Search

The correlation between microstructure and magnetic properties of 2:17 SmCo commercial magnets 32 H and 32 L is studied. The 32 H magnet with high coercivity has an excellent ($00l$) orientation texture, and its magnetic properties are (BH) $_{\mathrm {max}}= 29.47$ MGOe, $B_{\mathrm {r}} = 11.20$ kG, and $H_{\mathrm {cj}} = 29.45$ kOe. The magnetic properties of the 32 L magnet with low coercivity are (BH) $_{\mathrm {max}}= 29.28$ MGOe, $B_{\mathrm {r}} = 11.40$ kG, and $H_{\mathrm {cj}} = 9.829$ kOe. The electron probe microanalysis results show that a Cu-lean grain boundary region is obvious in the 32 H magnet, which affects the squareness of the demagnetization curve. The transmission electron microscope result shows that the cell size and cell boundary thickness of high and low coercivity magnets are 129.32 and 9.83, and 90.94 and 7.87 nm, respectively. But the cell structure of the 32 H magnet is more complete and clearer than that of the 32 L magnet. The difference of the Cu concentration between the cell boundary phase and cell phase in the 32 H magnet is bigger than that of the 32 L magnet, which is really responsible for the high coercivity. [ABSTRACT FROM AUTHOR]

Nd-Fe-B permanent magnets with a small amount of CuZn5 powders doping were prepared by conventional sintered method. The effects of CuZn5 contents on magnetic properties and microstructure, electrochemical corrosion resistance of sintered Nd-Fe-B magnets were systematically studied. The results show that the magnetic properties of magnets do not have a significant variation by CuZn5 powders doping; the coercivity of magnets rises gradually, while the remanence of the magnets decreases a little with increasing of the CuZn5 amount. The CuZn5 doped magnets have more positive corrosion potential, Ecorr, and much lower corrosion current density, icorr, than the magnets without CuZn5 doping, indicating CuZn5 doping could improve the corrosion resistance. Both Zn and Cu enrich mainly into the Nd-rich phase, fully improve the wettability between the Nd-rich phase and the Nd2Fe14B phase, and repair the defects of the main phase, so the coercivity of magnets doped with CuZn5 powders rises. Such microstructure modification effectively restrains the aggressive inter-granular corrosion. As a result, the CuZn5 doped magnet possesses excellent corrosion resistance in NaCl electrolyte. [ABSTRACT FROM AUTHOR]

Polarization curve and electrochemical impedance spectroscopy tests have been applied to investigate the corrosion kinetics for both spark plasma sintered (SPS) and conventional sintered Nd–Fe–B magnets in different electrolytes. Effect of microstructure modification on the chemical stability of the magnets was discussed. Both magnets exhibit evident passive behavior in alkaline electrolyte but are susceptible to corrosion in weak acid and saline electrolytes. SPS magnets are more corrosion resistant than conventional sintered magnets due to a remarkable microstructure modification formed in the SPS process. Further investigation shows that the unique microstructure in SPS magnets effectively restrains the aggressive intergranular corrosion which takes place along Nd-rich phases in conventional magnets. [ABSTRACT FROM AUTHOR]

The correlation between preparation conditions and microstructure as well as magnetic properties were studied for Sm(Co 0.665 Fe 0.25 Cu 0.06 Zr 0.025 ) 7 magnets. Increase of sintering temperature from 1468 to 1493 K promotes the amalgamation of small grains into big ones, resulting in an enlargement of average grain size from 10 to 25 μm and diffusion of Cu from inner grains to Cu-lean grain boundaries. As a result, the demagnetization curve squareness of the magnets improves substantially, leading to a remarkable enhancement of the maximum energy product from 21.3 to 27.1 MGOe. The coercivity of the magnets drops from 25.3 to 16.0 kOe at the same time, which was verified by the increase of the average domain width from 1.1 μm in small grains to 2.5 μm in big grains. It is therefore concluded that tuning of the preparation conditions causes a more homogeneous distribution of Cu, which is crucial for the improvement of magnetic properties. [ABSTRACT FROM AUTHOR]

Microstructure modification induced giant coercivity enhancement from 950 to 1688 kA/m has been achieved in Sm(Co 0.665 Fe 0.25 Cu 0.06 Zr 0.025 ) 7 permanent magnets via Cu powders doping. Microstructure observation shows that in original magnet there are Cu-lean regions along the grain boundary area, where regular cellular structure does not form. After Cu powders doping, most Cu-lean regions disappear. Comparison of magnetic domain reversal between the original magnet and doped magnet indicates that reversed domain propagation was substantially restrained due to the elimination of the Cu-lean region. Moreover, the pinning field increases by 83.3% after Cu doping, having good agreement with coercivity enhancement. [ABSTRACT FROM AUTHOR]

