Microstructure and magnetic properties of anisotropic Nd-Fe-B magnets prepared by spark plasma sintering and hot deformation

Bulk anisotropic Nd-Fe-B magnets were prepared from hydrogen-disproportionation-desorption-recombination（HDDR） powders via spark plasma sintering（SPS） and subsequent hot deformation. The influence of sintering temperature on the structure and magnetic properties of the spark plasma sintered Nd-Fe-B magnets were studied. The remanence Br, intrinsic coercivity Hcj, and the maximum energy product（BH）max, of sintered Nd-Fe-B magnets first increase and then decrease with the increase of sintering temperature, TSPS, from 650 °C to 900 °C. The optimal magnetic properties can be obtained when TSPS is 800 °C. The Nd-Fe-B magnet sinter treated at 800 °C was subjected to further hot deformation. Compared with the starting HDDR powders or the SPS treated magnets, the hot-deformed magnets present more obvious anisotropy and possess much better magnetic properties due to the good c-axis texture formed in the deformation process. The anisotropic magnet deformed at 800 °C with 50% compression ratio has a microstructure consisting of well aligned and platelet-shaped Nd2Fe14 B grains without abnormal grain growth and exhibits excellent magnetic properties parallel to the pressing axis.

Grain Growth Behavior in Sintered Nd-Fe-B Magnets

The Nd2Fe14B grain growth behavior in sintered Nd-Fe-B magnets was quantitatively described.The effects of sintering temperature and time,and alloy powder size and its distribution on grain growth process were analyzed.Hence,possible grain growth mechanisms in these magnets were qualitatively discussed.The Nd2Fe14B grain growth proceeded at quite a high rate in the initial 0~1 h of sintering and from then onwards the grain growth rate decreased.A large average particle size or a wide particle size distribution of initial alloy powders was found to remarkably accelerate the grain growth process and even result in the occurrence of abnormal grain growth.On the basis of experimental results,two grain growth mechanisms were considered to operate during sintering of Nd-Fe-B magnets,that is,dissolution and re-precipitation of Nd2Fe14B particles,and Nd2Fe14B particle growth by coalescence.It was believed that Nd2Fe14B particle growth by coalescence not only produced a large average grain size and a wide grain size distribution,but also was the fundamental reason for the formation of abnormally large grains in the microstructure of sintered Nd-Fe-B magnets.

Thermal Stability of Low Cost Nd-Fe-B Magnets

1.IntroductionSince the Nd-Fe-B magnets appearedin 1983,researchers,producers and users ofthe permanent magnetic materials have paidgreat attention to them.Because the mag-nets have low Curie temperature Tc andbad thermal stability as well as easeoxidation,their applications are limited insome fields.The researchers are greatly in-terested in increase energy product

Temperature stability and microstructure of ultra-high intrinsic coercivity 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 （α） 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.

Magnetic properties and thermal stability of anisotropic bonded Nd-Fe-B magnets by warm compaction

Anisotropic bonded magnets were prepared by warm compaction using anisotropic Nd-Fe-B powder. The forming process, magnetic properties, and temperature stability were studied. The results indicate that the optimal temperature of the process, which was decided by the vis-cosity of the binders, was 110°C. With increasing pressure, the density of the magnets increased. When the pressure was above 700 MPa, the powder particles were destroyed and the magnetic properties decreased. The magnetic properties of the anisotropic bonded magnets were as follows： remanence Br=0.98 T, intrinsic coercivity iHc=1361 kA/m, and maximum energy product BHmax=166 kJ/m3. The magnets had excellent thermal stability because of the high coercivity and good squareness of demagnetization curves. The flux density of the magnets was 35% higher than that of isotropic bonded Nd-Fe-B magnets at 120°C for 1000 h. The flux density of the bonded magnets showed little change with regard to temperature.

