Improvement of microstructure and magnetic properties of Nd–Fe–B alloys by Nb and Co additions

In order to establish the role of niobium on the hydrogenation, disproportionation, desorption and recombination (HDDR) behavior of near-stoichiometric alloys, two alloys: NdI3Fe8OB7 and Nd13Fe78Nb1Co1B7 (at%) were investigated before, during and after the HDDR process. The microstructure of the as-cast Nb-free alloy before employing the HDDR process was found to consist of three phases, the matrix Nd₂Fe₁₄B (φ) phase, Nd-rich phase and a significant amount of free iron; whereas, the microstructure of the Nb-containing alloy consisted of only the first two phases.     

Effects of the conventional HDDR process and the additions of Co and Zr on anisotropy of HDDR Pr-Fe-B-type magnetic materials

Effects of the conventional hydrogenation disproportionation desorption recombination (HDDR) process and the additions of Co and Zr on anisotropy of HDDR PrFeB-type magnetic materials are investigated. The results show that the degree of anisotropy in conventional HDDR Pr₁₃Fe₈₀B₇ materials decreases monotonically with the prolonged disproportionation time, and short disproportionation time is helpful for preparing highly anisotropic Pr₁₃Fe₈₀B₇ material. However, it is notable that the degree of anisotropy in conventional HDDR Pr₁₃Fe₈₀B₇ materials is significantly smaller than that in solid-HDDR Pr₁₃Fe₈₀B₇ materials with the same disproportionation time. At the same time, it is found that the addition of Co and Zr may make HDDR Pr-Fe-B materials that have higher anisotropy compared with HDDR pure ternary Pr₁₃Fe₈₀B₇ materials under the same HDDR process, but their degree of anisotropy will also decrease monotonically with the prolonged disproportionation time, and will be close to zero when the disproportionation time is greater than 20 h. Based on this, the origin of anisotropy is discussed by X-ray diffraction (XRD) investigations for the disproportionated products of the above alloys. The results show that the origin of anisotropy in HDDR Pr-Fe-B materials with the addition of Co or Zr may differ from that in HDDR pure Pr₁₃Fe₈₀B₇ materials, and the former maybe from the residual “Pr₂(Fe,Co,Zr)₁₄B” nucleus while the latter is not. Finally, it is also found that HDDR Pr-Fe-B materials with Co or Zr can obtain high-magnetic properties even if the high-desorption temperature is used, and this shows the addition of Co and Zr may make HDDR Pr-Fe-B materials that have a larger process temperature range.     

Origin of anisotropy for the HDDR Nd13.5Fe79.5B7 magnetic powders

For the HDDR Nd_13.5Fe_79.5B₇ magnetic powders, effects of disproportionation time and hydrogen pressure on the anisotropy were studied during the slow desorption stage. Studies showed that shorter disproportionation times caused the magnetic powders displaying higher anisotropy. With increasing disproportionation times, the degree of crystallographic alignment decreased. This in turn caused a drop in remanence and anisotropic character. Longer disporportionation times have also been correlated to a change in disproportionated microstructure from lamella to columnar. XRD (X-Ray Diffraction) studies showed that except NdH₂,α-Fe and Fe₂B, no other phases were included in the disproportionation mixture. This elucidated that the strong anisotropy is only related to a lamella disproportionation microstructure, which corresponds to a short disproportionation times. The lamella disproportionation microstructure may remain or inherit the alignment of original Nd₂Fe₁₄B grain, and may also be related to the alignment of the newly formed Nd₂Fe₁₄B grain. Thus, the anisotropic formation mechanism of ternary magnetic powders accords with “anisotropy-mediating phase” model. If the disproportionation mixture were carried out an optimum hydrogen pressure treatment during the HDDR process, the degree of crystallographic alignment can be further enhanced.     

