Effects of Nd–Ga intergranular addition on microstructure and magnetic properties of heavy-rare-earth-free Nd-Fe-B sintered magnets

In this study, we propose an approach of grain boundary modification that can significantly increase the coercivity of the B-lean Nd-Fe-B sintered magnets by intergranular addition of Nd–Ga. The coercivity is substantially enhanced from 1.51 to 2.04 T through optimizing the microstructure and adjusting the phase composition for the grain boundary phase in the annealed magnets. The matrix grains are covered by a continuous thin grain boundary phase accompanying the formation of intermetallic Nd₆Fe₁₃Ga phases. The analysis of magnetic behaviors above Curie temperature confirms that the grain boundary phase of annealed Nd–Ga doped magnets appears to be non-ferromagnetic, facilitating the intergrain exchange decoupling. Microstructure observation in grain boundary area indicates that some surface of the matrix grain is dissolved in the formation process of the Nd₆Fe₁₃Ga phase. It gives rise to a decrease in the proportion of matrix grains and saturation magnetization of the magnet. The detailed relationship between magnetic properties and microstructure is discussed based on these results.     

Improvement in magnetic properties,corrosion resistance and microstructure of Nd–Fe–B sintered magnets through intergranular addition of Tb68Ni32

New energy vehicles and offshore wind power industries have a high demand for sintered Nd–Fe–B magnets with high intrinsic coercivity and high corrosion resistance. In this study, the magnetic properties, anticorrosion properties, and microstructure of Nd–Fe–B sintered magnets with the intergranular addition of low-melting-point eutectic Tb₆₈Ni₃₂ alloy powders were investigated. The aim is to determine if the addition of Tb₆₈Ni₃₂ can improve these properties. A low melting-point eutectic alloy Tb₆₈Ni₃₂ powders was prepared as a grain boundary additive and blended with the master alloy powders prior to sintering. The coercivity of the resultant magnets gradually increases from 1468 to 2151 kA/m by adding increasing amounts of Tb₆₈Ni₃₂. At the same time, the remanence first increases and then slightly decreases. After studying the microstructure and elemental composition of the Tb₆₈Ni₃₂ added magnets, it is found that the significant increase in coercivity and the negligible reduction in remanence is due to densification, improved grain orientation, a uniform and continuous boundary phase distribution, as well as the generation of a (Nd,Pr,Tb)₂Fe₁₄B “core–shell” structure surrounding the main-phase grain. Moreover, the corrosion resistance of the magnet is greatly improved owing to the enhancement of electrochemical stability, as well as the optimization of the distribution and morphology of the intergranular phase.     

Toughness of Sintered NdFeB Magnets

The failure mechanism of sintered NdFeB magnets was studied from the view of fracture model and fracture mechanism, and the effect of alloying on strength and toughness of the magnets was researched too. The results showed that intercrystalline fracture was the main fracture model and cracks elongated along the interface between 2–14–1 phase and Nd-rich phase. Generally, the methods for improving toughness include enhancing bond strength of the interface between the two phases mentioned just, refining grains of 2–14–1 phase and increasing the amount of particles of Nd-rich phase localized at grain boundary. The fracture model of the Nd_15.5Fe_76.5Al₁Ti_0.5B_6.5 magnets was transcrystalline cleavage fracture, whose value of fracture toughness increased about 30% compared to that of the Nd_15.5Fe₇₈B_6.5 magnets. Otherwise, the value of fracture toughness of the magnets with a proper amount of addition of Ag increased about 70% compared to that of the Nd_15.5Fe₇₈B_6.5 magnets. Moreover, the two sorts of modified magnet mentioned just could be drilled successfully. In addition, development of the research about increasing the strength and toughness of sintered NdFeB magnets is reviewed in this article.     