Research of warm compaction technology on nylon bonded Nd-Fe-B magnets

Warm compaction and room temperature compaction were applied to prepare bonded Nd-Fe-B magnets. The results indicated that the density of magnet was determined by the compaction pressure and warm compaction temperature, whereas, the thermosetting temperature could hardly affect the density of magnet. The mechanical properties of magnets were the best when the thermosetting temperature was 200 ℃. The Br, Hob, and （BH）max of warm compaction magnet were higher than those of room compaction. When the warm compaction temperature and thermosetting temperature were invariable, the density of magnet increased with the increase of compaction pressure, which resulted in the increase of Br, Hcb, and （BH）max of magnet and the decrease of Hcj of magnet. When the warm compaction temperature and compaction pressure were invariable, the magnetic properties of magnets decreased with the increase of thermosetting temperature. The magnetic properties of warm compaction molding magnets were better than those of injection molding magnets.

Reduction of Sensitivity to Sintering Temperature for Nd-Fe-B Magnets through Zr and Nb Additions

To reduce the sensitivity of grain growth to sintering temperature for improving property consistency of sintered Nd-Fe-B magnets, combined additions of Zr and Nb were investigated. It was found that when Zr content was increased to 0.07 at. pct, abnormal grain growth was effectively hindered even when the sintering temperature reached 1100℃. With combined additions of 0.07 at. pct Zr and 0.07 at. pct Nb, the sensitivity of grain growth to sintering temperature was greatly reduced consistency than the magnets containing no Zr and also improved. The magnets sintered at 1100℃ showed higher property Nb. In addition, the magnetic properties of magnets were also improved.

MAGNETIZATION BEHAVIOR IN SINTERED Nd-Fe-B MAGNETS

Magnetization and demagnetization curves and hysteresis loops applied different magnetizing.fields in sintered Nd-Fe-B and Nd-Dy-Fe-B magnets from thermally demagnetized and dc field-demagnetized states were investigated at temperatures of up to 150℃.The first-quadrant remagnetization curves and the curves of coercive forces _MH_C versus rernagnetizing fietds H_m from dc field-demagnetized state at room temperature show a step around magnetizing field as absolute value of the maximum intrinsic coercivity.The steps of _MH_C-H_m curves shifted to lower remagnetizing fields and the shapes of magnetization curves changed from step type to precipitous type when temperature went up to 100～150℃ or after the specimen was thermally demagnetized at a temperature higher than the Curie temperature.The steep rise of knee coereivity with increasing magnetizing field is behind that of _MH_C.Note that the magnetic hardening in sintered Nd-Fe-B magnets is controlled by pinning of domain walls.

Effect of TbF_(3)diffusion on the demagnetization behavior and domain evolution of sintered Nd-Fe-B magnets by electrophoretic deposition

We studied the magnetic properties and domain evolution of annealed and TbF3-diffused sintered Nd-Fe-B magnets using the electrophoretic deposition method.After TbF_(3)diffusion,the coercivity increased significantly by 9.9 kOe and microstructural analysis suggested that Tb favored the formation of the(Nd,Tb)_(2)Fe_(14)B shell phase in the outer region of the matrix grains.The first magnetization reversal and the dynamic successive domain propagation process were detected with a magneto-optical Kerr microscope.For the TbF_(3)-diffused magnet,the magnetization reversal appeared at a larger applied field and the degree of simultaneous magnetization reversal decreased compared with an annealed magnet.During demagnetization after full magnetization,the occurrence of domain wall motion(DWM)in the reproduced multi-domain regions was observed by the step method.The maximum polarization change resulting from the reproduced DWM was inversely related to the coercivity.The increased coercivity for the diffused magnet was mainly attributed to the more difficult nucleation of the magnetic reversed region owing to the improved magneto-crystalline anisotropy field as a result of Tb diffusion.

Magnetic microstructure and coercivity mechanism of high performance Nd-Fe-B magnets

Magnetic microstructure of high performance Nd-Fe-B magnets was investigated by using magnetic force microscopy. The correlation between magnetic microstructure and coercivity for high performance Nd-Fe-B magnets was studied. It is found that the magnets with different coercivity mechanism take on different microstructures and magnetism. Moreover, the magnetic microstructures of high performance permanent magnets can be explained by the starting field theory model.