Microstructural evolutions of Pr13Fe80B7 alloys during solid HDDR process

Microstructural evolutions of Pr₁₃Fe₈₀B₇ alloys during solid hydrogenation disproportionation desorption recombination (HDDR) process is systematically investigated. The results show that the early-disproportionated products of Pr₁₃Fe₈₀B₇ alloys are mainly characteristic of rod-like morphology, and the rods are PrH₂ while the matrix is Fe. Moreover, all the PrH₂ rods have the same crystallographic orientation, and grow with a definite orientation related to the Fe matrix. However, it is notable that no iron boride phase except for NdH₂ and Fe is found. With the prolonged disproportionation time, the rod-like disproportionated products coarsen, and the Fe₂B come to form. When the disproportionation time is 17 h, the rod-like disproportionation morphology transforms into sphere, and a large amount of Fe₂B is found. Subsequent investigations for the recombination show that the recombination reactions start at the boundaries between PrH₂ rods and Fe matrix, and the rim-like Pr₂Fe₁₄B is formed on the PrH₂ rods. Moreover, the recombined PrFeB powder of the rod-like microstructure has strong magnetic anisotropy.     

Texture in a ternary Nd16.2Fe78.2B5.6 powder using a modified hydrogenation–disproportionation–desorption–recombination process

A modified hydrogenation–disproportionation desorption-recombination (HDDR) process consisting of (i) solid disproportionation and (ii) slow recombination under partial hydrogen pressure has been applied to a Nd_16.2Fe_78.2B_5.6 alloy. Scanning electron microscopy shows that an initially fine rod-like structure of NdH_2±x and Fe observed after 15 min of hydrogenation at 900°C is transformed into a granular morphology with prolonged annealing. Simultaneously, finely dispersed tetragonal Fe₃B particles of 10–50 nm diameter exist. XRD studies show that this metastable Fe₃B phase is transformed to Fe₂B and Fe on further annealing. Short solid-disproportionation times result in a higher degree of anisotropy after recombination, whereas long annealing times and conventional processing lead to isotropic material. It is concluded that the formation of the intermediate tetragonal Fe₃B phase after solid disproportionation is pivotal for the inducement of texture in HDDR processed ternary NdFeB-type alloys.     

The production and characterisation of highly anisotropic PrFeCoB-type HDDR powders

A study of the processing of highly anisotropic HDDR powder and PTFE-bonded magnets with the composition Pr_13.7Fe_63.5Co_16.7B₆M_0.1 (M=Zr or Nb) has been undertaken. These alloys were processed by an homogenising heat treatment for a range of times, and a subsequent HDDR treatment employing a range of disproportionation times. The optimum time for the homogenisation of the as-cast structure was found to be 20 h at 1100°C, while the optimum disproportionation time in the HDDR treatment was found to be 10 min at 860°C. Zr-additions appear to inhibit grain growth during the heat treatment process, whereas Nb-additions appeared to control more effectively the grain growth during the disproportionation and recombination stages of the HDDR process.     

Enhanced alignment and magnetic properties of die-upset nano-crystal Nd2Fe14B magnets with Nb addition

It is difficult to obtain the crystallographic alignment for stoichiometric Nd₂Fe₁₄B alloys by applying the melt-spun and subsequent hot-pressing and hot-deformation techniques. However, the enhanced alignment and magnetic properties of die-upset nano-crystal Nd₂Fe₁₄B magnets have been obtained by Nb addition in the present paper. The magnetic properties studies show that Nb addition leads to the remarkable increase of remanence B_r and intrinsic coercivity H_ci, which is due to the improvement of c-axis texture and refinement of microstructure. Microstructure studies using transmission electron microscopy (TEM) and X-ray diffraction (XRD) reveal that Nb atoms are enriched at grain boundary and the NbFeB phase is observed with increasing Nb content. Since some Fe atoms in the Nd₂Fe₁₄B phase participate in the formation of NbFeB phase, the excessive Nd atoms may be enriched at grain boundary, which may improve the physical property of grain boundary and provide a mass transport pass for preferential growth of oriented Nd₂Fe₁₄B grains, thus leading to the enhanced alignment and magnetic properties.     

