Temperature dependence of the flow stress and ductility of annealed and unannealed amorphous Fe40Ni40P14B6

The flow stress and ductility of amorphous Fe₄₀Ni₄₀P₁₄B₆ was investigated on asprepared and annealed ribbons over the temperature range 78 to 573 K. The embrittlement is characterized by a rise in the ductile-to-brittle transition temperatureT _dtb, such thatT _dtbTanneal and sets in at temperatures as low as 423 K. The flow stress of Fe₄₀Ni₄₀P₁₄B₆ at room temperature is 2.21 GPa (320 ksi), in good agreement with the extrapolation from hardness measurements. The flow stress increases with decreasing temperature. The increase can be accounted for in dislocation-based models of the deformation of a metallic glass but is more difficult to understand in terms of a free volume model.     

Formation of ferromagnetic bulk amorphous Fe40Ni40P14B6 alloys

Ferromagnetic bulk amorphous Fe₄₀Ni₄₀P₁₄B₆ alloy rods with a diameter of ∼ 1.2 mm can be prepared by means of a rapid quenching technique. If a fluxing technique is also used, amorphous rods with a diameter as large as ∼ 2.5 mm can be synthesized. The critical cooling rate R_c for the glass formation Fe₄₀Ni₄₀P₁₄B₆ is estimated to be on the order of 10² K s^− 1.     

Activation energies of the amorphous-crystalline transformation in the Fe40Ni40P14B6 metallic alloy

The amorphous — crystalline phase change of the metallic glass Fe₄₀Ni₄₀P₁₄B₆ has been studied by examining the change in the electric resistivity, using isochronal and isothermal heating. The isochronal measurements indicate that the crystallization occurs in two stages. One stage appears as a change with pre-annealing in the height of the resistivity-temperature curve at 600 K. This stage characterizes an induction period for recrystallization activated by (1.46+0.03) eV. The other stage, occurring at temperature above 600 K and manifesting itself as a remarkable drop in the resistivity indicates the onset of crystallization. From the isothermal studies of this stage, the energy activating the crystallization is found to be (3.58+0.03) eV. The minimum energy activating the vacancy formation in the crystallized matrix has been calculated to be (1.96+0.03) eV. These results are compared with the corresponding data obtained for the amorphous and crystalline metallic glasses Fe₄₀Ni₄₀B₂₀ and Fe₃₂Ni₃₆Cr₁₄P₁₂B₆.     

Hydrogen permeation in Fe40Ni38B18Mo4 and Fe81B13.5Si3.5C2 amorphous alloys

Hydrogen permeation in Fe₄₀Ni₃₈B₁₈Mo₄ and Fe₈₁B_13.5Si_3.5C₂ amorphous alloys was evaluated using electrochemical permeation measurement in this study. Due to the different alloying elements in these alloys, higher hydrogen permeation rate and effective diffusivity, and lower apparent solubility in Fe₄₀Ni₃₈B₁₈Mo₄, amorphous alloys were detected. In this study, the behavior of hydrogen permeation affected by the alloying elements of carbon, silicon, and boron in iron-based amorphous alloys was also described.     

Magnetic properties of metallic glass Fe39Ni39Mo4Si6B12

Magnetic metallic glass Fe₃₉Ni₃₉Mo₄Si₆B₁₂ has been studied by the⁵⁷Fe Mössbauer spectroscopy. Mössbauer spectra consist of broad and overlapping six-line pattern below the Curie temperature. The Curie and crystallization temperatures of this metallic glass have been determined to be 575 ± 3 K and 725 ± 3 K, respectively. The hyperfine magnetic field at room temperature is approximately 225 kOe. The reduced hyperfine fields of this sample decrease much faster than that observed for other iron-rich metallic glasses like Fe₄₀Ni₄₀B₂₀, Fe₈₀B₂₀ etc. This behaviour is attributed to the presence of molybdenum atoms in the sample.     

Two-stage enthalpy relaxation behaviour of (Fe0.5Ni0.5)83P17 and (Fe0.5Ni0.5)83B17 amorphous alloys upon annealing