Relationship between controllable preparation and microstructure of NdFeB sintered magnets

The double hard magnetic phase magnets with nominal compositions of Nd_30–xDy_xFe₆₉B₁(x=2, and 4) (wt.%) were prepared. The magnetic properties of the magnets were measured with a NIM-2000H hysteresigraph. The crystalline structures of the magnets were identified by X-ray diffraction (XRD). The Rietveld refinement was carried out using the FULLPROF software. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses were carried out in order to investigate the microstructure of the magnets. It showed that the magnets consisted mainly of Nd₂Fe₁₄B phase, and some Nd-rich phase. Two types of matrix-phase grains in dark grey and light grey were found in the magnets with x=2 and 4. The Dy content was obviously different in the two types of grains, which proved that the double hard magnetic phases (Dy-rich and Dy-lean phases) coexisted in the magnet. It revealed that the Nd-rich phases in junction regions had fcc structure, with the unit cell parameter of about 0.52–0.56 nm. The weak superlattice spots were found in the SAD patterns of the junction Nd-rich phases with large scale. The double hard magnetic phase structure seemed to improve the magnetic properties of NdFeB magnets with high coercivity, while decrease the consumption of Dy element, compared with the single alloy magnet.     

Effect of Terbium addition on the coercivity of the sintered NdFeB magnets

The effects of Tb addition on the microstructure and magnetic properties of the NdFeB magnets prepared by HD method were investigated by X-ray diffraction(XRD) and BH magnetometers.The results of the microstructure showed that both the Tb-doped and undoped permanent magnets were composed mostly of Tetragonal phase Nd2Fe14B(space group P42/mnm) and a trace amount of Nd-rich phase.Accordingly,addition of Tb led to a decrease of the pole density factor of(004),(006) and(008) crystal plane of the Nd2Fe14B phase calculated by Horta formula,but the coercivity of the magnets increased from 2038 kA/m up to 2302 kA/m as a consequence of Tb addition.The study of the Hc(T)/Ms(T) versus/Ms(T) behavior showed that the nucleation was the dominating mechanism for the magnetization reversal in both sintered magnets,and the microstructural parameters of αk and Neff were obtained also.The Kronmüller-plot showed an increase of the αk responsible for an increase of the coercivity.     

烧结NdFeB磁体强度和韧性研究现状

烧结NdFeB永磁材料具有磁性能高、价格低、生产工艺简单等突出优点，因而自其开发以来，生产和应用取得了快速发展，并且其市场还将不断扩大。但是，这种材料强度和韧性差，机械加工困难，应用过程中容易开裂。因此，改善烧结NdFeB永磁材料的力学性能已成为一个迫切的需要。综述了该种材料的断裂机制及在改善材料强度和韧性等方面所做的研究工作，认为通过添加合金元素，韧化晶界，细化主相晶粒，是一种有效的方法。研究表明，材料的断裂方式主要为沿晶断裂，晶界上的薄片富Nd相是引起沿晶断裂的主要原因，通过微量添加合金元素能有效改善磁体力学特性。     

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 NdH_3 and TbH_3 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 difficultnucleation(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.     

Effect of Cobalt on HDDR Anisotropic NdFeB

The effect of cobalt on the magnetic properties and anisotropy of HDDR anisotropic NdFeB was studied.It is found that Co is helpful for preparing anisotropic NdFeB with high coercivity. The research on the initial microstructure for NdFeB alloy indicates that Co tends to enter the crystal lattice of Nd-rich phase and some of Co atoms also enter the crystal lattice of Nd2Fe14B. The dissolution of Co changes the stability of Nd-rich phase and Nd2Fe14 B in H2 atmosphere and affects the kinetics of HDDR phase transformation. As a result, the NdFeB powder exhibits a high coercive force and a strong anisotropy.     

(Nd,Pr,Ce)-Fe-B磁体的组织与磁学性能

为开发低成本烧结钕铁硼磁体,用30% Ce替代(Nd_0.75Pr_0.25)_32.69Fe_66.25B_1.06磁体中的Nd和Pr,研究了磁体在烧结及回火过程中的组织结构和磁学性能变化.结果表明,取向压坯在1030~1080℃烧结2 h后,随烧结温度升高,磁学性能下降,烧结温度为1030℃时综合磁学性能均最好.烧结态Ce替代磁体的综合磁学性能优于未替代磁体.一级回火后,相组成和晶粒尺寸基本不变,边界结构也未发生明显变化,磁体性能基本不变,或有少量下降.二级回火后,晶界明显改善,获得较清晰且平直的晶界,磁体矫顽力均得到大幅提高.Ce替代磁体的剩磁、矫顽力和磁能积均稍低于未替代磁体.      