Magnetic structure and preferential occupation of Fe in the composite compound Nd2Co6Fe

The crystal and magnetic structures of the composite compound Nd₂Co₆Fe have been investigated by high-resolution neutron powder diffraction and X-ray powder diffraction. The compound crystallizes in the hexagonal Ce₂Ni₇-type structure consisting of Nd(Co,Fe)₂ and Nd(Co,Fe)₅ structural blocks alternately stacked along the c-axis. Multi-pattern Rietveld refinement of neutron diffraction and X-ray diffraction data at room temperature reveal that substitution of Fe for Co occurs exclusively in the Nd(Co,Fe)₅ structural blocks. The preferential occupation of the Fe atoms in the structure is discussed based on the mixing enthalpy between Nd and Fe atoms and on the lattice distortions. In agreement with the reported magnetic phase diagram of the Nd₂Co_7−xFe_x compounds, magnetic structure models with the moments of all atoms in the ab plane at 300 K and along the c-axis at 450 K provide a satisfactory fitting to the experimental neutron diffraction data. The refinement results show that the atomic moments of (Co,Fe) atoms within the Nd(Co,Fe)₅ blocks decrease slightly with temperature, whereas the atomic moments of Nd in the compound and of (Co,Fe) atoms at the interface between the Nd(Co,Fe)₂ and Nd(Co,Fe)₅ blocks are reduced significantly.     

Effect of element substitution on nanostructure and hard magnetism of rapidly quenched Nd2Fe14B

The structural and magnetic properties of Nd₁₂Fe₈₂B₆ and Nd₁₀M₂Fe₈₂B₆ (M = Nb, Ti, Zr, Cr) alloys prepared using arc melting and melt spinning have been investigated. All the samples are found to crystallise with a tetragonal Nd₂Fe₁₄B phase without any alloy or elemental impurities. There is a small decrease in the unit cell volume of Nd₂Fe₁₄B due to transition metal (M) addition. The substitution of Nb and Ti refines and homogenises the nanostructure of the alloys, promoting intergrain exchange coupling leading to an increase in the remanence and energy product. For example, the remanence and energy product of Nd₁₂Fe₈₂B₆ and Nd₁₀Nb₂Fe₈₂B₆ are 8.4 kG and 15 MGOe, and 9.9 kG and 20 MGOe, respectively. The J(T) curves are similar to those of a single phase ferromagnetic material suggesting no segregation of ferromagnetic impurities. The observed structural and magnetic properties are consistent with the fact that the substitutional transition metal atoms occupy the Nd site of the tetragonal Nd₂Fe₁₄B crystal lattice. The improvement of magnetic properties of nanocrystalline Nd₂Fe₁₄B alloys with the decrease in Nd concentration may be beneficial for the application of this material in bonded magnets.     

In situ Mössbauer and magnetization studies of Fe–Si nanocrystallization in Fe73.5Si13.5B9Cu1Nb1 X2, with X = Nb, Zr, Mo, amorphous alloys

The Fe–Si nanosized particles were obtained by controlled partial crystallization of Fe_73.5Si_13.5B₉Cu₁Nb₁X₂ (X = Nb, Zr, Mo) amorphous alloys. In situ Mössbauer spectroscopy and magnetization measurements have been used to follow the temperature-dependent magnetization of the amorphous as well as of the nanosized Fe–Si particles. Our results, for the residual amorphous and of nanoparticles phases, show that the temperature dependence of the hyperfine field and magnetization of both residual amorphous and nanocrystalline Fe(Si) phases are different from that of the as-quenched bulk amorphous or crystalline Fe₃Si alloys. Likewise, from the temperature dependence studies it was possible to determine that the onset temperature of the nanocrystallization process increases in the sequence Mo < Nb < Zr, for the same annealing conditions.     

Simultaneous enhancement in coercivity and remanence of Nd2Fe14B permanent magnet by grain boundary diffusion process using NdHx

We have successfully developed a Dy-free grain boundary diffusion process with neodymium hydride (NdH_x) alloy to the permanent magnet Nd₂Fe₁₄B powders using hydrogenation – disproportionation – desorption – recombination (HDDR) method. All the diffusion treatments were performed at 700–800 °C for various annealing time under the high vacuum with rotating diffusion method that effectively control the abnormal grain growth. The coercivities of Dy-treated Nd₂Fe₁₄B powders were varied from 9.5 kOe to 13.2 kOe but the remanence was decreased to 8.1 kG (10% reduction) depending on dysprosium hydride (DyH_x) content and diffusion treated time. However, the coercivity and remanence of Dy-free diffusion treated powder have been increased to 12.2 kOe (28.5% enhancement) and 11.1 kG (22% enhancement) at the optimal diffusion treatment (800 °C for 3 h), respectively. This unique simultaneous enhancement is to isolate the magnetic coupling between Nd₂Fe₁₄B grains by creating non-magnetic Nd grain boundaries and enhance the alignment of the Nd₂Fe₁4B hard magnetic phase, fabricated by optimal diffusion conditions.     

