The microstructure evolution of a Fe73.5Si13.5B9Nb3Cu1 nanocrystalline soft magnetic material

The microstructure evolution in the course of crystallization of a splat-quenched Fe_73,5Si_13.5B₉Nb₃Cu₁ amorphous alloy was investigated by atom probe field ion microscopy (APFIM) and high resolution transmission electron microscopy (HRTEM). All the alloying elements were found to be distributed homogeneously as an amorphous solid solution in the as-quenched state. At an initial stage of annealing, a concentration fluctuation of Cu was found to occur. Cu formed clusters of a few nanometer diameter and their composition was found to be approximately 30 at.% Cu at the beginning. In the later stage, a b.c.c. FeSi solid solution and the B and Nb enriched amorphous phase with the smaller Si content were found to coexist. In addition to these two phases, Cu enriched particles containing approximately 60 at.% Cu were found to be present in the intergranular regions, although we were not successful yet to determine whether this was a crystalline or amorphous phase. Based on these observations, we discuss the crystallization process of this alloy at 550°C which leads to the emergence of excellent soft magnetic properties.     

Influence of heat treatment on the efficiency of impact fragmentation of amorphous alloy Fe<Subscript>73.5</Subscript>Cu<Subscript>1</Subscript>Nb<Subscript>3</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript> ribbons

The mechanism of impact fracture of soft magnetic amorphous alloy Fe_73.5Cu₁Nb₃Si_13.5B₉ ribbons in a disintegrator after heat treatment at a temperature from the range 300–700°C and the fractional composition of the formed powder are studied. The temperature ranges of a change in the mechanism of ribbon fracture are determined. The particle size distribution is shown to change weakly within the revealed temperature ranges.     

Effects of Composition Changes on Strength, Bend Ductility, Toughness, and Flex-Bending Fatigue of Iron-Based Metallic Glass Ribbons

Vickers microhardness, nanohardness, tension tests, notch and fatigue precrack toughness tests, and controlled monotonic and cyclic strain experiments via bending over mandrels of different diameter have been performed on two different chemistries of Fe-based (Fe-Si-B) metallic glass ribbons. The tensile strengths of Fe_73.5Cu₁Nb₃Si_13.5B₉ were 2000?±?100?MPa and 1640?±?35?MPa for Fe₇₈Si₉B₁₃, which is consistent with the microhardness trends. High notch toughness (e.g., 89?±?0.9?MPam^1/2 - Fe_73.5Cu₁Nb₃Si_13.5B₉; 94.5?±?5.5?MPam^1/2 - Fe₇₈Si₉B₁₃) and fatigue precracked toughness of 76?MPam^1/2 for the Fe_73.5Cu₁Nb₃Si_13.5B₉ and 80?MPam^1/2 for the Fe₇₈Si₉B₁₃ were obtained. Flex-bending cyclic fatigue tests revealed a fatigue limit of 385?MPa for Fe₇₈Si₉B₁₃ ribbons, whereas the more brittle behavior of the Fe_73.5Cu₁Nb₃Si_13.5B₉ ribbons prevented the generation of flex-bending fatigue data. These results are discussed in the light of recent work on metallic glass systems.     

The oxidation behaviour of amorphous and crystalline Fe78Si9B13

A combination of thermogravimetry, scanning and transmission electron microscopy, electron probe microanalysis and differential scanning calorimetry has been used to investigate the oxidation kinetics, and oxide morphology, structure and composition in amorphous and crystalline Fe₇₈Si₉B₁₃ alloys. Kinetic data indicate that the oxidation reactions of both amorphous and crystalline Fe₇₈Si₉B₁₃ obey a parabolic rate law over the temperature range 300 to 450°C with activation energies of 120 and 86 kJ/mol respectively, indicating that grain boundary diffusion is probably the rate controlling process. The parabolic rate constant for oxidation of crystalline Fe₇₈Si₉B₁₃ is consistently higher than for amorphous Fe₇₈Si₉B₁₃ over the temperature range 300–450°C, so that the amorphous alloy always shows a better oxidation resistance. Electron microscopy and electron probe microanalysis show that the oxide scales formed on both amorphous and crystalline Fe₇₈Si₉B₁₃ consist of SiO₂, Fe₃O₄ and Fe₂O₃, but the detailed microstructure and compositions are different. The oxide scale formed on amorphous Fe₇₈Si₉B₁₃ contains more SiO₂ and has a small particle size, while the oxide scale formed on crystalline Fe₇₈Si₉B₁₃ contains more Fe₃O₄ and consists of larger particles. The difference in oxide growth between amorphous and crystalline Fe₇₈Si₉B₁₃ is caused by the difference in alloy microstructure.     

