface of Nd2Fe14B grains near the triple junctions of the magnet was observed after grain boundary diffusion. The formation of many non-ferromagnetic intergranular phases and Nd2Fe14B/(Nd,Tb)2Fe14B core-shell structure contributes to the significant increase of coercivity from 1.21 T to 2.7 T and the improvement of temperature coefficient of coercivity from −0.469%/℃ to −0.356%/℃.

Comprehensively improved magnetic performance of Nd-Fe-B magnets with high-efficiency grain boundary diffusion of heavy rare earth

Comprehensively improved magnetic performance Nd-Fe-B magnets with extremely limited amount of Heavy Rare Earth (HRE) elements are obtained to meet the increasingly demanding applications. In this work, Tb–Co thin film with uniform distribution, flat surface and strong adhesion is successfully prepared by magnetron sputtering, and the effects of Tb–Co film thickness on the magnetic properties are investigated. The magnets with optimal magnetic properties are achieved when the film thickness is about 3.5 μm, the coercivity of 1284.81 kA/m, the remanence of 1.48 T, and the maximum magnetic energy product of 437.32 kJ/m3, compared with the original magnets, enhancing by 47.9 %, 0.4 % and 5.7 %, respectively, so the magnetic properties have been comprehensively improved using little amount of HRE and Co elements. HRTEM and EPMA results revealed that Tb-rich shell distributed around the Nd2Fe14B grain. The mechanisms of the remanence and coercivity simultaneous enhancement of Nd-Fe-B magnets is elaborated in detail. Besides, the diffusion of Tb–Co thin film can enhance thermal stability of the magnets.

Coercivity saturation in Nd-Fe-B sintered magnets treated by grain boundary diffusion process of Tb-Ni-Al alloy

Microstructures of Nd-Fe-B sintered magnets treated by a grain boundary diffusion (GBD) process with different Tb coating amounts are investigated by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). Tb-substituted shell structures are formed in the outer portion of all Nd2Fe14B grains in the magnets. However, thick Tb-rich shells are formed on a micrometer scale on Nd2Fe14B grains in the region up to about 0.5 mm from the coating surface of the magnet, and thin Tb-rich shells several tens of nanometers in size are also formed in the inner region. As the Tb coating amount increases, the Tb concentration and thickness of the thick Tb-rich shells increase. At the same time, the Nd replaced by Tb forms Nd-rich grain boundary (GB) phases, and a large amount of those Nd-rich GB phases are discharged to the coating surface. The coercivity enhancement in GBD is mostly due to the formation of the thin Tb-rich shells, whereas the formation of thick Tb-rich shells wastes a large amount of Tb, causing coercivity saturation. Therefore, it is important to suppress the formation of thick Tb-rich shells and form thin Tb-rich shells with a high Tb concentration uniformly throughout the magnets.

Stability and magnetic properties of Nd[formula omitted]Fe[formula omitted]X (X = Al, Si, Cu, Zn, and Ga)

The stability and magnetic properties of the RE6Fe13X compounds, as well as their preferential localization in Nd-Fe-B magnets, have been investigated by first-principles calculations. We find that the RE6Fe13X (X = Al, Si, Zn, and Ga) have negative formation energies and are more likely to form in the triple junctions, while Nd6Fe13Cu with the formation energy of 0.73 eV tends to localize in the thin intergranular phase. These compounds contribute to the exchange decoupling of Nd2Fe14B grains and thus enhance the coercivity of Nd-Fe-B magnets. Such decoupling can be more pronounced in the cases of Nd6Fe13Cu and Nd6Fe13Zn, which exhibit antiferromagnetic ordering in the Fe sublattice according to our calculations. We propose that the partial substitution of Nd atoms with Pr can gradually reduce the formation energy of these phases and maintain their original antiferromagnetic states.