Effects of grain boundary diffusion process on magnetic properties enhancement and microstructure evolution of hot-deformed Nd-Fe-B magnets

Grain boundary diffusion(GBD) process is an important approach for producing Nd-Fe-B magnets with high coercivity and high thermal stability.The GBD for hot-deformed Nd-Fe-B magnets with nanocrystalline micro structure is more complicated compared to sintered magnets.Here,we investigated the effects of different GBD methods,i.e.,intergranular addition(in-situ GBD 1#),in-situ GBD from magnet surface during hot pressing and hot deformation(in-situ GBD 2#),and conventional GBD,on the magnetic prope...     

Al与Mo复合添加对NdFeB磁体矫顽力的影响

本文采用晶间合金化工艺将合金元素Mo和Al直接引入烧结Nd-Fe-B磁体晶间区域,改变晶间区域的合金体系和显微组织,以达到提高磁体矫顽力的目的.实验结果表明Mo在低温时效过程中可抑制晶间富Nd相与主相之间的平衡转变,使晶界区域析出细小二次主相晶粒,使矫顽力提高.     

Spark Plasma Sintering Nd-Fe-B Permanent Magnets With Good All-Around Property

In present study, sintered Nd-Fe-B permanent magnets with different compositions were fabricated by using both Spark Plasma Sintering (SPS) technique and conventional sintering technique. Microstructure and compositions of both magnets are observed by scanning electron microscope with energy dispersive X-ray detector. Magnetic properties, mechanical properties, and chemical stabilities of both Nd-Fe-B magnets are investigated. Compared with the conventional sintered magnets, SPS Nd-Fe-B magnets possess comparable magnetic properties, better corrosion resistance and mechanical properties. Further investigation shows that the good all-around properties of the SPS magnets result from their unique microstructure. In detail, the grain size of Nd₂Fe₁₄B main phase is fine and uniform, only a few Nd-rich phase forms along the grain boundaries of Nd₂Fe₁₄B, while most of them agglomerates into the triple junctions. As a result, SPS process is expected to be a promising method for the production of new Nd-Fe-B magnets with good all-around properties.     

Magnetic properties and microstructures of terbium coated and grain boundary diffusion treated sintered Nd-Fe-B magnets by magnetron sputtering

Tb coating on the surface of commercial sintered Nd-Fe-B magnet was prepared by DC magnetron sputtering.The secondary heat treatment was used to regulate the microstructure for the enhancement of coercivity,namely diffusion treatment and annealing treatment.The coercivity increases significantly from 18.3 to 28.0 kOe,the remanence decreases slightly from 14.1 to 14.0 kGs,and the comprehensive magnetic properties are higher than 75(Hcj+(BH)_max=76.7).SEM results indicate that,on the one hand,950℃is the optimal diffusion temperature.Lower diffusion temperature results in insufficient diffusion of Tb element.Higher diffusion temperature can lead to the main phase grain growth,the decrease of Nd-rich phase,and forming holes in the magnet.On the other hand,500℃is the optimal annealing temperature.Lower annealing temperature can result in the reduction of Nd-rich phase.Higher annealing temperature can generate the non-defined Nd-rich thin layer between grains.     

