The Effect of Annealing on the Electrochemical Behavior of Amorphous Alloys Fe73.5–Si13.5–B9–Nb3–Cu1in Acidic Perchlorate Media

The effect of annealing in a temperature interval of 300 to 600°C on the properties of an amorphous alloy Fe_73.5–Si_13.5–B₉–Nb₃–Cu₁is studied by potentiodynamic, x-ray diffraction, and x-ray photoelectron techniques, as well as by scanning probe microscopy. The highest corrosion resistance is characteristic of a nanocrystalline alloy completely crystallized at 600°C. The main factor enhancing the corrosion resistance of the alloy is shown to be not the alloy's crystallization degree but the high segregation activity of such components as silicon and boron.     

Enhanced microwave absorption properties of Fe73.5Cu1Nb3Si15.5B7 nanoflakes by moderate surface oxidization and rotational orientation in composites

Fe_73.5Cu₁Nb₃Si_15.5B₇ nanoflakes were fabricated by ball milling the annealed ribbons. The microwave absorption properties of Fe_73.5Cu₁Nb₃Si_15.5B₇ nanoflakes were improved by moderate surface oxidization and rotational orientation in composites. As for the oxide-coated nanoflakes composite, the permittivity decreased distinctly and the permeability maintained the initial value compared with the as-milled nanoflakes composite. Through rotational orientation, the lower permittivity and higher permeability were obtained, and consequently the microwave absorption properties were improved obviously. The minimum reflectivity for the absorber of the oriented composites with 35 vol% oxide-coated nanoflakes could reach −46.4 dB at 1.31 GHz with the thickness of 4.2 mm.     

Corrosion processes and their influence on the magnetic flux density of FeNbCuSiB alloys

This paper describes the effect of corrosion process in the magnetic flux density of FeNbCuSiB alloys and correlates with the Si content in the alloys and in the surface oxide layer after a corrosion process. The corrosion resistance of FeNbCuSiB alloys was studied by ordinary weight loss tests, the potentiodynamic polarization technique and spectroscopy impedance. The photoelectron spectroscopy was used in order to identify the compositional changes in the surface oxide layer. Two alloy compositions, Fe_77.5Cu₁Nb₃Si_2.5B₁₆ and Fe₇₃Cu₁Nb₃Si_16.5B_6.5, with different Si content were analyzed in two different conditions, amorphous and crystalline states, respectively. The Fe_77.5Cu₁Nb₃Si_2.5B₁₆ alloy displayed lower corrosion resistance than the Fe₇₃Cu₁Nb₃Si_16.5B_6.5 alloy. The increase of the Si amount in the alloy composition results in an improvement in the corrosion resistance. The enrichment of the Si content on the surface oxide layer was found to be responsible for the improved corrosion resistance. Losses in magnetic properties depend not only on the Si content but also on the structural state.     

Thermally induced crystallization of Fe73.5Cu1Nb3Si15.5B7 amorphous alloy

Thermally induced crystallization of Fe_73.5Cu₁Nb₃Si_15.5B₇ amorphous alloy occurs in two well-separated stages: the first, around 475 °C, corresponds to formation of α-Fe(Si)/Fe₃Si and Fe₂B phases from the amorphous matrix, while the second, around 625 °C, corresponds to formation of Fe₁₆Nb₆Si₇ and Fe₂Si phases out of the already formed α-Fe(Si)/Fe₃Si phase. Mössbauer spectroscopy suggests that the initial crystallization occurs through formation of several intermediate phases leading to the formation of stable α-Fe(Si)/Fe₃Si and Fe₂B phases, as well as formation of smaller amounts of Fe₁₆Nb₆Si₇ phase. X-ray diffraction (XRD) and electron microscopy suggest that the presence of Cu and Nb, as well as relatively high Si content in the as-prepared alloy causes inhibition of crystal growth at annealing temperatures below 625 °C, meaning that coalescence of smaller crystalline grains is the principal mechanism of crystal growth at higher annealing temperatures. The second stage of crystallization, at higher temperatures, is characterized by appearance of Fe₂Si phase and a significant increase in phase content of Fe₁₆Nb₆Si₇ phase. Kinetic and thermodynamic parameters for individual steps of crystallization suggest that the steps which occur in the same temperature region share some similarities in mechanism. This is further supported by investigation of dimensionality of crystal growth of individual phases, using both Matusita–Sakka method of analysis of DSC data and texture analysis using XRD data.     