Amorphous-crystalline nanostructured Nd-Fe-B permanent magnets using laser powder bed fusion: Metallurgy and magnetic properties

Laser powder-bed fusion (PBF-LB), a class of additive manufacturing (AM), has attracted wide interest in the production of Nd-Fe-B permanent magnets, benefiting from the minimisation of waste of rare-earth elements and the post-processing requirements. Most research on PBF-LB Nd-Fe-B has focused on reducing defects in printed parts alongside the improvement of the resultant magnetic properties. Detailed analysis of the microstructure that results in permanent magnetic properties is yet to be published. In this research, a combination of high-resolution microstructural investigations was conducted for this purpose. For the first time, an in-depth analysis of the grain structure in terms of morphology, size distribution, and texture is presented and correlated to the permanent magnetic performance. Melt pools showed a hierarchical grain size distribution of primary Nd2Fe14B phase grains with a polygonal morphology and random crystalline alignment, in addition to a small amount of Nd-rich and Nd-lean precipitates in the matrix of the Ti-rich amorphous grain boundaries. The permanent magnetic properties of this material are mainly determined by the nanostructured Nd2Fe14B grains and the amorphous Ti-rich iron-based intergranular phase but could be weakened by precipitates that act as magnetic pores. Remelting during PBF-LB led to the transformation of the coarse grains of the previously solidified layer to fine ones, favourable for the permanent magnetic properties. The mechanisms of these complex phase formations and transformations during processing and the development of the nanocrystalline microstructure are elucidated in this paper as a basis for informing the optimisation process for microstructural development.

Grain refinement and coercivity enhancement of sintered Nd–Fe–B alloys by doping eutectic alloy (Nd0.75Pr0.25)70Cu30.

ABSTRACT A significant enhancement in the coercivity and grain refinement was revealed for the spark plasma sintered NdFeB magnets by doping eutectic alloy (Nd0.75Pr0.25)70Cu30 powders. The commercially available NdFeB powders were mixed with (Nd0.75Pr0.25)70Cu30 powders (0, 2 and 5 wt.%) and then consolidated by spark plasma sintering at 700°C under 50 MPa. Microstructure analysis shows that the addition of (Nd0.75Pr0.25)70Cu30 alloy reduces the formation of coarse grain zone and decreases the average Nd2Fe14B grain size by intergranular phase modification. The RE-rich phase containing Nd, Pr at the Nd2Fe14B grain boundary caused the magnetic isolation effect. Hence, the coercivity increased from 18.3 kOe for un-doped NdFeB to 20.3 and 21.6 kOe with doping content of 2 and 5 wt.%, respectively. This study provides an economic way to fabricate sintered NdFeB magnets with enhanced coercivity and homogeneous nanostructure.

Influence of the grain boundary phase characteristics on the magnetic properties of Nd-Fe-B magnets

The effects of the grain boundary phase (GBP) thickness, ferromagnetism and orientation with the c-axis of the Nd2Fe14B phase on the magnetic properties of Nd-Fe-B magnets were investigated by micromagnetic simulation. Increasing the thickness of the non-ferromagnetic GBP parallel to the c-axis decreases the demagnetization field at the corners of magnets which act as the nucleation sites of initial reversed magnetic domain. However, the demagnetization field at the corners of magnets increases with the increased thickness of the non-ferromagnetic GBP perpendicular to the c-axis. This results in the high coercivity increasing rate with the same non-ferromagnetic GBP thickness increasing parallel to the c-axis than that perpendicular to the c-axis. For the magnet models with ferromagnetic GBP the initial nucleation of reversed magnetic domain starts at the ferromagnetic GBP. And the nucleation field of the reversed magnetic domain in the ferromagnetic GBP perpendicular to the c-axis is far higher than that in the ferromagnetic GBP parallel to the c-axis. Moreover, it is much easier for the reversed magnetic domain in the ferromagnetic GBP parallel to the c-axis to propagate to the neighboring Nd2Fe14B grains than that in the ferromagnetic GBP perpendicular to the c-axis. This leads to the much more substantial deteriorating coercivity with the increased thickness of ferromagnetic GBP parallel to the c-axis than that perpendicular to the c-axis. This work clarifies the influence mechanism of the GBP characteristics on the magnetic properties of Nd-Fe-B magnets.