晶界扩散Dy60Co35Ga5合金对烧结钕铁硼磁体 磁性能及热稳定性的影响

研究了晶界扩散Dy₆₀Co₃₅Ga₅合金对烧结钕铁硼磁体磁性能及其热稳定性的影响.随着扩散温度的升高,磁体的矫顽力（H_cj）呈现出先增加后减少的趋势,并在890 ℃扩散3 h,480 ℃回火5 h的工艺条件下,矫顽力达到较优,从1 209 kA/m提高到1 624 kA/m,磁体的剩磁只有轻微的下降,从1.38 T降低到1.32 T.高温下测试磁体的磁性能,原始磁体和890 ℃晶界扩散Dy₆₀Co₃₅Ga₅合金磁体的矫顽力都呈下降趋势,但晶界扩散Dy₆₀Co₃₅Ga₅合金磁体的矫顽力在高温下要明显优于原始磁体.原始磁体及890 ℃晶界扩散Dy₆₀Co₃₅Ga₅合金磁体在不同温度下保温2 h的不可逆磁通损失分别为63 %和45 %.且DSC结果显示,890 ℃晶界扩散Dy₆₀Co₃₅Ga₅合金磁体的居里温度（T_c）要明显高于原始磁体的居里温度,这表明晶界扩散磁体的热稳定性得到了很大的改善. XRD图谱显示,890 ℃晶界扩散磁体RE₂Fe₁₄B相的衍射峰较原始磁体向右偏移,说明Dy原子及Co原子少部分已进入主相晶粒.      

Effect of Copper and Titanium Addition on Microstructures and Magnetic Properties of Sintered Nd_(22) Fe_(71) B_7 Magnets

Ｉｔｉｓｗｅｌｌｋｎｏｗｎｔｈａｔｔｈｅｉｎｔｅｒｇｒａｎｕｌａｒｍｉ ｃｒｏｓｔｒｕｃｔｕｒｅｏｆｓｉｎｔｅｒｅｄＮｄ Ｆｅ Ｂｍａｇｎｅｔｓｐｌａｙｓａｋｅｙｒｏｌｅｉｎｄｅｔｅｒｍｉｎｉｎｇｔｈｅｉｒｃｏｅｒｃｉｖｉｔｙａｎｄｔｈｅｒｍａｌｓｔａｂｉｌｉｔｙ[1,2 ] .Ｔｈｅｒｅｆｏｒｅ ,ｉｔｉｓｉｍｐｏｒ ｔａｎｔｔｏｆｕｒｔｈｅｒｍｏｄｉｆｙｉｎｇｔｈｅｉｎｔｅｒｇｒａｎｕｌａｒｍｉ ｃｒｏｓｔｒｕｃｔｕｒｅｖｉａａｌｌｏｙｉｎｇｏｒｏｔｈｅｒｐｒｏｃｅｓｓｉｎｇｍｅｔｈｏｄｉｎｉｍｐｒｏｖｉ…     

晶界扩散中Tb对烧结NdFeB磁体微观组织和磁性能的影响

通过晶界扩散技术提升烧结钕铁硼(NdFeB)磁体矫顽力的方法已获得广泛应用,为了研究重稀土磁粉对磁体综合磁性能的影响,本文采用喷涂扩散的方法将重稀土Tb含量为6.0%(质量分数)的磁粉作为复合扩散源的一部分进行晶界扩散并制备了高性能烧结NdFeB磁体。结果表明,当主扩散源占比为60%(质量分数)时,Nd₄₀Tb₆₀对应扩散磁体的矫顽力最高达到21.52 kOe,矫顽力增幅明显。经过微观组织结构和XRD表征分析,重稀土元素Tb沿晶界相扩散进入磁体内部的同时发生了晶格取代反应,可在晶粒表层生成磁晶各向异性场更强的(Nd,Dy/Tb)₂Fe₁₄B硬磁相,显著增强了磁体矫顽力。当主扩散源占比为20%、40%和80%(质量分数)时,Nd₈₀Tb₂₀,Nd₆₀Tb₄₀和Nd₂₀Tb₈₀对应扩散磁体的矫顽力增幅较小,其中Nd₈₀Tb₂₀扩散...     