Magnetic, electrical and plastic properties of Fe76Nb2Si13B9, Fe75Ag1Nb2Si13B9 and Fe75Cu1Nb2Si13B9 amorphous alloys

Influence of 1 h annealing in vacuum on magnetic, electrical and plastic properties of Fe₇₆Nb₂Si₁₃B₉, Fe₇₅Ag₁Nb₂Si₁₃B₉ and Fe₇₅Cu₁Nb₂Si₁₃B₉ melt spun ribbons were carefully investigated. It was shown that in all cases soft magnetic properties can be significantly enhanced by applying 1-h annealing at characteristic temperatures T_op. This optimization annealing causes that permeability increases more than 15-times and magnetic losses (tangent of loss angle) achieves a minimum in relation to the as quenched state. Using structural examinations (X-ray and HRTEM) it was shown that for the Fe₇₅Cu₁Nb₂Si₁₃B₉ alloy the optimized microstructure corresponds to a nanocrystalline αFe(Si) phase whereas in other alloys to a relaxed amorphous phase free of iron nanograins. As a consequence of this fact the Fe₇₆Nb₂Si₁₃B₉ and Fe₇₅Ag₁Nb₂Si₁₃B₉ alloys show higher plasticity in comparison to the nanocrystalline Fe₇₅Cu₁Nb₂Si₁₃B₉ alloy. Temperatures of the first stage of crystallization, and related diffusion parameters were determined using measurements of resistivity versus temperature with different heating rates.     

The influence of Cr addition on the corrosion resistance of Fe73.5Si13.5B9Nb3Cu1 metallic glass in marine environments

The influence of Cr concentration on the corrosion resistance of Fe_73.5Si_13.5B₉Nb₃Cu₁ metallic glass in simulated marine atmospheres with chlorides (Cl⁻) was studied. Cyclic polarisation measurements were carried out to study susceptibility to localised corrosion. All the tests were carried out with the same material in different states: amorphous, nanocrystalline and crystalline. DC current electrochemical techniques were used to analyse the corrosion kinetics. Gravimetric tests were used to establish the reliability of the data obtained electrochemically. The study also focused on the modification of the magnetic properties in the material as a result of exposure to an aggressive medium and Cr addition.     

Microstrucural characterization of gas atomized Fe73.5Si13.5B9Nb3Cu1 and Fe97Si3 alloys

Powder particles of Fe_73.5Si_13.5B₉Nb₃Cu₁ and Fe₉₇Si₃ soft magnetic alloys have been prepared by gas atomization. The gas atomized powder was microstructurally characterized and the dependence of coercivity with the composition and powder particle size investigated. As-atomized powder particles of both compositions were constituted by a bcc α-Fe (Si) solid solution. The Fe_73.5Si_13.5B₉Nb₃Cu₁ powder particles presented a grain microstructure with dendrite structure, which dendrite arms were enriched in Nb. The coercivity increased as the particle size decreased, with a minimum coercivity, of 5 Oe, measured in the Fe₉₇Si₃ alloy in the range of 50–100 μm powder particle size. The coercive fields were quite higher in the Fe_73.5Si_13.5B₉Nb₃Cu₁ than in the Fe₉₇Si₃ powder, due to the Nb addition, which produced a phase segregation that leads to a noticeable magnetic hardening.     

