Morphology and magnetic properties of Sm2Co7/α-Fe nanocomposite magnets produced by high energy ball milling and spark plasma sintering

In this article,the Sm₂Co₇/α-Fe nanocomposite magnets were prepared by high energy ball milling and spark plasma sintering method.The effect of soft phase content on the magnetic properties was studied.Up to 30 wt% α-Fe was added into Sm₂Co₇ matrix without the decrease of remanence.Optimal energy product(BH)max of 9.2 MGOe was obtained with 20 wt% α-Fe.TEM observation shows that the grain size of α-Fe is 20-50 nm which ensures a good coupling effect between soft and hard phase.One more thing needs to be mentioned is that there exists inter-diffusion between Sm-Co phase and α-Fe phase.Moreover,our results can also illustrate that the Sm₂Co₇/α-Fe nanocomposite magnets are able to acquire better magnetic properties than the SmCo₅/α-Fe magnets prepared by the same process due to the large domain width of Sm₂Co₇ phase.     

Design of Permanent Magnet Systems Using Finite Element Analysis

Rare earth permanent magnets have a wide range of magnetic properties to meet the requirements of an extensive variety of applications. Sintered Sm₂Co₁₇-type magnets have the best thermal stability with high magnetic performance at temperatures up to 550 °C. Sintered NdFeB magnets have the highest maximum energy product, (BH)_max, but are limited to applications with relatively low operating temperatures. Bonded magnets offer some design flexibility at the expense of magnetic properties. In view of these complexities, it is very important to understand the critical factors when designing the magnetic circuit. Using design examples based on finite element analysis (FEA), we will discuss magnetic materials selection, magnetic circuit design principles and design trade-offs for various applications.     

Effect of niobium substitution on microstructures and thermal stability of TbCu7-type Sm–Fe–N magnets

This paper reports crystal structures, magnetic properties and thermal stability of TbCu7-type Sm_(8.5)Fe_((85.8-x)Co_(4.5)Zr_(1.2)Nb_x(x = 0-1.8) melt-spun compounds and their nitrides, investigated by means of X-ray diffraction, vibrating sample magnetometer, flux meter and transmission electron microscope. It is found that the lattice parameter ratio c/a of TbCu_7-type crystal structure increases with Nb substitution, which indicates that the Nb can increase the stability of the metastable phase in the Sm-Fe ribbons. Nb substitution impedes the formation of magnetic soft phase a-Fe in which reversed domains initially form during the magnetization reversal process. Meanwhile, Nb substitution refines grains and leads to homogeneous micro structure with augmented grain boundaries. Thus the exchange coupling pining field is enhanced and irreversible domain wall propagation gets suppressed. As a result, the magnetic properties are improved and the irreversible flux loss of magnets is notably decreased. A maximum value 771.7 kA/m of the intrinsic coercivity H_(cj) is achieved in the 1.2 at% substituted samples.The irreversible flux loss for 2 h exposure at 120 ℃ declines from 8.26% for Nb-free magnets to 6.32% for magnets with 1.2 at% Nb substitution.     

Anisotropic Sm2Col7 nano-flakes produced by surfactant and magnetic field assisted high energy ball milling

Anisotropic Sm2Co17 flakes with high aspect ratio were prepared by magnetic field assisted high energy ball milling in the presence of heptane and oleic acid(OA).The thickness of flakes was only tens of nanometers.Coercivity of 3 kOe was achieved in the nano-flakes.Most interestingly,the magnetic crystalline anisotropy of Sm2Co17 flakes was improved compared to that of particles made by traditional ball milling.These anisotropic Sm2Co17 nano-flakes could be the building blocks for the future high-performance nano-composite permanent magnets with an enhanced energy product.     

Superior corrosion resistance and corrosion mechanism of dual-main-phase (Ce15Nd85)30FebalB1M magnets in different solutions

The electrochemical corrosion behavior of both(Ce₁₅Nd₈₅)₃₀Fe_balB₁ M sintered magnets prepared with dual-main-phase method and N45-type magnets was studied in 3.5 wt% NaCl,1.1 wt% NaH₂ PO₄,and2.5 wt% NaOH solutions,respectively.The(Ce₁₅Nd₈₅)₃₀Fe_balB₁ M sintered magnets perform superior corrosion resistance than N45-type magnets in the tested solutions.In general,two ...     