Cast Fe-10Cr-1.5B-2Al alloy was quenched at different temperatures. The effects of quenching temperature on microstructure and hardness and wear-resistance of Fe-10Cr-1.5B-2.0Al alloy were investigated by means of the optical microscopy, the scanning electron microscope, X-ray diffraction, energy dispersive spectrometer, Vickers hardness and Rockwell hardness tester, and the MM-200 block-on-ring wear testing machine under dry friction condition. The results indicate that the as-cast microstructure of Fe-10Cr-1.5B-2.0Al alloy consists of ferrite, pearlite and netlike eutectics which are distributed in the grain boundary. The eutectics mainly include herringbone M2B and chrysanthemum M7(C, B)3. The matrix gradually turns into single martensite with the increase of the quenching temperature. The type of borocarbides has no obvious change after quenching. The netlike boride almost totally fractures and transforms from the fish-bone structure to the graininess. There is some retained austenite in the quenched structures when the quenching temperature is more than 1100 °C. When the quenching temperature is in a range of 1000 °C to 1100 °C, the hardness and wear resistance show a sharp increase with an increase of temperature, and show a slight decrease after surpassing 1100 °C. [ABSTRACT FROM AUTHOR]

The effects of quenching temperature on microstructure and hardness of cast Fe-8Cr-2B alloy containing 0.3 wt% C, 2.0 wt% B, 8.0 wt% Cr, 0.6 wt% Si, and 0.8 wt% Mn were investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Rockwell hardness and Vickers microhardness testers. The experimental results indicate that the as-cast microstructure of cast Fe-8Cr-2B alloy consists of M2B (M = Fe, Cr), M7(C, B)3, α-Fe, and γ-Fe. The dendritic matrix composed of lath martensite mixed with a small amount of retained austenite, and the netlike boride M2B distribute in the grain boundary. After quenching between 950 °C and 1100 °C, the netlike eutectic boride are broken up and a new phase-M23(C, B)6 which is distributed in the shape of sphere or short rod-like are precipitated from the matrix. Both the macrohardness and microhardness of specimens increase with the increasing quenching temperature. At about 1050 °C, the hardness reaches the maximum value. However, when the temperature exceeds 1050 °C, the hardness will decrease slightly. With the increase of tempering temperature, the hardness of cast Fe-8Cr-2B alloy quenching from 1050 °C decreases gradually and its impact toughness increases slightly. Crusher hammer made of cast Fe-8Cr-2B alloy quenching from 1050 °C and tempering from 300 °C has good application effect, and its service life improves by 150-180% than that of high manganese steel hammer. [ABSTRACT FROM AUTHOR]

An anisotropic Nd-Fe-B magnet was prepared by the spark plasma sintering (SPS) technique. A magnet with the same composition was produced using the conventional sintering method for comparison. The oxidation behavior in oxidizing environments for both magnets was studied. The rnicrostructure and composition of the magnets were investigated using scanning electron microscopy with an energy-dispersive x-ray detector. Compared with the conventional sintered Nd-Fe-B magnet, the SPS magnet possesses a lower oxygen diffusion coefficient, which leads to a lower oxidation rate, a thinner oxidation layer, and lower oxygen content. Microstructure observation shows that the SPS magnet possesses a unique microstructure due to the special sintering mechanism. In detail, the grain size of the Nd2Fe14B main phase isfine and untform. Only a little of the Nd-rich phase forms along the grain boundaries of the Nd2Fe14B phase, while most of it agglomerates into triple junctions as small particles. Therefore, the pathways for oxidation propagation are restricted and the intergranular oxidation process through the Nd-rich phase in the magnet is suppressed effectively. As a result, the SPS Nd-Fe-B magnet possesses better oxidation resistance than that of the conventional magnet. [ABSTRACT FROM AUTHOR]

