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Abstract
The influence of SPS sintering temperature on microstructure and magnetic properties of the sintering bulk alloy of melt quenching Fe70Cr8 Mo2Si5B15 powders by ball milling and MA Fe73.5Cu1 Nb3Si13.5B9 and Fe80Co4 Nb7B9 nanocrystalline powders was studied. The results show that (1) under 30MPa/5min conditions, the relative densities of bulk alloys increase with the increase of sintering temperatures. When Fe70Cr8 Mo2Si5B15 bulk alloy sintered at 1000°C, its relative density reaches above 99%, and when Fe73.5Cu1 Nb3Si13.5B9 and Fe80Co4 Nb7B9 bulk alloys sintered at 1050°C, their relative densities reach to 99%; (2) the main phase of sintering bulk alloys is α-Fe phase, and a little second phase intermetallic compound. The grain size of α-Fe phase in bulk alloys is in nanometer range, Fe70Cr8 Mo2Si5B15 bulk alloy has the finest grain size, its average grain size is about 50 nm; (3) with the sintering temperature increasing, the specific saturate magnetization Bs of these bulk alloys increases and coercive force Hc decreases, respectively. Fe70Cr8 Mo2Si5B15 bulk alloy has the lowest Hc, 4.1 kA·m-1.
Fe-rich and Co-rich amorphous glass-coated metal microwires with diameters in the range of 6.1-28.0 μm and 14.0-35.2 μm were synthesized by the Taylor-Ulitovsky method, respectively. The magnetic parameters of microwires samples with same metal core diameters, different glass coating thicknesses and same glass coating thicknesses, different metal core diameters were analyzed by vibrating sample magnetometer. The results show that the axial coercive fields and remnant magnetization ratios of the two types of microwires reduce with the increasing of metal core diameters, and rise with the increasing of glass coating thicknesses. The changes of radial remnant magnetization ratios of the microwires are reverse with that of the axial remnant magnetization ratios. The influences of metal core diameters and glass coating thicknesses on the magnetostatic properties of glass-coated microwires are due to the changes of the stress of metal cores via the changes of the volume fraction of the metal inner cores and metal outer shells with different magnetic domain framework.
Bulk metallic glasses (BMGs) Fe–B–Y–Nb–Cu, 2mm in diameter, were successfully annealed to become bulk nano-crystalline alloys (BNCAs) with α-Fe crystallite 11–13nm in size. A ‘crystallization-and-stop’ model was proposed to explain this behavior. Following this model, alloy-design criteria were elucidated and testified successful on another Fe-based BMGs Fe–B–Si–Nb–Cu, 1mm in diameter, with crystallite sizes 10–40nm. It was concluded that BNCAs can be designed in general by the proposed criteria.
Bulk metallic glasses (BMGs) Fe–B–Y–Nb–Cu, 2 mm in diameter, were successfully annealed to become bulk nano-crystalline alloys (BNCAs) with α-Fe crystallite 11–13 nm in size. A ‘crystallization-and-stop’ model was proposed to explain this behavior. Following this model, alloy-design criteria were elucidated and confirmed successful on another Fe-based BMG Fe–B–Si–Nb–Cu, 1 mm in diameter, with crystallite sizes 10–40 nm. It was concluded that BNCAs can be designed in general by the proposed criteria.
Microstructures and magnetic properties of Fe84Nb7B9, Fe80Ti8B12 and Fe32Ni36 (Nb/V)7Si8 B7 powders and their bulk alloys prepared by mechanical alloying (MA) method and hot-press sintering were studied. The results show that: 1) After MA for 20 h, nanocrystalline bcc single phase supersaturated solid solution forms in Fe84 Nb7B9 and Fe80Ti8B12 alloys, amorphous structure forms in Fe32Ni36Nb7Si8 B17 alloy, duplex microstructure composed of nanocrystalline γ-(FeNi) supersaturated solid solution and trace content of Fe2B phase forms in Fe32Ni36V7Si8 B17 alloy. 2) The decomposition process of supersaturated solid solution phases in Fe84Nb7B9 and Fe80Ti8Bi2 alloys happens at 710-780°C, crystallization reaction in Fe32Ni36Nb7Si8 B17 alloy happens at 530°C (the temperature of peak value) and residual amorphous crystallized further happens at 760°C (the temperature of peak value), phase decomposition process of supersaturated solid solution at 780°C (the temperature of peak value) and crystallization reaction at 431°C (the temperature of peak value) happens in Fe32Ni36V7Si8 B7 alloy. 3) under 900°C, 30 MPa, 0.5 h hot-press sintering conditions, bulk alloys with high relative density (94.7%-95.8%) can be obtained. Except that the grain size of FeNb7B9 bulk alloy is large, superfine grains (grain size 50-200 nm) are obtained in other alloys. Except that single phase microstructure is obtained in Fe80Ti8B12 bulk alloy, multi-phase microstructures are obtained in other alloys. 4) The magnetic properties of Fe80Ti8B12 bulk alloy (Bs=l.74 T,Hc=4.35 kA/ m) are significantly superior to those of other bulk alloys, which is related to the different phases of nanocrystalline or amorphous powder formed during hot-press sintering process and grain size.