Microstructure evolution,mechanical properties and creep mechanisms of Mg-12Gd-1MM-0.6Zr (wt%) magnesium alloy

In this research, the microstructure evolution, mechanical properties, and creep mechanisms of Mg-12Gd-1MM-0.6Zr (wt%) alloy under different conditions were systematically studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile creep tests. Regarding the microstructure of the as-cast sample, the average grain size is about 42 μm, and the eutectic compounds were determined to be Mg₅(Gd_0.8MM_0.2). During homogenization, these eutectic compounds gradually dissolve, and Mg₁₂MM particles are precipitated. During hot extrusion, complete dynamic recrystallization (DRX) occurs, resulting in equiaxial grains with an average grain size of about 12 μm and the formation of streamlines consisting of Mg₁₂MM particles along the extrusion direction (ED). After T5 treatment (225 °C for 7 h), a large number of β′(Mg₇Gd) phases are precipitated on the {11–20}α habit plane and are interconnected, forming an interlaced network structure. The ultimate tensile strength (R_m = 405 MPa) and yield strength (R_P0.2 = 288 MPa) of the T5 sample are significantly higher than those of the as-extruded sample (R_m = 289 MPa, R_P0.2 = 185 MPa), but the elongation (A = 4%) was remarkably lower than that of the as-extruded sample (A = 18%). When crept at 225 °C under 100 MPa, the steady-state creep rates of the as-cast, as-extruded, and T5 samples are 1.59 × 10^–8, 1.08 × 10^–8, and 1.40 × 10^–8 s^–1, respectively, and their total strains within 100 h are respectively breaking, 0.81%, and 0.92%, indicating that the as-extruded alloy exhibits the best creep resistance. TEM analysis reveals that, during the creep process of the T5 sample, the β′ particles coarsen and the precipitate-free zones (PFZs) widen, which increase the steady-state creep rate and the total strain within 100 h as compared with the as-extruded sample.     

Model for Calculating J(H) Curves of Ni Coated Nd-Fe-B Magnets

An analytical model to describe the influence of surface degradation and the Ni layer itself on the magnetic properties of Ni coated Nd-Fe-B magnets is presented. Starting from the bulk magnetic properties, the dimensions, the thickness of Ni coating and the affected surface layer, J(H) demagnetization curve is calculated. Subsequently the expected values of (BH)_max, and the reversible permeability are deduced from the calculated J(H) curves. For flat magnets the surface effects lead to a decrease of B_r and an increase of the permeability which lowers (BH)_max. For strait magnets a step in the J(H) curve appears at H = 0. The deteriorating effect of Ni coating and the surface layer scale with the dimensions of magnet and the thickness of these layers, which depend on the processing and the grain size of magnet. These effects can not be neglected if one or more dimensions of a Ni coated magnet are less than about 5 mm. SmCo₅ magnets show similar effects but the coercivity of the damaged surface layer is higher. Pinning type Sm₂Co₁₇ magnets show almost no deterioration on surface due to machining. As a result, Sm-Co magnets are better suited for applications with dimensions smaller than about 2 mm.     

Anisotropic magnetocaloric effect in a dysprosium(III) single-molecule magnet — Commemorating the 100th anniversary of the birth of Academician Guangxian Xu

Dysprosium compounds with high magnetic anisotropy are widely studied as single molecule magnets (SMMs). Here the anisotropic magnetocaloric effect in a Dy(III) SMM, {[Dy(OSiMe₃)₂(4-Mepy)₅(BPh₄)] 0.5Toluene}, was studied by single crystals. Angular dependent magnetization can be observed at 300 K because of its high magnetic anisotropy. SMM behavior measured along the easy axis direction is identical to that of the polycrystalline sample. Rotating magnetization from the easy axis to the hard plane gives a maximum magnetic entropy change (–ΔS_R) of 3.05 J/(kg∙K) at 19 K at ΔB = 5 T, which enables the Dy(III) SMM to be used as a low-temperature rotating magnetic refrigerant.     

Energy approach to the problem of calculating the stresses at the initial stages of plastic deformation of crystalline substances and the appearance of structural defects

The critical shear stress and its temperature dependence are calculated for 12 simple substances with different structures and types of bonding. The shear stress for stage II–III of deformation of single crystals (τ^II–III) and σ_{0, y}, i.e., the Hall–Petch relation extrapolated to an infinitely large grain size, are estimated. The energy of formation of lattice defects (vacancies) is calculated using a proposed expression. The results of calculation of the elastic shear energy of a matrix and regions with a high elastic anisotropy are used to estimate the role of elastic anisotropy in lattice stability and fracture. The calculated and experimental results agree satisfactorily with each other.     