Tetragonal polycrystalline Mn-Ga nanoflakes with a nominal composition of Mn1.33Ga were prepared by the surfactant-assisted high-energy ball milling method, and the effects of milling time and annealing treatment on structure and the magnetic properties were also investigated in detail. The irregular Mn1.33Ga flakes show typical shape anisotropy and a high aspect ratio with thicknesses in the range of 100 nm \sim 3 \mu m. Crystalline grain is reduced to 10 nm when increasing the milling time above 4 h. Coercivity is greatly improved, and the highest coercivity of 7.2 kOe is obtained for flakes milled for 2 h, but the saturation magnetization, Ms , and remanence, Mr , decrease sharply during the ball milling process. Medium-temperature annealing is beneficial to the crystallization and the enhancement of magnetization. After annealing at 773 K for 30 min, Ms and Mr are recovered to 68 and 41 emu/g, respectively; and the (BH)\mathrm {max} can reach 2.3 MGOe which is comparable with that of 2.5 MGOe for films or microparticles. [ABSTRACT FROM AUTHOR]

Melt spun ribbons of Nd4·5Fe77B18·5 alloy were consolidated into bulk permanent magnets by spark plasma sintering (SPS) method. Effects of heat treatment on the density, microstructure and magnetic properties of the magnets are studied. Further observation shows that the sample consolidated by SPS possesses ultrafine nanocrystalline structure and small fraction of amorphous phase. After proper heat treatment, magnets with fine crystal structure and good magnetic properties were obtained simultaneously. On the other hand, excessive grains growth, which will lead to undesirable magnetic properties, can occur due to improper heat treatment. [ABSTRACT FROM AUTHOR]

The corrosion resistance and microstructure of anisotropic Nd-Fe-B magnet prepared by spark plasma sintering (SPS) technique were studied. The SPS Nd-Fe-B has displayed more positive corrosion potential Ecorr and lower corrosion current density Icorr than those of the conventional sintered Nd-Fe-B with the same composition in several electrolytes, indicating better anti-corrosion properties. It also shows good stability in an oxidized atmosphere. The microstructure of SPS Nd-Fe-B magnet indicates that the grain size of Nd2Fe14B is fine (about 6 μm) and uniform, and the Nd-rich phase does not form along the grain boundaries of Nd2Fe14B, but agglomerates into the triple junctions. Therefore, the pathways for corrosion propagation are restricted and the inter-granular corrosion process through Nd-rich phase in the magnet is suppressed effectively. As a result, the SPS Nd-Fe-B magnet possesses excellent corrosion resistance. [Copyright &y& Elsevier]

The structure and thermal stability were studied for bulk nanorcystalline \Sm0.8{\Tm}0.2{\Co}5.2 permanent magnets produced by spark plasma sintering technique. X-ray diffraction results show that the magnet exhibits \CaCu5 crystal structures. Transmission electron microscopy observation indicates that the microstructure of the magnet was composed of fine and uniform grains with an average size of about 50 nm. Magnetic measurement shows that the Curie temperature (TC) of the magnet is 983 K, the magnetization and the remanence (Mr) under 9 T of the magnet are 71.72 and 47.82 emu/g, respectively, and the coercivity (Hci) of the magnet reaches as high as 25.70 kOe. Moreover, the Mr of the magnet keeps almost invariant as the temperature increases from 373 K to 573 K, and the Mr and Hci temperature coefficient in the range of 373 K–773 K are \-0.082\%/\K and \-0.144\%/\K, demonstrating the excellent thermal stability of the magnet. [ABSTRACT FROM PUBLISHER]

In this paper, bulk nanocomposite permanent magnets are prepared by spark plasma sintering (SPS) technique using melt-spun powders with a nominated composition of Pr4.2Tb0.3 Fe78B17.5. Effect of processing conditions on the density, microstructure, and magnetic properties of the magnets are studied. Higher sintering temperature improves the densification of the magnets, while deteriorates their magnetic properties simultaneously due to the excess crystal grain growth. An increased compressive pressure leads to better magnetic properties and higher density for the magnets. Magnet possesses a Br of 10.4 kGs and a Hci of 2.76 kOe under the optimal sintering condition. Observation shows the microstructure of the magnet contains fine grains with an average size of about 20 nm. The fine grain size in the microstructure and good magnetic properties are ascribed to the rapidly sintering speed and high compressive pressure of SPS, which effectively restrains the grain growth and allows the formation of fine crystalline materials with full dense in a short time. [ABSTRACT FROM AUTHOR]

Anisotropic Nd15.5Dy1.0FeBalCo3.0B6.8Al1.0 magnets were produced by the spark plasma sintering (SPS) technique. The effects of processing conditions on the microstructure, magnetic properties, dimensional precision and density of the magnets were studied. The magnetic properties, microstructure and constituents were investigated by means of a magnetic flux density - magnetic field strength (B-H) loopline instrument, scanning electron microscopy and energy dispersive X-ray analysis. The density of the magnets was determined by the Archimedes method, and the dimensional precision of the magnets was measured by micrometer. It was found that the microstructure of SPS processed Nd-Fe-B magnets is unique; the grain size is fine and uniform while distribution of the neodymium rich phase is heterogeneous. The optimal magnetic properties of SPS processed Nd-Fe-B magnets obtained so far are maximum energy product of 240 kJ m-3 and coercive force of 1260 kA m-1. The dimensional precision of the magnets is ~ 20 μm, and the density of the magnets reaches 7.58 g cm-3. [ABSTRACT FROM AUTHOR]