Electrochemical Behavior of an Amorphous and Nanocrystalline Fe<Subscript>79</Subscript>P<Subscript>13</Subscript>Si<Subscript>5</Subscript>V<Subscript>3</Subscript> Alloy in a Damp SO<Subscript>2</Subscript>-Contaminated Atmosphere

The electrochemical behavior of amorphous and nanocrystalline soft magnetic Fe₇₉P₁₃Si₅V₃ alloy in a 0.1 M Na₂SO₄ solution has been studied. Mössbauer studies show that the electrochemical characteristics of the alloy are comparable with those of an Finemet Fe₇₇Si₁₃B₇Nb_2.1Cu_0.9 alloy, whereas the studied alloy is inexpensive and can be prepared using natural alloy ferrophosphorus containing vanadium and silicon.     

Influence of Microstructure on Microhardness of Fe81Si4B13C2 Amorphous Alloy after Thermal Treatment

The influence of microstructure, and its changes, on microhardness of the amorphous Fe₈₁Si₄B₁₃C₂ alloy after thermal treatment at different temperatures from 298 K to 973 K (25 °C to 700 °C) was studied. The as-prepared alloy ribbon containing a small amount of crystalline phases, as well as domains of short-range crystalline ordering embedded in the amorphous matrix, exhibits unexpectedly high microhardness, mostly due to its composition. After thermal treatment above 723 K (450 °C), the alloy samples begin to crystallize, creating a nanocomposite structure involving nanocrystals embedded in an amorphous matrix, leading to an increase in microhardness. Further growth of the nanocrystals, as the heating temperature was increased to 973 K (700 °C), caused the change from nanocomposite structure into a more granulated and porous structure, with a dominant type of interface changing from amorphous/crystal to crystal/crystal, leading to a decrease in microhardness.     

Influence of high-pressure torsion on the structure of an FeCoSiBNb alloy with an aperiodic phase in the initial state

The transformation of the initial structure (cubic quasicrystal) of the Fe₆₀Co₁₅Nb₆Si₁₅B₄ (at %) alloy during severe torsion deformation under a high quasi-hydrostatic pressure is studied. It is found that, at the first stage, a substantially misoriented and fragmented structure forms without changes in the phase composition; at the final stage, the structure consists of a mixture of an amorphous phase and bcc α-Fe-based nanocrystals. The results are compared to the changes in the alloy structure under action of severe deformation of another type, namely, milling. The role of compressive stresses in structure formation is discussed.     

The oxidation behaviour of amorphous alloy Fe40Ni40P14B6

A combination of thermogravimetry, optical microscopy, scanning and transmission electron microscopy, electron probe microanalysis and differential scanning calorimetry has been used to investigate the oxidation behaviour of amorphous Fe₄₀Ni₄₀P₁₄B₆. The oxide layer formed on amorphous Fe₄₀Ni₄₀uB₆ has a whisker-like α-Fe₂O3 structure, which grows very rapidly to build up a thick layer of oxide. Kinetic data indicate that the oxidation of amorphous Fe₄₀Ni₄₀P₁₄B₆ obeys a parabolic rate law as long as the alloy remains amorphous and the rate controlling process is diffusion of iron in the amorphous alloy matrix. However, the oxidation rate drops sharply if crystallization of amorphous Fe₄₀Ni₄₀P₁₄B₆ takes place during oxidation annealing. Crystalline Fe₄₀Ni₄₀P₁₄B₆ also obeys a parabolic rate law but with as much smaller rate constant than the amorphous alloy. The rate controlling process for oxidation of crystalline Fe₄₀Ni₄₀P₁₄B₆ is diffusion of iron and nickel in the multiphase oxide layer, which consists of a fine scale mixture of NiO, Fe₃O₄, Fe₂O₃ and NiFe₂O₄, crystals. The difference in oxidation behaviour between amorphous and crystallized Fe₄₀Ni₄₀P₁₄B₆ is caused by the different alloy microstructures.     