Study on Key Techniques for Producing High Performance NdFeB Sintered Permanent Magnets

ＮｄＦｅＢｍａｇｎｅｔｓｈａｖｅｂｅｅｎｉｍｐｒｏｖｅｄｄｒａ ｍａｔｉｃａｌｌｙｓｉｎｃｅｉｔｗａｓｉｎｖｅｎｔｅｄｉｎ 1 983 .Ｔｗｏｔｙｐｅｓｏｆｓｉｎｔｅｒｅｄｍａｇｎｅｔｓ ,ｈｉｇｈｅｎｅｒｇｙｐｒｏｄ ｕｃｔｓ ((ＢＨ ) ｍａｘ)ａｎｄｄｏｕｂｌｅｈｉｇｈ (ｈｉｇｈ(ＢＨ ) ｍａｘａｎｄｈｉｇｈｃｏｅｒｃｉｖｉｔｙ (ｉＨｃ) )ａｒｅｔｈｅｔｗｏｔｅｎｄｅｎｃｉｅｓｆｏｒＮｄＦｅＢｓｉｎｔｅｒｅｄｍａｇｎｅｔｓ .Ｂｏｔｈｆａｃｔｏｒｉｅｓａｎｄｒｅｓｅａｒｃｈｉｎｓｔｉｔｕｔｅｓａｒｅｆｏｃｕｓ…     

Effects of Pr-Cu-Ti intergranular addition on microstructure and magnetic properties of heavy-rare-earth-free Nd-Fe-B sintered magnets

By intergranular addition of Pr-Cu-Ti alloy powders in the Nd-Fe-B sintered magnets with the normal B component, we propose an approach to the optimization of grain boundary and local Nd-Fe-B composition system. The coercivity is enhanced from 1.42 to 1.86 T, while further addition leads to a reduction in remanence and coercivity. The analyses of phase composition reveal that Ti mainly exists in the form of metallic Ti alloy, and part of Ti combines with B to form the TiB₂ phase after the liquid phase sintering process. This process results in a consumption of B in the local Nd-Fe-B composition system and a change of the grain boundary component, which contributes to the formation process of the RE₆(Fe,M)₁₄ phase after the annealing process. Therefore, with the modification of grain boundary and composition system, the intergranular addition of Pr-Cu-Ti induces the generation of continuous thin grain boundary phases. It promotes the intergrain exchange decoupling, increasing the coercivity in the annealed magnet. While the excess addition results in the segregation of TiB₂, as well as the precipitation of TiB₂ into the Nd-Fe-B phase, which leads to structural defects. Thus, the further effort for the addition alloy with Ti to reduce the deterioration of the microstructure will lead to further improvement in magnetic properties.     

Effect of Terbium addition on the coercivity of the sintered NdFeB magnets

The effects of Tb addition on the microstructure and magnetic properties of the NdFeB magnets prepared by HD method were investigated by X-ray diffraction (XRD) and BH magnetometers. The results of the microstructure showed that both the Tb-doped and undoped permanent magnets were composed mostly of Tetragonal phase Nd₂Fe₁₄B (space group P42/mnm) and a trace amount of Nd-rich phase. Accordingly, addition of Tb led to a decrease of the pole density factor of (004), (006) and (008) crystal plane of the Nd₂Fe₁₄B phase calculated by Horta formula, but the coercivity of the magnets increased from 2038 kA/m up to 2302 kA/m as a consequence of Tb addition. The study of the H_c(T)/M_s(T) versus H^min_N/(M_s(T) behavior showed that the nucleation was the dominating mechanism for the magnetization reversal in both sintered magnets, and the microstructural parameters of α_k and N_eff were obtained also. The Kronmüller-plot showed an increase of the α_k responsible for an increase of the coercivity.     

Constitution of melt-spun NdFeB-magnets studied by analytical field ion microscopy

An analytical field ion microscope was applied to melt-spun NdFeB magnets with various cobalt substitutions. The microstructure of melt-spun NdFeB magnets consists of three phases. Whereas the Nd₂Fe₁₄B- and the metastable Nd₇Fe₃-phase determine the magnetic properties of optimally quenched specimens the Nd_1.1Fe₄B₄-phase additionally influences the magnetic behaviour of overquenched and annealed material. With Co substitution the Nd-rich phase transforms via the metastable Nd₂₃(Fe_1−xCo_x)₇₅B₂ intermediate stage of optimally quenched specimens to the stable Nd(Fe_1−xCo_x)₂-phase in over-quenched and annealed ribbons. From our experimental data we conclude that a nucleation model of coercivity is consistent with the magnetic measurements.