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.     

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.     

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.     

Effects of additives on hydrogen absorption and desorption characteristics of Nd(Fe,Mo)12 alloys

Effects of such additives as Co, Zr, Nb or Ga on hydrogen absorption and desorption characteristics of Nd(Fe,Mo)₁₂ alloys are investigated. It is found that Zr or Nb addition increases the disproportionation temperature of Nd(Fe,Mo)₁₂ alloys, and Co or Ga addition decreases the recombined temperature of its disproportionated products. This shows that Zr or Nb addition retards the disproportionation, while Co or Ga addition is effective for improving the recombination, which is similar to the effects of the additives on the hydrogen absorption and desorption characteristics of Nd₂Fe₁₄B alloys. However, according to X-ray diffraction (XRD) investigations for the magnetic-oriented samples, the final hydrogenation disproportiontation desorption recombination (HDDR) NdFe_10.5−XM_XMo_1.5 (M=Co, Zr, Nb or Ga) products are isotropic. The effects of additives on hydrogen absorption and desorption characteristics of Nd₂Fe₁₄B and Nd(Fe,Mo)₁₂ alloys are very similar, but the magnetic anisotropy of the final two HDDR products are different. In order to investigate this, similarities and differences of the two alloy systems and their corresponding HDDR phenomena are further studied. The results show that the formation of anisotropic powders may be related to the disproportionated products and crystal growth direction of the Nd₂Fe₁₄B and Nd(Fe,Mo)₁₂ system.     

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.     

Coercivity variations with Pr- and Zr-substituted NdDyFeB-based HDDR powders

As part of an industry-based project, we studied the effects of Pr and Zr substitutions to a basic Nd₁₅DyFe₇₆B₈ material. We processed the materials using a conventional hydrogenation-disproportionation-desorption-recombination (HDDR) process and a simple rotary-pump vacuum; however, we also experimented with high-vacuum conditions in order to see what effect these had. The Pr and Zr substitutions were observed to have a positive and cost-effective influence on the coercivity of the processed powders: the optimum Pr substitution was the replacement of three-quarters of the neodymium, and for Zr, a much smaller 0.1 at% was found to be the best.Microstructural observations of the as-cast structures revealed significant differences between the Zr- and Pr-substituted materials and the additive-free NdDyFeB alloy, but post-HDDR microstructures were all very similar and provided little help for optimizing the processing conditions. By combining the substitutions of Pr and Zr in a relatively rare-earth-rich alloy, we were able to produce a coercive powder of >1000 kA/m and have a process which can now be quickly and easily transferred to the factory.     

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.     

Complete suppression of metastable phase and significant enhancement of magnetic properties of B-rich PrFeB nanocomposites prepared by devitrifying amorphous ribbons

The effect of refractory element addition on phase transformation, crystallization behavior and magnetic properties of Pr_8.5Fe_81.5B₁₀ (addition-free) and Pr_8.5Fe_81.5M₂B₁₀ (M=V, Cr, Nb, Zr, Ti) ribbons has been investigated. The annealed addition-free ribbon as well as the samples with V or Cr additions are mainly composed of the metastable Pr₂Fe₂₃B₃ phase, whereas annealed ribbons with Nb, Zr or Ti additions primarily consist of Pr₂Fe₁₄B and a minor amount of Fe₃B/boride. The complete suppression of the metastable Pr₂Fe₂₃B₃ phase due to Nb, Zr or Ti additions leads to a significant enhancement of the magnetic properties. For example, the remanence, the coercivity and the energy product are remarkably increased from 2.5 kG, 0.4 kOe and 0.2 MG Oe for the addition-free material to 9.2 kG, 4.7 kOe and 7.6 MG Oe for the specimens with Nb addition. The successful elimination of the metastable Pr₂Fe₂₃B₃ phase is believed to profit from two factors: (a) Nb, Zr or Ti atoms substitute the Pr site, comparatively increase the Pr content, and thus inhibit the nucleation of Pr-lean Pr₂Fe₂₃B₃ phases, and (b) the formation of Nb, Zr, or Ti borides consumes some part of B, which hinders the generation of the B-rich Pr₂Fe₂₃B₃ phase.     

Influence of RE-rich phase distribution in initial alloy on anisotropy of HDDR powders

The influence of the RE-rich phase distribution in the precursor alloys on the anisotropy of the hydrogenation disproportionation desorption recombination(HDDR) processed powders is investigated. The homogenized ingot alloy and the as-cast strip casting(SC) alloy with a uniform RE-rich grain boundary phase lead to high anisotropy of the refined powders,acquiring degrees of alignment(DOA) of 0.62 and 0.54, respectively. The RE-rich phase aggregation results in a deteriorated DOA of the powders due to the drastic disproportionation rate, while a thin and uniform RE-rich phase distribution is beneficial for DOA. A reaction model of the initial particle microstructure is proposed for optimizing the HDDR powder anisotropy.     