Improvement of coercivity and thermal stability of Nd-Fe-B sintered magnets by intergranular addition of Tb80Fe20 alloy

To improve the coercivity and temperature stability of Nd-Fe-B sintered magnets for high-temperature applications, the eutectic Tb₈₀Fe₂₀ (wt%) alloy powders were added into the Nd-Fe-B sintered magnets by intergranular method to enhance the coercivity (H_cj) and thermal stability. The microstructure, magnetic properties and thermal stability of the Nd-Fe-B magnets with different Tb₈₀Fe₂₀ contents were studied. The experimental results demonstrate that the coercivity (H_cj) of the sintered Nd-Fe-B magnet is significantly enhanced from 14.12 to 27.78 kOe, and the remanence (B_r) decreases not obviously by introducing 4 wt% Tb₈₀Fe₂₀ alloy. Meanwhile, the reversible temperature coefficients of coercivity (β) and remanence (α) of the Nd-Fe-B magnets are increased from ?0.5634%/℃ to ?0.4506%/℃ and ?0.1276%/℃ to ?0.1199%/℃ at 20–170 ℃, respectively. The Curie temperature (T_C) of the Nd-Fe-B magnet is slightly enhanced with the increase of Tb₈₀Fe₂₀ content. Moreover, the irreversible flux magnetic loss (h_irr) is obviously reduced as Tb₈₀Fe₂₀ addition increases. Further analysis of the microstructure reveals that a modified microstructure, i.e. clear and continuous RE-rich grain boundary layer, is acquired in the sintered magnets by introducing Tb₈₀Fe₂₀ alloy. The associated mechanisms on improved coercivity and thermal stability were comprehensively researched.     

Mechanical properties,thermal conductivity and defect formation energies of samarium immobilization in Gd2Zr2O7:First-principles study and irradiation experiment

A density functional theory(DFT) study was employed to investigate the mechanical property,thermal conductivity,Debye temperature,electronic structure and defect chemistry of(Gd_1-xSm_x)₂Zr₂O₇.All the(Gd_1-xSm_x)₂Zr₂O₇ compounds exhibit an excellent structural and mechanical stability(Gd_0.25Sm_0.75)₂Zr₂O₇ has the lowest Young’s modulus of...     

Preparation and magnetic properties of anisotropic Nd_2Fe_(14)B/Sm_2Co_(17) hybrid-bonded magnets

In the present work, anisotropic Nd_2 Fe_(14) B/Sm_2 Co_(17) hybrid-bonded magnets were prepared with different Nd-Fe-B contents. It is found that the particle distributions and ratios between the two magnetic phases have important roles in the magnetic properties, microstructures and thermal stability of the magnets. With increase of Nd-Fe-B content, the saturation magnetization of the anisotropic hybrid magnet increases significantly, however, coercivity decreases, and the demagnetization curves show magnetically single-phase behavior. The anisotropic Nd_2 Fe_(14) B/Sm_2 Co_(17) hybrid-bonded magnets exhibit a maximum energy product and remanence of 14.15 MGOe and 99.53 A·m~2/kg, respectively, when the NdFe-B content is 70 wt% at room temperature. Furthermore, the hybrid magnets also have better thermal stability at elevated temperatures due to the interaction between the two magnetic particles.     

Corrosion resistance and mechanical properties of (Ho,Nd)FeB magnets

(Ho,Nd)FeB magnets with different Ho contents were prepared by Ho substitution for part of Nd during the casting process. Effects of Ho contents on the corrosion resistance and mechanical properties of (Ho,Nd)FeB magnets were analyzed by a highly accelerated aging tester, an electrochemical workstation, a microhardness tester, a bending tester, a scanning electron microscope and an X-ray diffractometer. Results show that the addition of Ho can change the main phase structure, optimize the distribution of rare-earth rich (RE-rich) phases in grain boundary, and improve the corrosion resistance and mechanical properties of NdFeB magnets. When the content of Ho increases from 0 to 21.0 wt%, the weight loss of magnets decreases from 2.672 to 0.933 mg/cm², and the microhardness and bending strength increase from 528.74 HV and 374.92 MPa to 633.84 HV and 459.80 MPa, respectively.     