Phase composition, microstructures and magnetic properties of melt-spun Nd1.2Fe10.5Mo1.5 ribbons and their nitrides

The effect of wheel speed on the phase compositions and microstructures of melt-spun Nd_1.2Fe_10.5Mo_1.5 ribbon was investigated. It is found that the Nd(Fe,Mo)₁₂ phase can be obtained at the wheel speed of 10 m/s, and TbCu₇-type Nd(Fe,Mo)₇ phase is formed with the wheel speed higher than 10 m/s. The amorphous phase is achieved at 65 m/s. The average grain size of phases decreases linearly with increasing wheel speed. The Nd(Fe,Mo)₁₂N_1.0 nitride obtained from annealed ribbons quenched at 65 m/s shows a coercivity much higher than that from the ribbons quenched at 10 m/s, which is due to the smaller grain size in the former ribbons.     

Mössbauer spectroscopy applied to the study of Nd-Fe-B permanent magnets corrosion

Powdered Nd-Fe-B-type permanent magnets were heated at 200 C for times up to 32 days. The evolution of the Nd₂Fe₁₄B magnetic phase, followed by⁵⁷Fe Mössbauer spectroscopy, evidenced the positive influence of both the hydrogen decrepitation process and Co, Nb and V additives on the corrosion resistance.     

Coercivity enhancement of anisotropic Dy-free Nd–Fe–B powders by conventional HDDR process

Coercivity enhancement of Dy-free Nd–Fe–Co–B–Ga–Zr powders was studied using the conventional hydrogenation–decomposition–desorption–recombination (HDDR) process. It was found that the addition of Al together with the proper Nd content and the slow hydrogen desorption of the HDDR treatment can induce high coercivity in the powder. For example, the 14.0 at% Nd–2.0 at% Al powder exhibits H_cJ of 1560 kA/m, B_r of 1.22 T, and (BH)_max of 257 kJ/m³. The high coercivity inducement of the powder is thought to be attributed to the formation of Nd-rich phase, which continuously surrounds fine Nd₂Fe₁₄B grains.     

Co和稳定元素对Nd3(Fe,Co,M)29(M=Ti,V,Cr) 化合物结构和磁性的影响

利用x射线衍射和磁测量研究了不同稳定元素Co以及Ti,V和Cr替代对Nd₃Fe_29-x-yCo_xM_y(M=Ti,V,Cr)化合物结构和磁性的影响.研究发现:每一个稳定元素都有一替代量极限,在此极限以内所有化合物均为Nd₃(Fe,Ti)₂₉型结构,A2/m空间群.不同稳定元素的溶解极限不同.Co的替代量与稳定元素有关,当以Cr作为稳定元素时,Cr的替代量随着Co含量的提高而提高 关键词： 3(Fe')" href="#">Nd₃(Fe Co 29')" href="#">M)₂₉ 结构 磁性     

Nd2Fe14B的价电子结构分析和磁性计算

应用固体与分子经验电子理论计算了Nd₂Fe₁₄B的价电子结构、磁矩和居里温度,计算结果与实验值相符.计算表明:该合金的磁性与3d磁电子数成正比.从Fe(c)晶位到Fe(k₂)晶位磁矩增加,其机理源于价电子、哑对电子和3d磁电子之间的转化,有78%的哑对电子和18%的3d共价电子转化成了磁电子.居里温度和磁矩与Fe原子配位数成正比,与加权等同键数I^σ成反比,Nd原子 关键词： 2Fe₁₄B')" href="#">Nd₂Fe₁₄B 价电子结构 居里温度     