FeSiBPNbCu alloys with high glass-forming ability and good soft magnetic properties

Effects of Cu addition on the glass-forming ability (GFA), thermal stability, magnetic properties and crystallization process of (Fe_0.76Si_0.09B_0.1P_0.05)_99−xNb₁Cu_x (x = 0, 0.25, 0.5, 0.75, 1) alloys were investigated. The introduction of Cu effectively stimulates the precipitation of the α-Fe(Si) without obvious deterioration of the GFA, and successfully modifies the simultaneous precipitation of α-Fe(Si), Fe₂B and Fe₃(B,P) phases in (Fe_0.76Si_0.09B_0.1P_0.05)₉₉Nb₁ alloy into separable precipitation of each phase at different temperatures during annealing, leading to the enhancement of soft magnetic properties. The saturation magnetic flux density of the representative (Fe_0.76Si_0.09B_0.1P_0.05)_98.25Nb₁Cu_0.75 alloy could be enhanced from 1.43 to 1.51 T after annealing at 530 °C for 10 min due to the precipitation of α-Fe(Si) nanoparticles with a diameter of about 22 nm dispersing randomly in the amorphous matrix. The integration of high GFA and excellent soft magnetic properties makes the FeSiBPNbCu alloys promising soft magnetic materials for industrial applications.     

Structural transformations of amorphous iron-based alloys upon abrasive and thermal treatments

Wear resistance and structural changes have been investigated in amorphous alloys Fe₆₄Co₃₀Si₃B₃ and Fe_73.5Nb₃Cu₁Si_13.5B₉ upon wear using a fixed abrasive. The structural studies have been performed by the methods of metallography, electron microscopy, and Mössbauer spectroscopy. It has been shown that the abrasive resistance of amorphous alloys is 1.6–3.1 times lower than that of high-carbon tool steels, which have a close level of hardness. The low abrasive wear resistance of amorphous alloys is caused by the deformation softening of the alloy surface in the process of wear. The major volume of the deformed surface layer of the alloys preserves the amorphous state. Its structural changes upon wear are characterized by the formation of inhomogeneities (fragments with a size of 10–50 nm) and by a decrease in the width of the strongest “halo” in the selected-area electron-diffractions patterns. In the amorphous matrix of the Fe₆₄Co₃₀Si₃B₃ alloy, a strong magnetic texture is formed and a redistribution of atoms occurs, which leads to an increase in the local shortrange order corresponding to FeB, Fe₂B, Fe₃B and α-Fe phases. In microvolumes of a thin (several μm) surface layer, the formation of a nanocrystalline structure (on the order of several volume %) was revealed. A tempering of the Fe_73.5Cu₁Nb₃S_13.5B₉ alloy at temperatures below 500°C does not affect the hardness and wear resistance of the alloy. At 500°C, there occurs an increase in microhardness and wear resistance of the Fe_73.5Cu₁Nb₃S_13.5B₉ alloy as a result of the formation in it of a nanocrystalline structure with the retention of a certain amount of the amorphous phase. The complete crystallization of the alloy at 540°C increases the brittleness of the alloy, which leads to a sharp reduction in its wear resistance.     

The Effect of Manufacturing Conditions on the Structure and Electrochemical Properties of Metallic Glasses Fe76.1Cu1.0Nb3.0Si13.8B6.1

By using x-ray diffraction analysis, small-angle x-ray scattering, resistometry, and electrochemical voltammetry, the effect of thermally pretreating the melt on the structure, as well as crystallization and electrochemical behavior, of rapidly quenched bands of an alloy Fe_76.1Cu_1.0Nb_3.0Si_13.8B_6.1 is investigated. It is shown that, by varying the production conditions of rapidly quenched alloys, one can substantially affect their amorphous-nanocrystalline structure and increase the corrosion resistance.     

The process of crystallization from amorphous state in ribbons of Fe–Si–B–based alloys under the effect of a high DC magnetic field

The effect of a high dc magnetic field (up to 29 T) applied during the crystallizing annealing of amorphous ribbons on the structure of Fe₈₁Si₇B₁₂ and Fe_73.5Cu₁Nb₃Si_13.5B₉ alloys has been studied. In the Fe₈₁Si₇B₁₂ alloy, an increase in the average size of grains that form during magnetic annealing has been revealed; in the Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy, a small decrease is observed in the average grain size. The possible reason for this may be the differences in the specific features of the processes of crystallization of these alloys. No effect of the magnetic field on the crystallographic orientation of the arising grains has been revealed.     