Optimization of both coercivity and knee-point magnetic field of Sm2Co17-type magnets via solid solution process

It is confirmed that phase homogenization is very important for improving the magnetic properties of 2:17-type Sm–Co sintered magnets. In this work, the influence of solid solution process on microstructure and magnetic properties of the Sm(Co_balFe_0.233Cu_0.073Zr_0.024)_7.6 sintered magnets was systematically studied. With the solid-solution treating duration (t_s) increasing from 0 to 4 h, intrinsic coercivity (H_cj) increases from 12.83 to 36.54 kOe, magnetic field at knee-point (H_knee) increases from 2.76 to 19.14 kOe, and the maximum energy product increases from 19.79 to 29.48 MGOe. The electron probe microanalyzer results reveal that there mainly exist gray and dark regions besides “white” rare earth-rich phase, and the content of Sm, Fe and Cu elements for the two kinds of regions changes a lot for the specimens. Furthermore, with t_s increasing up to 4 h, the elements content deviation between the gray and dark regions becomes small gradually from 3.94 at% to 0.27 at%, 7.66 at% to 0.21 at% and 7.27 at% to 0.16 at% for Sm, Fe and Cu elements, respectively. Moreover, transmission electron microscopy results show that the distribution of cell size is much more concentrated for aged specimens when t_s is 4 h. It is also found that the Cu concentration at cell boundaries for the 4 h solid-solution treatment case shows relatively higher values and greater concentration gradient (1.94 at%/nm). It is verified that sufficient solution treatment duration is prerequisite to form these homogeneous microstructural features, which are the key points for obtaining both high H_cj and H_knee.     

Progress of Research on Bonded Nd-Fe-B Magnets

Some progress of research on bonded Nd-Fe-B magnets in National University of Defense Technology(NUDT) is presented in this paper. The contents include B-rich R₂Fe₁₄B-based nano composite with good performance; a model to determine of the least amount of binder; resin for high temperature application; resin encapsulating magnetic powders for long-term storage; thermoplastic polymer used for mold-pressing magnets; hybrid bonded Nd-Fe-B/Sm₂Co₁₇ magnet with a potentially useful improvements in remanence and magnetic energy product.     

Simultaneous increase in remanence and coercivity during grain refining of Nd-Fe-B deformed magnets

To explore the feasibility of deep grain-size refinement in overcoming the problem of traditional B_r-H_ci trade-off in bulk permanent magnets, the effect of deformation temperature on the texture strength, grain refining, and magnetic properties of slow-deformed Nd-Fe-B magnets was systematically studied. As a result, strong textures with gradual grain refining from conventionally large size of D_L = 650 nm to small nanoscale size of D_L = 53 nm (D_L denotes the lateral size of the aligned plate-like grains) are observed as the temperature reduces from T_d = 650 °C to T_d = 450 °C. Moreover, magnetic observations show a simultaneous increase in remanence and coercivity as the grain refines from D_L = 650 nm to D_L = 127 nm. The increase in coercivity results from the grain size refinement as well as the smaller aspect ratio of the plate-like grains, and the increase in remanence results from the improved texture homogeneity. As compared with the D_L = 650 nm magnets, the simultaneous increase in remanence and coercivity leads to optimum enhancements of 57% in coercivity, 10% in remanence, and 25% in energy product, which demonstrate the feasibility of near-nanoscale grain refinement in overcoming the traditional B_r-H_ci trade-off for improved (BH)_max values. The failure of higher coercivities in the D_L ≤ 80 nm magnets is closely related with the defect effects of the grain boundaries. Mechanisms explaining the grain refining and texture changing behavior were also discussed.     

Influence of Pr-Al-Co alloy diffusion source on magnetic properties and microstructure of sintered Nd-Fe-B magnets processed by grain boundary diffusion

In this study, the influence of the content of Al and Co in the diffusion source on the magnetic performance and microstructure of the diffused magnet was studied by grain boundary diffusion treatment with Pr₇₀Al_30–xCo_x (x = 0 at%, 10 at%, 15 at%, 20 at%, 30 at%) alloys. When the Co content in the diffusion source increases from 0 at% to 10 at%, the coercivity enhancement in the Pr₇₀Al₂₀Co₁₀ diffused magnet is the highest, increased from 1.62 to 2.24 T, higher than 2.01 T of the Pr₇₀Al₃₀ diffused magnet. With further increase of Co content in the diffused source, the coercivity of the diffused magnet decreases gradually, the coercivity of Pr₇₀Al₁₅Co₁₅, Pr₇₀Al₂₀Co₁₀ and Pr₇₀Co₃₀ diffused magnet is 2.15, 1.99 and 1.81 T, respectively. Microstructural analysis shows that plenty of continuous grain boundary phases (CGBPs) can be formed in the Pr₇₀Al₂₀Co₁₀ diffused magnet under the synergistic effect of Al and Co, which leads to the enhancement of magnetic isolation between more adjacent grains. However, the amount of CGBP in the diffused magnets gradually decreases with the further increase of Co content in the diffusion source.     