Elimination of precipitate free zones in an Fe−Nb creep-resistant alloy

Precipitation of the Fe₂Nb intermetallic compound has previously been found to cause substantial hardening during aging of Fe rich Fe-Nb alloys. However, the formation of a wide precipitate free zone adjacent to the grain boundaries caused a degradation of creep resistance. In an effort to decrease the precipitate free zone width, thereby improving the creep resistance, an extensive study was made of the precipitation behavior of an Fe-1.7 at. pct Nb(Cb) alloy quenched from the δ-phase field. The quenched alloy was found to decompose via a two step reaction during aging at temperatures below 550°C. The first step in the decomposition reaction is thought to occur by clustering of Nb atoms in the ferrite matrix, similar to the clustering of Mo atoms which is known to occur during aging of Fe-Mo alloys. The second step in the reaction is not well understood. The precipitate free zones were formed by solute depletion in the vicinity of the grain boundary and the subsequent difficulty of nucleation of the Fe₂Nb precipitates in the regions of lowered solute concentration. Using two step aging treatments, an initial low temperature step to develop the Nb atom clusters followed by a higher temperature step to cause Fe₂Nb precipitation, the precipitate free zones were eliminated from the aged alloys. The origin of this effect is thought to be the heterogeneous nucleation of Fe₂Nb precipitates on the clusters developed during the initial aging step.     

Hydrogen effects on the spall strength and fracture characteristics of amorphous Fe-Si-B alloy at very high strain rates

A novel approach is suggested, using laser-induced shock wave measurements to estimate the effects of cathodic hydrogen charging on the mechanical properties and fracture characteristics of materials. This approach is applied to (1) determine the dominant mechanism of hydrogen embrittlement (HE) in an amorphous Fe₈₀B₁₁Si₉ alloy; and (2) estimate the effects of the high pressures involved in cathodic charging. The dynamic spall strength of an amorphous Fe₈₀B₁₁Si₉ alloy shocked before and after hydrogenation by a high-power laser to very high pressures (tens of giga Pascals) is measured. The dynamic spall strength of crystalline iron is measured as well for comparison. An optically recording velocity interferometer system (ORVIS) is used to measure the profile of the free surface velocity in time. The spall strength and the strain rate are calculated from the measurement of the free surface velocity as a function of time. Fracture characteristics are studied by scanning electron microscopy (SEM). The main conclusions are (1) the most reasonable mechanism of HE in the amorphous Fe-Si-B alloy is the high-pressure bubble formation; (2) the high pressures involved in cathodic hydrogen charging or laser-induced shock waves measurements may have similar effects on fracture characteristics; and (3) at very high strain rates, the spall strength is determined mainly by the interatomic bonds.     

Nanocrystal Growth in Thermally Treated Fe75Ni2Si8B13C2 Amorphous Alloy

Thermal treatment of amorphous Fe₇₅Ni₂Si₈B₁₃C₂ alloy leads to crystallization of the stable ??-Fe(Si) and Fe₂B as well as to the metastable Fe₃B phase. The study of the mechanism of crystal growth of the ??-Fe(Si) phase revealed that the mechanism of ??-Fe(Si) growth changes from two dimensional in the early stage to one dimensional in the later stage of crystallization. The Fe₂B phase was found to crystallize through two independent routes: from the amorphous phase and from the metastable Fe₃B phase, which leads to a different mechanism of crystal growth for each route. Both routes exhibit a change in the mechanism of crystal growth: from two dimensional to one dimensional and from three dimensional to two dimensional, respectively. The respective mechanisms of crystal growth correlate well with the observed changes in preferential orientation of the crystallites of the Fe₂B phase.     

Mechanical properties of Fe-Si-B amorphous wires produced by in-rotating-water spinning method

Amorphous wires with high strength and good ductility have been produced in Fe-Si-B alloy system by the modified melt-spinning technique in which a melt stream is ejected into a rotating water layer. These wires have a circular cross section and smooth peripheral surface. The diameter is in the range of about 0.07 to 0.27 mm. Their Vickers hardness (H_v) and tensile strength (σ_f) increase with silicon and boron content and reach 1100 DPN and 3920 MPa, respectively, for Fe₇₀Si₁₀B₂₀, exceeding the values of heavily cold-drawn steel wires. Fracture elongation(ε _f ), including elastic elongation, is about 2.1 to 2.8 pct. An appropriate cold drawing results in the increase of σ_f and ε_f by about eight and 65 pct, respectively. This increase is interpreted to result from an interaction among crossing deformation bands introduced by cold drawing. The undrawn and drawn amorphous wires are so ductile that no cracks are observed, even after closely contacted bending. Further, it is demonstrated that the σ_f of the Fe₇₅Si₁₀B_l5 amorphous wire increases by the replacement of iron with a small amount of tantalum, niobium, tungsten, molybdenum, or chromium without detriment to the formation tendency of an amorphous wire. Such iron-based amorphous wires are attractive as fine gauge, high strength materials because of their uniform shape and superior mechanical qualities.     