Effect of bonding process on the properties of isotropic epoxy resin-bonded Nd-Fe-B magnets

Bonded NdFeB magnets were prepared by compression molding. The effect of preparation technology on their magnetic and mechanical properties was studied through the analysis of density, Br, Hcj, （BH）max, bending strength, and compressive strength of the bonded magnets. The results showed that the magnetic properties decreased with increasing binder content, whereas the mechanical properties increased. Br and （BH）max increased with rising pressure, whereas Hcj decreased. For a fixed mass fraction of the binder, the optimal pressure was 620 MPa and the best thermosetting temperature was 160°C. These conditions made the bonded magnets have the optimal mechanical properties. Scanning electron microscopy （SEM） analyses of the fracture surfaces indicated that the epoxy resin bonded magnets exhibited brittle behavior.

Microstructure and Magnetic Properties of Anisotropic Nd-Fe-B Magnets Fabricated by Single-Stage Hot Deformation

Anisotropic Nd-Fe-B magnets were fabricated by the single stage hot deformation (SSHD) method. The magnetic properties of the anisotropic Nd-Fe-B magnets are as follows: the maximum energy product is 234.7 kJ·m-3 , remanence 1.16 T and coercivity 684.3 kA·m-1. A study of the relationship between microstructure and magnetic properties for the anisotropic Nd-Fe-B magnets was carried out. The results show that the grains of Nd2Fe14B have grown up preferentially along the direction perpendicular to the pressing direction.

Investigation of Coercivity Mechanism for Nd-Fe-B Magnets Prepared by Combination of Strip-Casting with Hydrogen Decrepitation Techniques

The coercivity mechanism of Nd Fe B magnets prepared by combination of strip casting with hydrogen decrepitation techniques was investigated. The microstructure of (Nd 0.935 Dy 0.065 ) 14.5 Fe 79.4 B 6.1 magnet was observed. The average grain size is about 6～12 μm. The magnetizing field dependence of the hardmagnetic properties for the (Nd 0.935 Dy 0.065 ) 14.5 Fe 79.4 B 6.1 and the temperature dependence of the coercivity for the Nd 14.5 Fe 79.4 B 6.1 were investigated. Results show that the coercivity for magnets prepared by the combination of strip casting with hydrogen decrepitation techniques is controlled by the nucleation mechanism.

Effects of Ti and(or) Cu on microstructures and magnetic properties of sintered Nd-Fe-B magnets

The alloying elements Ti and(or) Cu were added into the intergranular regions of sintered Nd Fe B magnets and their effects on microstructures and magnetic properties of the magnets were investigated. The results showed that a small amount of Ti and(or) Cu additions can enhance the coercivity and have little effect on the remanence of Nd Fe B magnets. Compared with individual addition of pure Ti or Cu elements, Ti and Cu co addition of intergranular region is more efficient to improve the coercivity of the magnets. The improvement of the coercivity can be attributed to the segregation of Cu element on the surface of the magnetic phase (Nd 2Fe 14 B) and the occurrence of fine Nd Fe Ti particles near grain boundaries. The former can prevent the magnetic coupling of Nd 2Fe 14 B grains to a certain degree and impede effectively the propagation of reversed domain walls through the magnetic phase grains. The latter can inhibit the growth of magnetic phase grains during the sintering process, resulting in a finer grain size. Both are beneficial to the coercivity enhancement. With increasing Ti content above 0.8%, a strip Ti rich phase appears in the intergranular region, resulting in the dramatic reduction of the remanence of Nd Fe B magnets.

Coercivity and microstructure of sintered Nd-Fe-B magnets diffused with Pr-Co,Pr-Al,and Pr-Co-Al alloys

The commercial 42 M Nd-Fe-B magnet was treated by grain boundary diffusion(GBD)with Pr_(70)Co_(30)(PC),Pr_(70)Al_(30)(PA)and Pr_(70)Co_(15)Al_(15)(PCA)alloys,respectively.The mechanism of coercivity enhancement in the GBD magnets was investigated.The coercivity was enhanced from 1.63 T to 2.15 T in the PCA GBD magnet,higher than the 1.81 T of the PC GBD magnet and the 2.01 T of the PA GBD magnet.This indicates that the joint addition of Co and Al in the diffusion source can further improve the coercivity.Microstructural investigations show that the coercivity enhancement is mainly attributed to the exchange-decoupling of the GB phases.In the PCA GBD magnet,the wider thin GB phases can be formed and the thin GB phases can still be observed at the diffusion depth of 1500μm due to the combined action of Co and Al.At the same time,the formation of the Pr-rich shell can also be observed,which is helpful for the coercivity enhancement.