The microstructure, crystal structure, and magnetic properties were studied for Nd-Fe-B nanoflakes prepared by surfactant-assisted high-energy ball milling (HEBM) with heptane and oleic acid as the milling medium. The microstructure, crystal structure, and magnetic properties of the nanoflakes were examined with scanning electronic microscopy, x ray diffraction, and vibrating sample magnetometer, respectively. Effect of ball-milling time on the c-axis crystallographic alignment and coercivity of Nd-Fe-B nanoflakes were systematically investigated. Microstructure observation shows that the Nd-Fe-B nanoflakes have an average thickness of tens of nanometers, an average diameter in the range of 500 ∼ 1000 nm, and an aspect ratio as high as 100. As the ball-milling time increases from 2 h to 12 h, the intensity ratio between (006) and (105) reflection peaks, which indicates the degree of c-axis crystal texture of the Nd2Fe14B phase, drops gradually, indicating that the long time-milling process undermines the c-axis crystal texture of Nd2Fe14B phase in the nanoflakes. On the other hand, the coercivity of the nanoflakes firstly increases, peaks at 3.8 kOe for 8 h, and then decreases again along with the ball milling. [ABSTRACT FROM AUTHOR]

Highlights • The Sm(Co bal Fe x Cu 0.06 Zr 0.025) z magnets with different Fe content and z value were investigated. • The correlation of Fe content and z value was established to achieve excellent magnetic properties. • Increasing Fe content promoted the irreversible microstructure evolution in magnets. • Magnetic domain patterns of magnets affected by increasing Fe content indirectly. Abstract Correlation between Fe content and z value on the structural and magnetic properties of the Sm(Co bal Fe x Cu 0.06 Zr 0.025) z magnets (x = 0.20, 0.25, 0.28, and z = 7, 9.5) has been investigated. It is found that the cellular phases in the magnets grow up with increasing Fe content, so the cell boundary phases could hardly form well without enough Sm atoms, resulting in coercivity decreasing. After supplement of Sm content by decreasing z value, cell boundary phases formed continuously, and coercivity was strongly enhanced due to the remarkable concentration gradient of Cu between the 1:5H cell boundary phase and 2:17R cellular phase. It is thus suggested that as the Fe content increases, the z value of magnets need reducing simultaneously in order to keep high coercivity of the magnets. [ABSTRACT FROM AUTHOR]

In present study, sodium silicate, a kind of heat-resistant binder, was used to prepare bonded Nd-Fe-B magnets with improved thermal stability and mechanical strength. Effect of curing temperature and curing time of the new binder to the magnetic properties, microstructure, and mechanical strength of the magnets was systematically investigated. Fracture surface morphology observation show that sodium silicate in bonded magnets could completely be cured at 175 °C for 40 min, and the magnets prepared under this condition exhibit optimal properties. They exhibit usable magnetic properties of B r of 4.66 kGs, H cj of 4.84 kOe, and (BH) max of 4.06 MGOe at 200 °C. Moreover, the magnets possess high compressive strength of 63 MPa. [ABSTRACT FROM AUTHOR]

Highlights • We proposed here a new method to make bulk composites of La-Fe-Si hydrides using sodium silicate. • This method only needs a relatively low solidification temperature and can create a homogenous microstructure. • The bulk composites possess good mechanical, thermal and magnetocaloric properties. Abstract We adopted sodium silicate to prepare composites of La(Fe, Si) 13 hydrides in this work. A homogenous and continuous matrix of sodium silicate was formed at a relatively low solidification temperature. No counter diffusion between the matrix and the grains of La(Fe, Si) 13 hydrides was observed. The magnetocaloric effect of pure La(Fe, Si) 13 hydrides is largely preserved in the composites, showing a high tolerance to sodium silicate. These composites possess high thermal conductivity and decent compressive strength compared with the epoxy resin bonded samples. These results suggest that sodium silicate is a promising binder material for producing bulk La(Fe, Si) 13 hydrides for their application in magnetic refrigeration. [ABSTRACT FROM AUTHOR]