Role of Alloying Elements in the Mechanical Behaviors of An Mg-Zn-Zr-Er Alloy

The mechanical behavior of the as-extruded and heat-treated Mg-1.5Zn-0.6Zr and Mg-1.5Zn-0.6Zr-2Er alloys was investigated and correlated with microstructure evolution. Deformation mechanisms are detailed. No evidence of twinning was observed under compression in the Er-bearing alloy throughout the grain size range of ~5 to 27 μm at a strain rate of 0.001 or 1/s. The compressive yield strength followed a Hall–Petch relation with a slope of ~10.3 MPa/mm^1/2. Er played a major role in the pyramidal 〈c+a〉 slip that was identified as a dominant plastic deformation mechanism. The CRSS for 〈c+a〉 slip system was greatly reduced and was 98 MPa in the as-extruded alloy. While it did not change the mechanical response of the Mg-1.5Zn-0.6Zr-2Er alloy, annealing was found to promote dissolution of Zn in the Mg matrix, leading to an increase in CRSS for extension twinning in the heat-treated Mg-1.5Zn-0.6Zr alloy. As a result, twinning was only observed under a higher strain rate of 1/s in compression. The CRSS for extension twinning for the heat-treated alloy with a grain size of ~28 μm was estimated to be 40 MPa, a bit lower than that for the Er-bearing alloy of the same grain size, which was 42 MPa.     

Coercivity kinetics upon step annealing of sintered Sm(Co_(0.88-x)Fe_xCu_(0.09)Zr_(0.03))_7 magnets

Behavior of the coercivity of the high-temperature Sm(Co_(0.88-x)Fe_xCu_(0.09)Zr_(0.03))_7 magnets depending on the temperature and time of annealing with the temperature decreasing stepwise from 700 to 400℃ was investigated.It is shown that the growth rate of coercivity abruptly increases at the initial stage of annealing in the vicinity of the Curie temperature of the SmCo_5 phase.The origin of the effect is the counter diffusion of Cu and Co atoms through dislocation tubes,which form because of enhanced stresses and a partial breakage of coherent coupling at the interface of the Sm_2 Co_(17) and SmCo_5 phases.Diffusive enrichment of the SmCo_5 phase in Cu close to the interface with Sm_2 Co_(17) leads to relaxation of stresses and increases in the gradient of the magnetic domain-wall energy and coercivity.     

A comparative study of NdY-Fe-B magnet and NdCe-Fe-B magnet

Low cost and high abundance rare earth elements Y and Ce have attracted tremendous interests of the industrial and scientific societies for fabricating the highly cost-performance efficient rare earth permanent magnets. However, the effect of separate replacement of Nd by Y or Ce on the performances of NdFeB-type magnet under the same atomic ratio and preparation conditions is still unclear. In this work, we systematically investigated the magnetic properties, thermal stabilities and service performances of (Nd_0.8Y_0.2)_13.80Fe_balAl_0.24Cu_0.1B_6.04 (atomic fraction, denoted as 20Y) and (Nd_0.8Ce_0.2)_13.80Fe_balAl_0.24Cu_0.1B_6.04 (atomic fraction, denoted as 20Ce) magnets. The results demonstrate that the μ₀M_r, μ₀H_c and (BH)_max of 20Y magnet are respectively 1.325 T, 1.173 T and 343.467 kJ/m³, which are comprehensively higher than those of 20Ce magnet (μ₀M_r = 1.310 T, μ₀H_c = 0.948 T, (BH)_max = 321.105 kJ/m³). Moreover, the 20Y magnet has higher thermal stability compared with 20Ce magnet which is favorable for the magnetic performances at elevated temperatures. The investigation of microstructure and elemental distribution indicates that the excellent magnetic performances of NdY-Fe-B magnet can be attributed not only to the preferable intrinsic properties 4πM_s, H_a and T_c of Y₂Fe₁₄B, but also to the in-situ core–shell structure of the 2:14:1 matrix phase grain with Y-rich core and Nd-rich shell, along with the thicker grain boundary layer between the adjacent matrix phase grains in NdY-Fe-B magnet. Furthermore, the 20Y magnet exhibits better mechanical property and higher corrosion resistance than 20Ce magnet, which are helpful for prolonging the service life of the magnet in practical application.