Atomic structure of the crystalline/amorphous interface in a directionally crystallized Pd80Si20 alloy

An amorphous ribbon of Pd₈₀Si₂₀ alloy was directionally crystallized under an imposed temperature gradient of 25 K/mm with a growth velocity of 0.0785 mm/s, and the structure of the crystalline/amorphous interface was investigated by conventional and high-resolution transmission electron microscopy (TEM). Under these conditions, the amorphous Pd₈₀Si₂₀ crystallizes into a broken-lamellar eutectic of the Pd₃Si and Pd₉Si₂ equilibrium phases. The Pd₃Si phase is faceted and grows along the [010] direction by nucleation and propagation of unit-cell ledges parallel to the (010) terrace plane. The Pd₉Si₂ phase is largely coherent with Pd₃Si and grows along a high-index crystallographic direction. Microscopic facets were not observed on the Pd₉Si₂ phase either by conventional or high-resolution TEM, indicating that its crystalline/ amorphous interface is comparatively rough. These observations are related to the crystallography and interphase boundary (IPB) energies of the phases and discussed in terms of mechanisms of lamellar growth. Formerly Graduate Research Assistant, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

Deformation and fracture of a composite material based on a high-strength maraging steel covered with a melt-quenched Co<Subscript>69</Subscript>Fe<Subscript>4</Subscript>Cr<Subscript>4</Subscript>Si<Subscript>12</Subscript>B<Subscript>11</Subscript> alloy layer

Multifractal analysis is used to study the deformation and fracture of a promising composite material consisting of a wire base made of K17N9M14 maraging steel covered with a surface layer made from a Co₆₉Fe₄Cr₄Si₁₂B₁₁ amorphous alloy. As compared to its components, this material has a substantially better set of the mechanical properties.     

Hydrogen effects on an amorphous Fe-Si-B alloy

Hydrogen absorption in and desorption from an amorphous Fe₈₀B₁₁Si₉ alloy, hydrogen effects on the microstructure of this alloy, and the possible mechanism of hydrogen embrittlement (HE) in this alloy have been studied. Ribbons were electrochemically charged with hydrogen at room temperature. The interaction of hydrogen with structural defects and the characteristics of hydrogen desorption were studied by means of thermal desorption spectroscopy (TDS). The effects of hydrogen on the microstructure and thermal stability were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), electrical resistivity measurements, and differential scanning calorimetry (DSC). The phenomenon of HE was investigated using scanning electron microscopy (SEM) and various mechanical testing techniques. The absence of hydride-forming elements resulted in low hydrogen solubility and low desorption temperatures. Hydrogenation at room temperature is reported for the first time to lead to either local nanocrystallization of the amorphous phase or transformation of nanocrystalline phases such as Fe_∼3.5B, originally present in the uncharged material, to a new nanocrystalline Fe₂₃B₆ phase. The susceptibility of this alloy to HE is explained in terms of high-pressure bubble formation.     

Effect of Alloying Additions on Microstructure,Mechanical and Magnetic Properties of Rapidly Cooled Bulk Fe-B-M-Cu (M = Ti,Mo and Mn) Alloys