[Display omitted] • Novel forward-extrusion method was used to prepare anisotropic NdFeB magnets. • The hot-extruded NdFeB magnet showed heterogeneous microstructure characteristics. • The formation of the c-axis texture was studied for the hot-extruded NdFeB magnet. In this paper, the microstructure and texture formation mechanism of nanocrystalline anisotropic Nd-Fe-B magnets prepared by a one-step heating forward-extrusion method were systematically studied. The hot-extruded (HE) Nd-Fe-B magnet showed the same heterogeneous microstructure characteristics as the traditional die-upsetting magnet. With the decrease of extrusion width (w1), the c-axis texture gradually enhanced, accompanied by an increasing proportion of fine-grain region and the improvement of the ordered arrangement of platelet grains. In addition, strong c-axis texture was formed by the ordered arrangement of platelet grains benefiting from the difference of stress distribution caused by the difference of side length of the extrusion port. By adjusting the dimension of the rectangle extrusion port, the magnetic properties of HE Nd-Fe-B magnets were optimized. The optimized magnetic properties with (BH) max of 241.3 kJ/m3, H ci of 1025.3 kA/m, M r of 0.94 MA/m can be acquired for the hot extrution (HE) Nd-Fe-B magnet with extrusion width (w1) of 2.5 mm. In this method, the demoulding process can be avoided successfully by using cold pressing instead of hot pressing, which is of great significance for continuous and high-volume production. [ABSTRACT FROM AUTHOR]

The microstructure, crystal structure and magnetic properties were studied for Sm 0.6 Pr 0.4 Co 5 nanoflakes prepared by surfactant-assisted high-energy ball milling (SAHEBM). Effect of ball-milling time on the c -axis crystallographic alignment, morphology and magnetic properties of Sm 0.6 Pr 0.4 Co 5 nanoflakes was systematically investigated. With increasing milling time from 1 h to 7 h, the intensity ratio between (002) and (111) reflection peaks indicating degree of c -axis crystal texture of the (Sm, Pr)Co 5 phase increases first, peaks at 3 h, then drops again, revealing that the strongest c -axis crystal texture was obtained in the nanoflakes milled for 3 h. On the other hand, the coercivity ( H ci ) of the flakes increases gradually from 1.71 to 14.65 kOe with the increase of ball milling time. As a result, an optimal magnetic properties of M r of 10.23 kGs, H ci of 11.45 kOe and (BH) max of 24.40 MGOe was obtained in Sm 0.6 Pr 0.4 Co 5 nanoflakes milled for 3 h, which also displayed a high aspect ratio, small in-plane size, pronounced (001) out-of-plane texture. [ABSTRACT FROM AUTHOR]

The structure and magnetic properties of Fe nano-particles prepared by the sonochemistry method were systematically studied. The power of sonicator played an important role in the microstructure and magnetic properties of the nano-particles. Initial X-ray powder diffraction patterns of Fe nano-particles prepared with a power of 180W and 450W exhibited amorphous-like diffraction peaks. TEM images showed that the average particle size of Fe nano-particles made with 180W is 2-3nm and increases to 5-6nm with 450W. The amorphous and superparamagnetic nature of the Fe nano-particles has been confirmed by TEM, SAD and magnetometry measurements. The saturation magnetization (Ms) of the 2-3nm Fe nano-particles is only 0.414T. Increasing the power to 450W, the Ms of Fe nano-particles with the size of 5-6nm can reach the value of 0.958T. To obtain fully crystalline particles, the Fe nano-particles were annealed at 923K for 2min. After annealing, the Ms increased to 1.807T. TEM results show that the amorphous-like Fe nano-particles have been transformed to crystalline bcc α-Fe. [ABSTRACT FROM AUTHOR]

Abstract: Nd–Fe–B permanent magnets with a small amount of Dysprosium (Dy) nanoparticles doping were prepared by conventional sintered method, and the microstructure and magnetic properties of the magnets were studied. Investigation shows that the coercivity rises gradually, while the remanence decreases simultaneously with increased Dy doping amount. As a result, the magnet with 1.5wt.% Dy exhibits optimal magnetic properties. Further investigation presumed that Dy is enriched as (Nd, Dy)2Fe14B phase in the surface region of the Nd2Fe14B matrix grains indicated by the enhancement of the magneto-crystalline anisotropy field of the Nd2Fe14B phase. As a result, the magnet doped with a small amount of Dy nanoparticles possesses remarkably enhanced coercivity without sacrificing its magnetization noticeably. [Copyright &y& Elsevier]