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.     

Cracking of amorphous Fe40Ni38Mo4B18 induced by static charging with hydrogen

Cracking in amorphous ribbon of Fe₄₀Ni₃₈Mo₄B₁₈ without external loading could be induced by cathodic charging with hydrogen in 0.1 N H₂SO₄ + 5 mg/L NaAsO₂. However, before cracking was initiated, the effect of hydrogen on the mechanical properties could be eliminated if hydrogen had been removed. A series of static charging experiments was carried out to study the cracking characteristics in this alloy. The diffusivity and concentration of hydrogen were obtained from both permeation and cathodic charging /thermal evolution experiments. The cause of cracking by static charging could be attributed to the build up of an internal hydrogen pressure around heterogeneous sites. The critical pressures for crack initiation were calculated based on the diffusivity and concentration data. Formerly Graduate Student, Department of Materials Science and Engineering, National Tsing Hua University     

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.     

Concentration dependence of hydrogen diffusivity in amorphous Fe40Ni38Mo4B18 alloy

Hydrogen permeation in amorphous Fe₄₀Ni₃₈Mo₄B₁₈ alloy was studied using an electrochemical technique in the temperature range 303–348 K. Hydrogen diffusivities were calculated by several methods. They increased as the charging current density increased. The surface, concentration, and trapping effects on hydrogen diffusion were carefully examined. It was suggested that the surface impedance was not existent and diffusivity was affected by the “intrinsic” concentration and trapping effects. When diffusivities were correlated with hydrogen concentration, the effective activation energy decreased from 36.8 to 34.8 kJ/mol as the surface hydrogen concentration increased from 1000 to 6000 mol H/m³. Under the same charging current density, hydrogen concentration in the alloy decreased as temperature increased. Therefore, lower effective activation energies would be obtained if diffusivities were correlated with the charging current density. The concentration dependence of hydrogen diffusivity was rationalized on the basis of the structural features of amorphous alloys.     

Comparison of the Crystallization Behavior of Fe-Si-B-Cu and Fe-Si-B-Cu-Nb-Based Amorphous Soft Magnetic Alloys

The role of the solute elements, copper, and niobium, on the different stages of de-vitrification or crystallization of two amorphous soft magnetic alloys, Fe_73.5Si_13.5B₉Nb₃Cu₁, also referred to as FINEMET, and a Fe_76.5Si_13.5B₉Cu₁ alloy, a model composition without Nb, has been investigated in detail by coupling atom probe tomography and transmission electron microscopy. The effects of copper clustering and niobium pile-up at the propagating interface between the α-Fe₃Si nanocrystals and the amorphous matrix, on the nucleation and growth kinetics have been addressed. The results demonstrate that while Cu clustering takes place in both alloys in the early stages, the added presence of Nb in FINEMET severely restricts the diffusivity of solute elements such as Cu, Si, and B. Therefore, the kinetics of solute partitioning and mobility of the nanocrystal/amorphous matrix interface is substantially slower in FINEMET as compared to the Fe_76.5Si_13.5B₉Cu₁ alloy. Consequently, the presence of Nb limits the growth rate of the α-Fe₃Si nanocrystals in FINEMET and results in the activation of a larger no. of nucleation sites, leading to a substantially more refined microstructure as compared to the Fe_76.5Si_13.5B₉Cu₁ alloy.     

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.     

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.     

Effect of annealing on the structure and mechanical properties of the maraging steel N9K17M14-nanoamorphous alloy Co69Fe4Cr4Si12B11 composite material

The effect of annealing on the structure, mechanical properties, and fracture of a one-dimensional composite material consisting of a microwire made of maraging N9K17M14 steel with a surface layer of a eutectic soft magnetic Co₆₉Fe₄Cr₄Si₁₂B₁₁ alloy is studied. The optimum temperature of annealing of the composite material is found; as a result, high strength characteristics are achieved at good plasticity. The composite material with a nanoamorphous layer is shown to have the high strength characteristics of the matrix maraging steel at significantly higher plasticity. When the Co₆₉Fe₄Cr₄Si₁₂B₁₁ alloy is deformed in the composition of the composite material, it exhibits a plasticity effect, and this alloy fails in a brittle manner when deformed in the form of a wire or a ribbon. This effect becomes more pronounced upon annealing.     

Fabrication and Characterization of Melt-Extracted Co-Based Amorphous Wires

Amorphous Co_68.15Fe_4.35Si_12.25B_15.25 wires with smooth surface and circular cross section were fabricated by melt extraction technology using a copper wheel with a knife-edge cross section angle of 60 deg. The effect of some process parameters such as wheel circumference velocity, molten alloy feed rate, and temperature on the geometry and weight, i.e., melt extracted layer thickness, of wire was examined carefully. An optimum process parameter to produce high-quality circular wires was presented. A high resolution CCD video camera recorder was used to monitor the changing of the surface shape of molten alloy contacting the wheel tip under different conditions. It was found that the mechanism of the wire formation during the optimum process condition was controlled by the momentum mechanism, while in the low wheel speed region, heat transfer turned out to be a dominant factor. Some characteristics of the circular wires such as amorphous nature and tensile strength were also studied.     