Temperature stability and fracture toughness of Nd-Fe-B magnets

Temperature stability and toughness of magnets are very important properties especially for application in motor. In this paper, it is found that temperature stability and toughness of magnets are improved when Fe is substituted with Co and heavy rare earth is substituted for Nd in part and suitable rich B grain-boundary phase is added. In addition, heavy rare earth substitution decreases the remanence temperature coefficient greatly, but has a little effect on Curie temperature of the magnets, which is beneficial to Nd-Fe-B magnets for the application in motor.

Effect of Weak Magnetic Intergranular Phase on the Coercivity in the HDDR Nd-Fe-B Magnet

Assuming that intergranular phase (IP) existing between adjacent grains is a weak magnetic phase, we study the effect of IP on the coercivity in the HDDR Nd-Fe-B magnet. The results indicate that the coercivity increases with the increasing IP’s thickness d, but decreases with increasing its anisotropy constant K1(0). When the structure defect thickness r0 =6nm, d=1nm and K1(0)=0.15K1 (K1 is the normal anisotropy constant in the inner part of a grain), our calculated coercivity is in agreement with available experimental data.

Homogeneity of H_(cj) in Sintered Bulk Nd-Fe-B Magnets

High performance magnets not only have high magnetic properties, but also have good homogeneity. The homogeneous of Nd14.2DyxAl0.8Cu0.1B6.2Febal. bulk magnets was studied. The sizes of the samples produced by conventional sintering process were 53×50.5×25.3/40.2/50.4/61.0 mm. The magnetic character and microstructure of the samples were determined by NIM-10000H hysteresigraph, optical metaloscope, and SEM respectively. The results show that the Hcj reduces with increasing C content at the same process parameters. And the Hcj shows dependence not only on the ageing processing but also on the size of the bulk and loading capacity etc. The Hcj increases with prolonging ageing time and the ageing time should be different with different size of the bulk. With the increase of the loading capacity, the Hcj reduces promptly and is very inhomogeneous. The microstructure of the samples shows that the grain of the hard magnetic phase is finer and the neodymium-rich boundary is distributed homogeneously with prolonging ageing time. And so is the sample in a small amount of the bulks.

SIMULTANEOUS INCREASE OF REMANENCE AND COERCIWTY OF Nd-Fe-B MAGNETS BY Nb ADDITION

The Nd-Fe-B magnets with Nb addition prepared by a new method exhibit an unusual increase of Br instead of normal monotonous decreasing tendency when the Nb content exceeds 1.75wt.%.The microstructure of these samples were studied,and it u found that the Nd-rich phase,which usually exists in Nd-Fe-B magnets,is very hard to be detected.

Microstructure and properties of low-temperature aged Dy-doped Nd-Fe-B magnet

In this work, a low temperature ageing process for the high coercivity Dy-doped Nd-Fe-B magnets was developed by the optimizing ageing process. The experimental results show that there is no difference in microstructures, crystal orientation, magnetic and mechanical properties between the low-temperature aged and the two-stage aged sintered Dy-doped Nd-Fe-B magnets. Because of the uneven stress distribution in the sintered Dy-doped Nd-Fe-B magnet and the high activation of Dy element, Dy atoms could diffuse into the main crystal phase and the grain boundary phases of the magnets during low-temperature ageing process, which results in the reasonable distribution of Dy element and formation of the thin and uniform grain boundary phases, which are the main reasons to improve intrinsic coercivity of the Dy-doped Nd-Fe-B magnets by the low-temperature ageing.