The influence of alloying additions on the microstructure, mechanical, and magnetic properties of bulk Fe₇₉B₂₀Cu₁, Fe₇₉B₁₆Ti₄Cu₁, Fe₇₉B₁₆Mo₄Cu₁ and Fe₇₉B₁₆Mn₄Cu₁ (at. pct) alloys was investigated. Nanocrystalline samples in the form of 3 mm rods were prepared directly by suction casting without additional heat treatment. Mössbauer spectroscopy, transmission electron microscopy and scanning electron microscopy studies confirmed that the investigated alloys consist α-Fe and Fe₂B nanograins embedded in an amorphous matrix. The addition of alloying elements, such as Ti, Mo and Mn to Fe₇₉B₂₀Cu₁ alloy increases the amount of amorphous phase and decreases the presence of Fe₂B phase in all examined alloys. The mechanical properties of the samples, such as hardness, elastic modulus, and elastic energy ratio, were analysed by an instrumented indentation technique performed on a 12 × 12 nanoindentation grid. These tests allowed to characterise the mechanical properties of the regions observed in the same material. For the Fe₇₉B₂₀Cu₁ alloy, the hardness of 1508 and 1999 HV, as well as Young’s modulus of 287 and 308 GPa, were estimated for the amorphous- and nanocrystalline-rich phase, respectively. The addition of Ti, Mo, and Mn atoms leads to a decrease in both hardness and elastic modulus for all regions in the investigated samples. Investigations of thermomagnetic characteristics show the soft magnetic properties of the studied materials. More detailed studies of magnetisation versus magnetic field curves for the Fe₇₉B_20−xM_xCu₁ (where x = 0 or 4; M = Ti, Mo, Mn) alloy, recorded in a wide range of temperatures, followed by the law of approach to magnetic saturation revealed the relationship between microstructure and magneto-mechanical properties.

     

The effect of microstructure on properties and behaviors of annealed Fe78B13Si9 amorphous alloy ribbon

Changes in various properties with annealing of amorphous alloys have been widely studied by many researchers. The present work examines the annealing treatment dependence of mechanical, relaxation, thermal, and magnetic properties and behaviors in light of sample microstructure, as determined by TEM. The experimental results show that both the onset of embrittlement and the change in DC coercive field with sample annealing are consistent with microstructural development. An interrelationship among the various experimental measurements and observations is advanced as a unifying construct of the annealing treatment dependence of various properties in Fe₇₈B₁₃Si₉ amorphous alloy.     

Nucleation and growth kinetics of stable and metastable eutectics in FeSiB metallic glasses

Two iron-based metallic glasses, Fe₇₈Si₁₀B₁₂ and Fe₇₂Si₁₀B₁₈, have been examined in detail after partial crystallization at a series of temperatures. The number and size of the eutectic cells—of both stable and metastable eutectics—have been determined as functions of time and temperature and the results compared with theoretical nucleation models. It has been found that the extent to which quenched-in nuclei from the original planar flow casting operation influence the subsequent crystallization behaviour depends on alloy composition and on the annealing temperature. The results indicate that it is possible to control the type, number and size of the crystallization products in the microstructure by suitable choice of annealing conditions.     

Effect of the nanocrystallization of a soft magnetic amorphous Fe–P–Mo alloy on its corrosion resistance in a damp industrial SO<Subscript>2</Subscript>-contaminated atmosphere

The study of the electrochemical behavior of a soft magnetic amorphous Fe–P–Mo alloy in a 0.1M Na₂SO₄ solution, which simulates a damp SO₂-contaminated atmosphere, shows that the corrosion resistance of the nanocrystalline Fe_80.2P_17.1Mo_2.7 alloy is comparable to that of a FINEMET alloy. No molybdenum is required for manufacturing the Fe_80.2P_17.1Mo_2.7 alloy, because it can be prepared using natural alloy ferrophosphorus containing molybdenum.     

Effect of High-Pressure Torsion Deformation and Subsequent Annealing on Structure and Magnetic Properties of Overquenched Melt-Spun Nd9Fe85B6 Alloy

The influence of a high-pressure torsion deformation (HPTD) and subsequent annealing on the magnetic properties and phase transformations in the melt-spun Nd₉Fe₈₅B₆ alloy has been investigated. Because of the low density of nucleation centers and formation of the metastable NdFe₇ and Nd₂Fe₂₃B₃ phases, the crystallization of the nearly amorphous ribbons leads to the non-uniform and coarse Nd₂Fe₁₄B grains resulting in the lower values of the coercivity H_c and remanence B_r. The HPTD prior to annealing creates numerous α-Fe nanograins, which apparently serve as nucleation sites for Nd₂Fe₁₄B. In the ribbons subjected HPTD and annealed at 600 °C, the α-Fe and Nd₂Fe₁₄B grains are uniform and fine (13 and 22 nm, respectively) leading to the increase of H_c by 23% and the increase of B_r by 16%. Thus, HPTD is a powerful tool for improvement of the magnetic hysteresis properties in the overquenched Nd₉Fe₈₅B₆.