Grain size and coercivity tuning in Nd2Fe14B-based magnets prepared by high pressure hydrogen milling

Nd2Fe14B-based permanent magnets play important role in the emerging green energy technologies due to their superb energy product (BH)max. High coercivity at elevated operating temperatures is crucial for device performance and as an extrinsic property it is largely determined by the microstructure of the magnet. In this work, we use high pressure hydrogen milling to reduce the average grain size in Nd2Fe14B powders towards the critical single domain regime to study the influence on the resultant coercivity. In addition, Nd content has been varied in a broad range to investigate how it affects the coercivity. Milling in 100 bar hydrogen enables complete decomposition of the Nd2Fe14B phase into α-Fe, NdH2 and Fe2B at nominally room temperature. A subsequent hydrogen desorption heat treatment leads to the recombination to the parent phase, now with nearly two orders of magnitude reduction in the grain size. The results show that indeed Hc peaks around the critical single domain grain size of ≈200 nm of Nd2Fe14B. Higher contents of Nd-rich grain boundary phase lead to a continuous increase in coercivity up to μ0Hc = 1.5 T, likely due to the suppression of long-range magnetostatic interactions between the nanocrystalline Nd2Fe14B grains.

Magnetic properties of Nd6Fe13Cu single crystals

The understanding of a coercivity mechanism in high performance Nd–Fe–B permanent magnets relies on the analysis of magnetic properties of all phases present in magnets. By adding Cu in such compounds, a new Nd6Fe13Cu grain boundary phase is formed; however, the magnetic properties of this phase and its role in the magnetic decoupling of matrix Nd2Fe14B grains are still insufficiently studied. In this work, we have grown Nd6Fe13Cu single crystals by the reactive flux method and studied their magnetic properties in detail. It is observed that below the Néel temperature (TN = 410 K), Nd6Fe13Cu is antiferromagnetic in zero magnetic field; whereas when a magnetic field is applied along the a-axis, a spin-flop transition occurs at approximately 6 T, indicating a strong competition between antiferromagnetic and ferromagnetic interactions in two Nd layers below and above the Cu layers. Our atomistic spin dynamics simulation confirms that an increase in the temperature and/or magnetic field can significantly change the antiferromagnetic coupling between the two Nd layers below and above the Cu layers, which, in turn, is the reason for the observed spin-flop transition. These results suggest that the role of an antiferromagnetic Nd6Fe13Cu grain boundary phase in the coercivity enhancement of Nd–Fe–B–Cu magnets is more complex than previously thought, mainly due to the competition between its antiferro- and ferromagnetic exchange interactions.

High-quality TbF3 coating prepared by suspension plasma spraying technique for improving the grain boundary diffusion efficiency of sintered Nd-Fe-B magnets

High-quality TbF3 coating used for the grain boundary diffusion process (GBDP) of sintered Nd-Fe-B was prepared by the suspension plasma spraying (SPS) technique. The obtained TbF3 coating with SPS process showed a dense and continuous structure with the uniform distribution as well as a good adhesion with the Nd-Fe-B substrate. Compared with the uncoated Nd-Fe-B magnet without heavy rare earth, the coercivity of TbF3 coated Nd-Fe-B magnet enhanced from 1160 kA/m to 1824 kA/m after the GBDP heat treatment. The microstructure and composition analysis indicated that the (Nd, Tb)2Fe14B shell formed at the periphery of Nd2Fe14B grains throughout the whole sample magnet. The shell with high magnetocrystalline anisotropy effectively suppressed the nucleation of reverse domain and repaired the defects on the grain surface of the sintered Nd-Fe-B magnets, thus enhancing the coercivity. As a consequence, the SPS technology has advantage of high depositing rate, flexible production process, good boundary adhesion and homogeneous-dense structure, which is expected to become a promising candidate way of depositing diffusion coating for the GBDP of sintered Nd-Fe-B in industrial production.