Theoretical investigation on the phase stability of Nd2Fe14B and site preference of V,Cr, Mn,Zr and Nb

The stability of the excellent permanent magnetic compound Nd₂Fe₁₄B and substitution of Fe in the compound by V, Cr, Mn, Zr and Nb are investigated by using interatomic pair potentials which are converted from lattice-inversion method. Calculation shows that the substitution always makes the cell volume larger, and the increase of the volume is almost linear with substituent concentration. The calculated cohesive energy shows that the preferential order of substitution of Fe is Nb, V, Cr, Mn, Zr. Nevertheless, all the five substituting elements should most preferentially replace Fe in the j₂ site, which has the greatest space among all six Fe sites.     

Preparation,microstructure, and magnetic properties of a nanocrystalline Nd12Fe82B6 alloy by HDDR combined with mechanical milling

Nanocrystalline Nd₁₂Fe₈₂B₆ (atomic ratio) alloy powders with Nd₂Fe₁₄B/α-Fe two-phase structure were prepared by HDDR combined with mechanical milling. The as-cast Nd₁₂Fe₈₂B₆ alloy was disproportionated via ball milling in hydrogen, and desorption–recombination was then performed. The phase and structural change due to both the milling in hydrogen and the subsequent desorption–recombination treatment was characterized by X-ray diffraction (XRD). The desorption–recombination behavior of the as-disproportionated alloy was investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The morphology and microstructure of the final alloy powders subject to desorption–recombination treatment were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The results showed that, by milling in hydrogen for 20 h, the matrix Nd₂Fe₁₄B phase of the alloy was fully disproportionated into a nano-structured mixture of Nd₂H₅, Fe₂B, and α-Fe phases with average size of about 8 nm, and that a subsequent desorption–recombination treatment at 760 °C for 30 min led to the formation of Nd₂Fe₁₄B/α-Fe two-phase nanocomposite powders with average crystallite size of 30 nm. The remanence B_r, coercivity H_c, and maximum energy product (BH)_max of such nanocrystalline Nd₁₂Fe₈₂B₆ alloy powders achieved 0.73 T, 610 kA/m, and 110.8 kJ/m³, respectively.     

The Mössbauer study of Nd−Fe−Co−B permanent magnetic alloys

Sintered Nd₁₇ (Fe_1?x Co_x)₇₅B₈ permanent magnetic alloys have been studied by Mössbauer effect, X-ray diffraction and electron probe micro-analysis, The results show that the alloys are composed of the tetragonal phase Nd₂(Fe,Co)₁₄B, the B-rich phase Nd₁₁₁(Fe,Co)₄B₄ and the Nd-rich phase Nd₈₀(Fe,Co)₁₉B. In the tetragonal phase, Co atoms occupy preferentially the k₂ and j₂ sites, and Fe atoms occupy randomly the k₁ site and preferentially the j₁ site while the e and c sites seem to be completely occupied by Fe atoms.     

Mössbauer study of intergranular phase in Nd 8 Fe86 − x Nb x B6(x = 0, 1, 2, 3) nanocomposite magnet

Nd₈Fe_86???x Nb _x B₆ (x = 0, 1, 2, 3) nanocomposite magnet has been studied by Mössbauer spectroscopy and nanostructure observation. It was found that intergranular phase formed between α-Fe and Nd₂Fe₁₄B phase in NdFeNbB alloys plays a significant role on the magnetic properties. By the addition of Nb into Nd₈Fe₈₆B₆ composition, coercivity was found to increase by 25% due to the grain refinement of both the soft and hard magnetic phases which was decreased from 50 nm of virgin Nd₈Fe₈₆B₆ to 25 nm in Nd₈Fe₈₅Nb₁B₆ alloys. The role of Nb addition was confirmed to stabilize the Nd₂Fe₁₄B lattice preventing from thermal vibration of the corresponding sites at where Fe atoms are substituted by Nb in the Nd₂Fe₁₄B lattice. The enhanced coercivity was originated from the exchange hardening of soft and amorphous phases surrounding the hard magnetic Nd₂Fe₁₄B crystal.