The effects of annealing at 1000 °C for 24 h on the extrinsic properties of Pr-Fe-B and Nd-Fe-B-type sintered magnets

Recent studies have shown the effects of a post sintering heat treatment at 1000 °C for 24 h on the microstructure and magnetic properties of Pr-Fe-B/Nd-Fe-B magnets based on Nd₁₆Fe₇₆B₈ and Pr₁₆Fe₇₆B₈. In an attempt to understand the influence of environmental factors, an investigation into the effects of annealing under different degrees of vacuum for both types of sintered magnets has been carried out. The effect of annealing the Pr-Fe-B magnets at 1000 °C for 24 h resulted in a general increase in the magnetic properties, especially the intrinsic coercivity, although the degree of improvement appeared to be dependent on the initial annealing conditions (ambient pressure). Oxygen analysis of sintered and annealed magnets indicates a change in the nature of the grain boundary phases after the annealing treatment. The effect of annealing the Nd-Fe-B magnets at 1000 °C for 24 h resulted in a general decrease in the magnetic properties, especially the intrinsic coercivity.     

Magnetic properties of Pr-Fe-B/Mn films with perpendicular anisotropy

The influence of a Mn layer on the magnetic properties of sputtered Pr-Fe-B/Mn films with Cu spacer layer has been investigated for various Mn layer thicknesses. The Pr-Fe-B/Mn films all possess perpendicular anisotropy. An enhancement of the intrinsic coercivity _iH_c is observed for suitable Mn layer thickness and _iH_c exhibits an oscillatory dependence on the thickness of the Mn layer with a period of about 60 nm. The average size of the columnar Pr₂Fe₁₄B grains is about 100 nm. A highest _iH_c value of 22.1 kOe and an optimal (BH)_max value of 18.2 MGOe are reported for these Pr-Fe-B/Mn films.     

纯三元及含Zr,Ga元素磁粉的各向异性

未经均匀化热处理的纯三元及含Zr,Ga元素的SC合金铸片经优化的HDDR工艺处理都可以制备各向异性Nd Fe B磁粉.这表明:元素的添加及SC铸片是否进行了均匀化热处理都不是HDDR磁粉各向异性形成的必要条件.磁粉各向异性形成的关键因素在于HDDR工艺的调节,即适当地加快歧化反应过程,减缓脱氢再结合过程以及控制脱氢再结合时的合适氢气压强均有利于磁粉各向异性的形成.本文将为制备低成本高各向异性磁粉提供重要的指导.     

纯三元及含Zr, Ga元素磁粉的各向异性

未经均匀化热处理的纯三元及含Zr, Ga元素的SC合金铸片经优化的HDDR工艺处理都可以制备各向异性NdFeB磁粉。这表明：元素的添加及SC铸片是否进行了均匀化热处理都不是HDDR磁粉各向异性形成的必要条件。磁粉各向异性形成的关键因素在于HDDR工艺的调节,即适当地加快歧化反应过程,减缓脱氢再结合过程以及控制脱氢再结合时的合适氢气压强均有利于磁粉各向异性的形成。本文将为制备低成本高各向异性磁粉提供重要的指导。     

Magnetic properties,microstructure, and phase evolution of PrxFebal.TiyB20−x (x=4–9; y=2.5–5) nanocomposites

Magnetic properties, microstructure, and phase evolution of Pr lean and boron-enriched Pr_xFe_bal.Ti_yB_20−x (x=4–9; y=2.5–5) melt-spinning ribbons with nanostructures have been investigated. Based on thermal magnetic analysis (TMA), for y=2.5, two phases, namely Pr₂Fe₁₄B and α-Fe, were found for ribbons with x=9, while additional two metastable phases, Pr₂Fe₂₃B₃ and Fe₃B, existed for x=4, 7 and 8. With the decrease of Pr content, the remanence increases but coercivity decreases. The optimal properties of B_r=9.5 kG, _iH_c=10.7 kOe, and (BH)_max=17.8 MG Oe are achieved in Pr₉Fe_bal.Ti_2.5B₁₁ nanocomposites. On the other hand, higher Ti substitution for Fe in Pr₇Fe_bal.Ti_yB₁₃ ribbons could refine the grain size and suppress the metastable Pr₂Fe₂₃B₃ and Fe₃B phases effectively. The excellent permanent magnetic properties are mainly dominated by the nanoscaled microstructures and the coexistence of sufficient magnetically soft phases, Fe₃B, Pr₂Fe₂₃B₃ and α-Fe, with magnetically hard Pr₂Fe₁₄B phase.     