Microstructure and coercivity modification of HDDR (Pr, Nd, Ce)-Fe-B magnetic powders by grain boundary diffusion with Pr-Fe-Al-Ga

The coercivity of HDDR (Pr, Nd, Ce)-Fe-B magnetic powders can be significantly enhanced from 3040 Oe to 7271 Oe through a unique grain boundary diffusion process known as rotating adhering diffusion, which uses Pr64Fe7Al9Ga20 as a diffusion source. Microstructural analysis revealed that the diffusion process resulted in a substantial increase in grain boundaries, as well as the formation of a Pr-rich shell in the Nd2Fe14B grains. Furthermore, the replacement of partial Nd element by Pr element in the diffusion source also caused a decrease in the thermal stability of the powders by 8.06 K. This research provides significant insight into the further development of high-performance Nd-Fe-B materials.

Anisotropy optimization of HDDR magnetic powders by precursor alloy annealing

The anisotropy of hydrogenation–disproportionation–desorption–recombination (HDDR) method to form Nd2Fe14B powders was effectively improved by annealing the strip casting (SC) alloy precursors at 1080 °C. According to the analysis of the evolution of microstructure, the excellent anisotropy was attributed to the nearly consistent orientation of the nanoscale Nd2Fe14B grains in the HDDR magnetic powders, which perfectly inherited that of the coarse grains in the treated SC alloy flakes. In contrast, for the untreated specimen, more nanoscale clusters of different orientations were generated after smashing and HDDR treatment of fine columnar Nd2Fe14B grains with different orientations in the original SC alloy, severely affecting the anisotropy of the magnetic powders. This study provides new insights into the development of high-anisotropy and high-performance Nd-Fe-B magnets.

Substantial coercivity enhancement in Dy-free Nd-Fe-B sintered magnet by Dy grain boundary diffusion

To date, the highest coercivity reported for Dy-free Nd–Fe–B sintered magnets under the Dy grain boundary diffusion (GBD) process is ∼2.4 T. Therefore, exploring various approaches for achieving higher coercivity is beneficial for eliminating the dependence on the scarce and expensive heavy rare-earth elements, Dy and Tb. In this study, we report record-breaking high coercivity by separately diffusing Dy70Cu30 and then Pr68Cu32 eutectic alloys through grain boundaries in fine-grained Dy-free sintered magnets. The two-step GBD-processed sample containing a low Dy content (only 0.9 wt%) exhibited a high coercivity of 2.8 T and an excellent temperature coefficient of coercivity of β = -0.447%/ °C. The second GBD process facilitated Dy infiltration from the bulk surface toward the center by chemically induced liquid film migration to form a uniform Dy-rich shell throughout the bulk. Pr was also enriched in the Dy-rich shell near the surface of Nd2Fe14B grains in the two-step GBD-processed sample. However, the Pr-enrichment in the shell did not cause the degradation of the coercivity at high temperatures. This unique shell structure strengthened the anisotropy field to achieve 2.8 T coercivity without alloying Dy to the initial sintered magnet.

Effect of quenching rate on the structural and hard magnetic properties of Nd-Fe-B melt-spun ribbons

The phase structure, microstructure, magnetic and thermomagnetic properties of nanostructured Nd–Fe–B melt-spun ribbons were investigated. The melt-spun ribbons have been prepared at different wheel speeds varying from 17 to 25 m/s. The hard magnetic Nd2Fe14B phase with (00l) texture, indicating preferred crystallographic orientation, was observed in all the ribbons with some α-Fe(Co) as the minor phase. Nd2Fe14B grains are uniformly distributed with grain sizes ranging from 50 to 150 nm. A decrease in the average grain size of Nd2Fe14B and fading away of texture formation in the ribbons were found with the increase in the wheel speeds. The best combination of magnetic properties with a coercivity of 14.5 kOe, the saturation magnetization of 132 emu/g, and the energy product of 16 MGOe was achieved at 23 m/s and these ribbons are suitable for the fabrication of hot deformation Nd–Fe–B magnets.