High temperature oxidation behavior of FeCo‐based nanocrystalline alloys

This paper describes the dynamic and isothermal oxidation behavior of three different FeCo‐based Fe_38.5Co_38.5Nb₇Cu₁B₁₅, Fe₃₆Co₃₆Nb₇Si₁₀B ₁₁ and Fe_33.5Co_33.5Nb₇Si₁₅B₁₁ alloys and one traditional FINEMET Fe₇₃Nb₃Cu₁Si_15.5B_7.5 alloy. Dynamic and isothermal oxidation measurements in controlled oxidizing atmosphere were performed and the oxidation apparent energy as well as the oxidation behavior was obtained. SEM observations were carried out in order to characterize the oxide layer formed during the oxidation measurements. The apparent activation oxidation energy found for the Fe₃₆Co₃₆Nb₇Si₁₀B₁₁, Fe_33.5Co_33.5Nb₇Si₁₅B₁₁ and Fe₇₃Nb₃Cu₁Si_15.5B_7.5 alloys was about 35 kJ/mol and for the Fe_38.5Co_38.5Nb₇Cu₁B₁₅ alloy was about 70 kJ/mol.     

Influence of thermal treatment on structure development and mechanical properties of amorphous Fe73.5Cu1Nb3Si15.5B7 ribbon

Ribbon-shaped amorphous samples with the stoichiometric composition Fe_73.5Cu₁Nb₃Si_15.5B₇ prepared by the melt spinning process were annealed at temperatures ranging from 693 K to 1123 K for 1 h under vacuum. In the early annealing stage, the alloy undergoes a specific nucleation process where Cu clusters precipitate from an amorphous matrix. Further heating initiates the partial crystallization of alloy forming the α-Fe–Si nanocrystallites. Subsequent Vickers hardness tests showed high values depending on the annealing temperature. It was found that the hardening process includes two stages. This behavior correlates well with results of density dislocation calculations. A crystallite size of 10 nm for the α-Fe–Si particles correlated very well with a maximum hardness of the material.     

Micromagnetic simulation of the distribution of magnetization in semiinfinite single crystals

The enrichment of the Fe_73.5Si_13.5B₉Cu₁Nb₃ amorphous alloy in iron was found to change the kinetic regime of primary crystallization, which becomes two-stage rather than single-stage. The observed peculiarities of the crystallization behavior of the alloy were explained assuming that niobium plays a key role in the formation of its amorphous structure.     

The influence of structural peculiarities of rapidly quenched bands of the Fe76.1Cu1.0Nb3.0Si13.0B6.1 alloy on their electrochemical behavior

X-ray diffraction analysis, auger spectroscopy and electrochemical measurement showed that, by varying the temperature of starting melt, it is possible to change the surface crystallization degree of quenched metallic glass of the Fe_76.1Cu_1.0Nb_3.0Si_13.8B_6.1 composition, while the presence of crystallites of an ordered α-Fe(Si) solid solution and the air-oxide film formed during the quenching of bands allows improving their corrosion-electrochemical characteristics.     

Effects of Si, W and W–Mo on isothermal oxidation behaviors of Nb/Nb5Si3 in situ composites at high temperature

The effects of Si, W and W–Mo on the isothermal oxidation behaviors of Nb/Nb₅Si₃ in situ composites in static air at 1000 and 1200 °C for 20–100 h were investigated on as-cast materials. The results show that the oxidation kinetics of each alloy was not changed whether at 1000 or 1200 °C, and the oxidation mechanism were not changed. The oxidation resistance of Nb/Nb₅Si₃ in situ composites was sensitive to Si content, and the oxidation rate of Nb-10Si alloy was more than twice as many that of Nb–20Si alloy. By alloying of W, the oxidation resistance of Nb–20Si–10W alloy was improved significantly, because the WO₃ scale can provide the adherence for the creaked Nb₂O₅ scale and reduce the diffusion of oxygen through the scale. Comparing to alloying with W, the poor oxidation resistance of Nb–20Si–10W–10Mo alloy was attributed to the evaporation of MoO₃ and highly porous scale.     