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.     

Preparation,mechanical strengths,and thermal

Ni-based amorphous wires with good bending ductility have been prepared for Ni₇₅Si₈B₁₇ and Ni₇₈P₁₂B₁₀ alloys containing 1 to 2 at. pct Al or Zr by melt spinning in rotating water. The enhancement of the wire-formation tendency by the addition of Al has been clarified to be due to the increase in the stability of the melt jet through the formation of a thin A1₂O₃ film on the outer surface. The maximum wire diameter is about 190 to 200 μm for the Ni-Si (or P)-B-Al alloys and increases to about 250 μm for the Ni-Si-B-Al-Cr alloys containing 4 to 6 at. pct Cr. The tensile fracture strength and fracture elongation are 2730 MPa and 2.9 pct for (Ni_0.75Si_0.08B_{0.17 99}Al₁) wire and 2170 MPa and 2.4 pct for (Ni_0.78P_0.12B_0.1)₉₉Al₁ wire. These wires exhibit a fatigue limit under dynamic bending strain in air with a relative humidity of 65 pct; this limit is 0.50 pct for a Ni-Si-B-Al wire, which is higher by 0.15 pct than that of a Fe₇₅Si₁₀B₁₅ amorphous wire. Furthermore, the Ni-base wires do not fracture during a 180-deg bending even for a sample annealed at temperatures just below the crystallization temperature, in sharp contrast to high embrittlement tendency for Fe-base amorphous alloys. Thus, the Ni-based amorphous wires have been shown to be an attractive material similar to Fe- and Co-based amorphous wires because of its high static and dynamic strength, high ductility, high stability to thermal embrittlement, and good corrosion resistance.     

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.     

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.     

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.     

Effect of the state of a melt on the glass-forming ability,structure, and properties of a melt-quenched bulk amorphous nickel-based alloy

The glass-forming ability, viscosity, phase composition, and microhardness of a melt-quenched Ni_64.4Fe₄Cr_4.9Mn₂B_16.2C_0.5Si₈ bulk amorphous alloy quenched from various melt temperatures at near-critical rates are studied. The melt-quenching temperature ensuring the maximum glass-forming ability and microhardness (HV= 13 GPa) of the alloy is found to be 1200–1230°C. The melt viscosity is shown to behave anomalously near the solidification temperature, and the related factors favoring this anomalous behavior are detected.     

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.     

Diagnostics of the structure and phase composition of amorphous-nanocrystalline soft magnetic materials by magnetic methods

Various structural states, such as microcrystalline, submicrocrystalline, nanocrystalline (with various sets of grain sizes), and amorphous, are considered in soft magnetic materials intended for preparing sensitive elements for instruments used in high-tech processes. The effect of the structural state of amorphous Fe₅Co₇₀Si₁₅B₁₀, Fe₆₀Co₂₀Si₅B₁₅, and Co_81.5Mo_9.5Zr₉ alloys on their magnetic characteristics is studied under various nanocrystallization conditions. A correlation between the specific features of the fine structure and grain sizes in the alloys and the Barkhausen effect parameters is found.     

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.     

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.     

Effects of Phosphorus and Carbon Contents on Amorphous Forming Ability in Fe-based Amorphous Alloys Used for Thermal Spray Coatings

Cost-effective Fe-based amorphous alloys used for thermal spray coatings were developed by varying contents of P and C, and their microstructure, hardness, and corrosion resistance were analyzed. In order to achieve chemical compositions having high amorphous forming ability, thermodynamically calculated phase diagrams of Fe-Al-P-C-B five-component system were used, from which compositions of super-cooled liquid having the lowest driving force of formation of crystalline phases were obtained. The thermodynamic calculation results showed that only phases of Fe₃P and Fe₃C were formed in the Fe₇₈Al₂P_(18.3?x)C _x B_1.7 alloy system. Considering driving force curves of Fe₃P and Fe₃C, the carbon contents were selected to be 6.90 and 7.47 at. pct, when the thermodynamic calculation temperatures were 697 K (414 °C) and 715 K (442 °C), respectively. According to the microstructural analysis of suction-cast alloys, the Fe₇₈Al₂P_10.83C_7.47B_1.7 alloy showed a fully amorphous microstructure, whereas the Fe₇₈Al₂P_11.40C_6.9B_1.7 and Fe₇₈Al₂P_10.3C_8.0B_1.7 alloys contained Fe₃P and Fe₃C phases. This Fe₇₈Al₂P_10.83C_7.47B_1.7 alloy showed the better hardness and corrosion resistance than those of conventional thermal spray coating alloys, and its production cost could be lowered using cheaper alloying elements, thereby leading to the practical application to amorphous thermal spray coatings.