Fe73.5Cu1Nb3 Si13.5B9 soft magnetic alloy powders and its bulk alloys were prepared by the mechanical alloying (MA) method and cryogenical high pressure solidification process. Phase components, average grain size, thermal stability of powders and relative density of bulk alloys of the samples were analyzed by XRD, DSC, SEM and TEM. The results showed that (1) after MA for 70 hours, bcc single phase α-Fe nanocrystalline supersaturate solid solution powders with average grain size 9.5 nm can be obtained; (2) four exothermal peaks with different intensity appeared in DSC heating-up curves, these peaks corresponded to structural relaxation of distorted nanocrystalline supersaturate solid solution, crystallization of small quantity amorphous and phase precipitation of supersaturate solid solution, respectively. Phase precipitation of supersaturate solid solution included two stages; (3) under the sintering conditions of P = 5.5 GPa, t = 5 min and PW = 980 W, single phase α-Fe nanocrystalline bulk alloys with 98.4% relative density and about 12.5 nm average grain size can be obtained, soft magnetic properties of the bulk alloys were specific saturation magnetization Ms = 1.71 × 10-7 Wb·m/g, coercive force Hc = 8.22 × 103 A/m.
Fe65Co10Ga5P12C4 B4 glassy alloy powders were produced by high pressure Ar gas atomization of Fe65Co10Ga5P12C4 B4 alloy ingot prepared by induction melting. The glass transition temperature (Tg), supercooled liquid region (ΔTx (=Tx-Tg)) of the glassy alloy powders are 730K and 50K. The glassy alloy powders have ferromagnetism with a Curie temperature (Tc) of 640 K and exhibits saturation magnetization (I2) of 1.18T. No appreciable difference in the thermal stability and magnetic properties is seen between the glassy alloy powders and the melt-spun ribbon. By spark-plasma-sintering (SPS) these glassy alloy powders under the pressure of 80 MPa and at the temperature of 723 K below Tg, a bulk glassy Fe65Co10Ga5P12C4 B4 alloy with a size of 20 mm in diameter and 5 mm in thickness was synthesized. Its relative density was about 96%. There was also no appreciable difference in the thermal stability between the bulk glassy alloy and the glassy alloy powders. This bulk glassy alloy exhibited saturation magnetization (I2) of 1.19T and coercivity (Hc) of 115 A/m.
For the production of nanocrystalline soft-magnetic compacts with high saturation magnetization, we investigated the densification behavior of amorphous and nanocrystalline Fe86Zr7B6Cu1(at. %) powders using a hot-pressing machine, and the magnetic properties of the compacts. The density of the compacts produced from the nanocrystalline powder was 90.4%. However, nanocrystalline compacts with a high density of 99.9% were obtained by hot pressing the amorphous powder under 1.5 GPa and 853 K. We constructed the densification map of the amorphous and nanocrystalline powders. The pressure required to produce dense compacts, which was estimated from this map, is 3.0 GPa for the production of amorphous compacts, and 4.4 GPa for the consolidation of the nanocrystalline powder into nanocrystalline compacts, and only 1.5 GPa for the direct consolidation of the amorphous powder into nanocrystalline compacts. The consolidation of the amorphous powder through the crystallization is the most appropriate method for the production of dense nanocrystalline compacts. The dense nanocrystalline compacts exhibited good soft-magnetic properties which were superior to those of Fe-based amorphous compacts, in addition to a high saturation magnetization of 1.56 T. The coercive force, effective permeability at 1 kHz and 0.8 A/m, and core loss at 10 kHz and 0.1 T were 33 A/m, 1300 and 37 W/kg, respectively.
Nanocrystalline bulk Fe90Zr7B3 alloys with grain sizes of 20 to 30 nm were prepared by spark-plasma sintering of amorphous powders and subsequent annealing. The bulk alloys sintered at the temperature (Ts) between 673 and 723 K are composed of a mostly single amorphous phase and the alloy sintered at Ts of 873 K is composed of a mostly single bcc-Fe phase in as-sintered state. The former alloys show better soft magnetic properties in spite of porous consolidated state than the latter alloy after annealing at 923 K. For the bulk alloy sintered at Ts of 723 K, the effective permeability (μc) increases by accelerating the heating rate (α) in sintering and the density becomes higher by increasing the sintering pressure (Ps). The nanocrystalline alloy sintered at Ts of 723 K, Ps of 780 MPa and α of 1.7 K/s shows the maximum permeability of 29800, effective permeability of 3430 at 100 Hz and coercivity of 12 A/m with a relative density of 97 %.