High-resistivity anisotropic hot-deformed Nd-Fe-B magnets prepared from DyF3 electrophoretic deposited powders

The electrical resistivities of anisotropic Nd-Fe-B-based magnets were successfully increased in both parallel and perpendicular directions to the c-axis by hot-deforming Nd-Fe-B melt-spun flakes that were coated with DyF3 using electrophoretic deposition. Consequently, the operating temperature under high-frequency AC magnetic field was reduced by 20 °C because of the reduction of eddy current loss. The resistive layer formed at the interface of original ribbons in the hot-deformed magnet was found to be NdF3 rather than DyF3. Transmission electron microscopy and atom probe tomography revealed that liquid Nd-rich intergranular phase reduces DyF3 to form NdF3 at the ribbon interfaces during hot-deformation at elevated temperature. Excess Dy diffuses into the original ribbon flakes through grain boundaries to form a Dy-rich shell in Nd2Fe14B grains, giving rise to a high coercivity of 1.8 T. This study demonstrates an alternative way to solve the thermal demagnetization problem of Nd-Fe-B-based permanent magnets during operation in electric motors via the resistivity increase of the magnet.

Simultaneous enhancement of coercivity and saturation magnetization in high-performance anisotropic NdFeB thick films with a Dy diffusion layer.

Generally, the addition of the Dy element leads to a decrease of the saturation magnetization and the remanent magnetization in NdFeB films due to its antiferromagnetic coupling with Fe. However, in this study, upon increasing the ratio of Dy in the Nd-Dy diffusion layers of NdFeB thick films, the saturation magnetization has an anomalously slight enhancement, while the coercivity and remanent magnetization have a large enhancement. The increase of coercivity is attributed to the decoupling between Nd2Fe14B grains and the enhanced pinning effect. Microstructural analysis revealed a layered structure composed of spherical Nd2Fe14B grains at the location of the Dy diffusion layer, which is attributed to the Dy diffusion layer reacting with the region of Nd element aggregation during the annealing process, facilitating transformation into Nd2Fe14B grains. The increase of the proportion of Nd2Fe14B grains results in a slight enhancement of saturation magnetization. By this method, we obtained a high-performance anisotropic NdFeB thick film of 28.7 μm with a coercivity of 2.46 T and a surface field of 163 Oe. This work establishes a microscale growth model for NdFeB thick films and helps to prepare high-performance NdFeB thick films applicable directly to microelectromechanical systems.

A Novel Magnetic Hardening Mechanism for Nd‐Fe‐B Permanent Magnets Based on Solid‐State Phase Transformation

AbstractPermanent magnets based on neodymium‐iron‐boron (Nd‐Fe‐B) alloys provide the highest performance and energy density, finding usage in many high‐tech applications. Their magnetic performance relies on the intrinsic properties of the hard‐magnetic Nd2Fe14B phase combined with control over the microstructure during production. In this study, a novel magnetic hardening mechanism is described in such materials based on a solid‐state phase transformation. Using modified Nd‐Fe‐B alloys of the type Nd16Febal‐x‐y‐zCoxMoyCuzB7 for the first time it is revealed how the microstructural transformation from the metastable Nd2Fe17Bx phase to the hard‐magnetic Nd2Fe14B phase can be thermally controlled, leading to an astonishing increase in coercivity from ≈200 kAm−1 to almost 700 kAm−1. Furthermore, after thermally treating a quenched sample of Nd16Fe56Co20Mo2Cu2B7, the presence of Mo leads to the formation of fine FeMo2B2 precipitates, in the range from micrometers down to a few nanometers. These precipitates are responsible for the refinement of the Nd2Fe14B grains and so for the high coercivity. This mechanism can be incorporated into existing manufacturing processes and can prove to be applicable to novel fabrication routes for Nd‐Fe‐B magnets, such as additive manufacturing.