Fe-based soft magnetic amorphous alloys with high saturation magnetization above 1.5 T and high corrosion resistance

Fe-rich amorphous alloys with minor-addition of Cr and/or Nb were examined with the aim of developing Fe-based amorphous alloys exhibiting simultaneously high saturation magnetization above 1.5 T and good corrosion properties. Fe₈₂Cr₂B₈P₄Si₃C and Fe₈₂NbB₉P₄Si₃C amorphous alloys were found to exhibit high saturation magnetizations of 1.49 T and 1.57 T, respectively, and rather good corrosion resistance in 3.5 mass% NaCl solution at 298 K. The minor-addition of Cr or Nb enables the formation of amorphous alloy particles without harmful oxide layer by water atomization process which makes these alloys suitable for applications as soft magnetic core materials. The addition of 1 at% Nb improved the corrosion resistance through the increase in E_corr value, which makes easy to reach passive state, and the suppression of pitting corrosion. Besides, it has been proved that the simultaneous addition of Nb and Cr has an effect on forming protective passive film.     

Characterization of the effect of alloying additions on the crystallization of an amorphous Fe73.5Si13.5B9Nb3Cu1 alloy

An initially amorphous Fe_73.5Si_13.5B₉Nb₃Cu₁ alloy was partially crystallized by heat treatment. EDX measurements of the crystals and amorphous matrix were performed. These measurements suggested that there is a repulsion between Cu and Nb resulting in the formation of Cu and Nb clusters which served as heterogeneous nucleation sites. The experiments observation suggested that in regions with a low crystal density the growth rate was reduced by the strong interaction of Cu, Si, Nb and Fe atoms. In regions with high crystal density soft impingement due to higher Nb content hindered the growth of the nanocrystals.     

The effect of natural ageing of quickly quenched amorphous Fe<Subscript>75</Subscript>Si<Subscript>12</Subscript>B<Subscript>10</Subscript>Nb<Subscript>1</Subscript>Cu<Subscript>2</Subscript> alloys on their corrosion behavior sin acidic sulfate environments

The effect of natural ageing on the electrochemical and corrosion behavior of an amorphous Fe₇₅Si₁₂B₁₀Nb₁Cu alloy in acidic sulfate environments is studied. In its shape, the polarization curve is similar to that of the alloy, which was heat treated at 400°C. The ageing made the alloy brittle and unable to be soldered with acidic fluxes.     

Effects of chemical composition on nanocrystallization kinetics, microstructure and magnetic properties of finemet-type amorphous alloys

In this study, the kinetics of nanocrystallization of amorphous Fe_73.5Si_13.5B₉Nb₃Cu₁ (F1) and Fe₇₇Si₁₁B₉Nb_2.4Cu_0.6 (F2) alloys is investigated. The microstructure and magnetic properties of the nanocrystalline alloys are compared. The crystallization temperature of F2 alloy is shifted towards lower temperatures with respect to F1. Thus, the crystalline volume fraction and the crystalline grain size at specific annealing temperature for the F2 alloy are higher than for the F1 alloy, accounting for the higher coercive force of F2 alloy with respect to the one of F1 alloy. According to isoconversional methods, the activation energy for crystallization is variable as a function of transformed fraction because of the continuous changes in chemical composition during the transformation. Mean values of 350 and 290 kJ/mol are obtained for F1 and F2, respectively. Microstructural observations confirm that minor changes in chemical composition affect the kinetics and final microstructure of the nanocrystalline alloy, that determine the observed magnetic properties.