Effect of Substitution of Zr and Pr on magnetic properties of R2Co17(R=Er,Yb)

Magnetic properties (saturation magnetizations, Curie temperatures and anisotropy fields) have been measured for the ternary systems Er₂Co_17-xZr_x and Yb₂Co_17-xZr_x and for the quaternary system Er_2-yPr_yCo_16.4Zr_0.6. Introduction of Zr expands the lattice and diminishes the Curie temperature, both indicating Zr replaces Co. Saturation magnetization is diminished, when Zr replaces Co, more rapidly than simple dilution, suggesting Co d-band filling by electron transfer from Zr. The anisotropy field (H_A) is sharply increased when Co replaces Zr, indicating that both preferential substitution and band structure effects are involved. Replacement of Er by Pr enhances H_A, presumably because of preferential replacement of Er at the 2d site. H_A of 42 kOe (at 295 K) was achieved for Er_1.6Pr_0.4Co_16.4Zr_0.6.     

Effect of substitution of Zr on the magnetic properties of R2Co17 (R = Ce and Sm)

The ternary systems Ce₂Co_17?xZr_x and Sm₂Co_17?xZr_x have been studied to ascertain the effect of Zr substitution on the magnetic properties. X-ray diffraction experiments show that single phase materials can be obtained only in the range of x ≤ 1 and that Zr stabilizes the hexagonal Th₂Ni₁₇ structure. The unit cell volume is found to increase by substitution of Zr. The Curie temperature T_c decreases monotonically with increasing x at an approximate rate of 100 and 90 deg/atom for the Ce and Sm systems, respectively. The saturation magnetic moments also decrease monotonically with increasing x for both the systems. The rate of decrease is larger than that expected as a simple dilution. The anisotropy fields H_A are significantly increased by Zr substitutions for both the systems, reaching 70 and 200 kOe at 77 K in the cases of Ce₂Co₁₆Zr₁ and Sm₂Co₁₆Zr₁, respectively. Among all the substituents studied until now, Zr seems to be the most effective for permanent magnet applications. Crystallographic and magnetic characteristics indicate that Zr replaces Co when the ternaries are formed.     

Resistivity and magnetoresistance of Fe80B20 and Fe78B13Si9 amorphous glasses

Summary The electrical resistivity of Fe₈₀B₂₀ and Fe₇₈B₁₃Si₉ amorphous glasses as a function of temperature from 293 K down to 15 K was measured, and it was found to fit quite well with the model given by Cote and Meisel. Comparison between our resistivity measurements of Fe₈₀B₂₀ and others was made, where some differences were found. These resistivity differences are evidence for a variety of amorphous atomic arrangements of the samples. The longitudinal magnetoresistance of Fe₈₀B₂₀ and Fe₇₈B₁₃Si₉ at 293K and 77K was measured in a low magnetic field. The observed magnetoresistance shows a typical field dependence known for ferromagnetic materials.     

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.     

Beneficial effect of Co substitution on the structure and magnetic properties of mechanically alloyed (Pr,Tb)–Fe–(C,B) magnets

The effect of Co substitution on the structure and magnetic properties of mechanically alloyed Pr₁₄Tb₂Fe_76−xCo_xC₆B₂ and Pr₁₆Fe_76−xCo_xC₆B₂ (x=0–20$x=0–20$) alloys has been studied systematically. The main phase in the alloys is Pr₂Fe₁₄C-type carbide, coexisting with a small amount of α-Fe and rare-earth-rich phases. In addition to the increasing of the Curie temperature of the Pr₂Fe₁₄C-type phase, Co substitution can affect the magnetic properties by adjusting the α-Fe fraction of the alloys. The increase of both coercivity and remanence has been realized in a certain composition range. This increase may be attributed mainly to the enhancement of the effective anisotropy constant _Keff$K_{\mathrm{eff}}$ of the magnets due to the reduced α-Fe fraction with a small Co addition. The highest coercivity _iH_c of 20.3 kOe and the optimum energy product (BH)_max of 10.3 MG Oe have been obtained for the Pr₁₄Tb₂Fe_69.5Co_6.5C₆B₂ alloy.     

Dependence of Magnetic Properties on Composition of Nanocrystalline Fe–M–B–Cu (M: Zr,Nb, Mo,Ti, Ta) Alloys

The specialized rf-Mössbauer technique is used to elucidate the magnetic properties of NANOPERM-type nanocrystalline alloys. The influence of alloy composition on the soft magnetic properties is studied for the Fe₈₀M₇B₁₂Cu₁ (M: Ti, Ta, Nb, Mo, Zr) alloys. The rf-Mössbauer experiments allowed us to distinguish magnetically soft nanoclusters from magnetically harder microcrystalline phases. The measurements performed as a function of the rf field intensity allowed the determination of the distribution of anisotropy fields related to the size distribution of bcc nanoclusters. Smaller anisotropy fields in the nanocrystalline phase were found in Nb-, Zr-, and Mo-containing alloys as compared with the alloys which contain Ti and Ta. The Mössbauer measurements were supplemented by X-ray diffraction determination of the size of nanocrystalline grains.