In order to clarify the dominant factors for excellent soft magnetic properties of nanocrystalline bcc Fe–Zr–B alloys, we examined the change in the microstructure, soft magnetic properties and magnetostriction (λs) of an Fe86Zr7B6Cu1 amorphous alloy upon crystallization. The amorphous phase changed to a mostly single bcc phase with a grain size of about 10 nm by annealing for 3.6 ks at temperatures between 773 and 923 K. The phase transition causes a marked reduction of λs from 16.6×10−6 to an extremely low value of 10−7. The permeability (μe) also increased significantly by the transition to the nanocrystalline bcc phase and reached a maximum value of 48000 at Ta=873 K at which the λs approaches zero value. Thus, the excellent soft magnetic properties seem to result from the simultaneous satisfaction of small λs and nanocrystalline bcc structure. Consequently, it appears important for the achievement of the good soft magnetic properties that the nanocrystalline structure remains almost unchanged up to the high temperature range where the nearly zero λs value is obtained. Based on the data for the temperature dependence of magnetization and μe, it is presumed that the high thermal stability of the nanocrystalline bcc phase is due to the presence of an amorphous thin layer with higher Zr and B contents along the grain boundary of the bcc phase.
The addition of 17 at.% of elemental Ni powders to the cryogenic attritor milling of Metglas Fe78B13Si9 slowed the mechanical crystallization of the α-Fe and Fe2B phases. Transmission electron microscopy (TEM) observation and energy dispersive spectroscopy (EDS) analysis indicated that no more than 3.60 at.% of Ni dissolved into the Metglas, which was well within the equilibrium solubility limit of Ni in α-Fe. It is proposed that the addition of Ni impede mechanical crystallization during attritor milling by inhibiting bending and wear-like processes which could otherwise cause crystallization.
A bulky Fe84Nb7B, alloy was obtained by extruding the powders ground from amorphous ribbon, at temperatures T(e) between 563 K and 723 K at pressures P(e) between 824 MPa and 1208 MPa and at a speed of 5 mm s-1. The bulk Fe84Nb7B9 alloy extruded at T(e) = 698 K and P(e) = 1208 MPa is in a fully consolidated state and has a density of about 99%. The bulk alloy is composed of a mostly single b.c.c. phase with a grain size of about 10 nm after annealing for 3.6 ks at temperatures between 873 K and 973 K, and its soft magnetic property tends to be better for the bulks extruded at lower pressures. The bulk alloy extruded at T(e) = 698 K and P(e) = 824 MPa exhibits a high saturation magnetization B(s) of 1.40 T and a low coercive force H(c) of 64 A m-1 after annealing at 698 K.
In this paper we present some results concerning the preparation of nanocrystalline Fe86Cu1Nb3Zr4B6 powders by high-energy ball milling of elemental powders, in argon atmosphere and their magnetic properties. By controlled heat treatment of powders obtained by mechanical alloying an improvement in the magnetic properties can be noticed.
A study is conducted to explain the magnetic behavior of amorphous fibers (AF) and amorphous glass-covered wires (AGCW) with the nominal composition Co71Fe4Si14.5Nb4B6.5, in order to establish the role played by the differences between the local anisotropy distributions in the two types of amorphous materials. The Co-rich amorphous glass-covered wires and wires after glass removal with small magnetostriction are magnetically softer in the axial direction, while Co-rich amorphous fibers have better soft magnetic properties in the circumferential direction. The results reveal the importance of intrinsic creep-induced anisotropy in such materials.
The magnetic properties of Fe‐Si‐B‐M (M: additives) alloys prepared by annealing amorphous alloys made by the single roller method over their crystallization temperature have been investigated for development of new Fe‐based soft magnetic alloys. Excellent soft magnetic properties were obtained by adding the two elements Cu and Nb to Fe‐Si‐B alloys. It was found that these new alloys, called ‘‘FINEMET,’’ have an ultrafine grain structure composed of bcc Fe solid solution. They are suitable for many kinds of magnetic components such as saturable reactors, choke coils, and transformers, because they have superior soft magnetic properties and a high saturation flux density, and because different types of B‐H hysteresis loops are obtained by magnetic field annealing.
The soft magnetic properties of Fe73.5Cu1Nb3Si13.5B9 (Finemet) alloys prepared by mechanical alloying were studied using various experimental techniques including Mössbauer spectroscopy. The study includes the influence of the milling atmosphere. The results are compared with mechanically alloyed Fe–Si alloys as well as with melt-spun Finemet alloys. The coercivity of the mechanically alloyed powder is much larger than that of the melt-spun ribbons, though the saturation magnetisation is the same. This is ascribed to the absence of a grain boundary amorphous ferromagnetic phase resulting in a weakening of the exchange coupling between the nanograins. This study suggests that the interfacial component in Finemet alloys plays a crucial role in achieving good soft magnetic properties. The critical grain size for single domain particle was found to be 10 nm. For very small grain sizes, the existence of superparamagnetism was also studied using Fe-57 Mössbauer spectroscopy.