The anneal-induced enthalpy relaxation behaviour for (Fe_0.5Ni_0.5)₈₃P₁₇ and (Fe_0.5Ni_0.5)₈₃B₁₇ amorphous alloys was examined calorimetrically. Upon heating the sample annealed at temperatures belowT _g, an excess endothermic reaction (enthalpy relaxation) occurs aboveT _a. The C _p,endo evolves reversibly in a continuous manner with Int _a. The changes in the magnitude of C _p,endo and H _endo withT _a show clearly two distinct stages; a low-temperature one which peaks at aboutT _g-200 K and a high-temperature peak just belowT _g. The activation energy,Q _m, increases with the peak temperature of C _P,T _m, from 1.7 to 2.5 eV for the Fe-Ni-P and from 1.8 to 2.0 eV for the Fe-Ni-B for the low-temperature peak, and from 2.6 to 5.0 eV for the Fe-Ni-P for the high-temperature peak. The reversible change inT _c for the Fe-Ni-P alloy pre-annealed for 1 min at 640 K as a function ofT _a was also found to show two stages; a low-temperature stage ranging from 400 to 550 K whereT _c rises, and a high-temperature one above 550 K whereT _c lowers. From the almost complete agreement of the temperature region and the peak temperature for each stage as well as the marked contrast in the change ofT _c, it was proposed that the low-temperature endothermic peak is attributed to local and medium range rearrangements of metallic (iron and nickel) atoms and the high-temperature reaction to the long-range cooperative regroupings of metallic and metalloid atoms. The mechanism for the appearance of the two-stage enthalpy relaxation was investigated based on the new concept of two-stage distribution in relaxation times (or glass transitions) centring atT _g1 andT _g2 which arise respectively from the metal atoms and from the metal-metalloid atoms and the distinct two-stage splitting was interpreted to generate by the distinctly distinguishable difference in the easiness of structural relaxation between metal-metal and metal-metalloid. It was further found that the present irreversible and reversible enthalpy relaxation results are fairly well interpreted by a possible mechanism of two-stage relaxation processes consisting of as-quenched amorphous local and medium range relaxation of metal atoms cooperative relaxation of metal and metalloid atoms.     

X-ray photoemission study of Fe40Ni40B20 metallic glass

X-ray photoelectron spectroscopic (XPS) study of the valence band and the core levels of amorphous Fe₄₀Ni₄₀B₂₀ are presented. The oxides which formed at the surface of as-received sample are due to oxidation of iron and boron. For etched samples, the presence of oxide is not discernible, and the chemical environment is predominantly iron-boron-like, while nickel remains unassociated. The valence band has a high density of states at Fermi levels of amorphous Fe₄₀Ni₄₀B₂₀ are presented. The oxides which formed at the surface of B2s and Fe3d states, and metalloids-states respectively.     

Effect of quench rate on the structural relaxation of a metallic glass

Measurements of the ferromagnetic Curie temperature, T _c, have been used to monitor the structural relaxation of the metallic glass Fe_81.5B_14.5Si₄. The glass was produced by melt-spinning to various ribbon thicknesses at consequently different quench rates. It is shown that the faster quenched (i.e. thinner) ribbons have lower initial T _c values, corresponding to less relaxed structures. All the materials, however, tend to the same equilibrium value on annealing. An effect which is particularly marked at low annealing temperatures is that the faster quenched ribbons relax more quickly. The results are interpreted in terms of two relaxation mechanisms.     

Crystallization behaviour of Metglas 2826 MB (Fe40Ni38Mo4B18)

Differential scanning calorimetry, x-ray diffraction and transmission electron microscopy have been employed to determine the thermal stability and identify the crystalline phases on devitrification of Metglas 2826 MB. The glass crystallizes intoγ-FeNiMo and fcc (FeNi)₂₃B₆ with activation energies of 270 and 375 kJ mol⁻¹ respectively. The reactions are primary and polymorphic in nature. The influence of Mo towards crystallization of Fe₄₀Ni₄₀B₂₀ has been to enhance the formation of the fcc (FeNi)₂₃B₆ phase in preference to orthorhombic (FeNi)₃B phase and to raise the thermal stability of the amorphous state.     

Two-gap superconductivity in R2Fe3Si5 (R=Lu,Sc) and Sc5Ir4Si10

Abstract

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.     

Influence of proton irradiation on the critical current densityj c(B,T) of NbTi

The critical current densityj _c of a copper-coated single-core wire of Nb-50 wt.% Ti has been measured as a function of the magnetic flux density (1B8T) and the temperature (2.5 KTT _c). Irradiation with 3.1-MeV protons at 25 K up to a flux of1·10 ¹⁷ cm ^–2 led to aT _c drop of 0.17 K and an almost uniform degradation of 19% over the completej _c(B, T) surface. After annealing for 1 h at 285 K only 40% of thej _c degradation remained. Thej _c(B, T) surface can be described, independent of the irradiation dose, approximately byj _c(B, T) B _c2(T) – B, giving a pinning force density ofF _p=j _cB4F _p,max b(1–b), whereb=B/B _c2 andF _p,max B _c2 ² . The defect structure responsible for the vortex pinning is not affected by irradiation. Thej _c decrease on irradiation results from approximately 70% from a decrease ofF _p,max and 30% from theT _c decrease.This work has been supported by the technological program of the Federal Department of Research and Technology of the FRG. The author alone is responsible for the contents.     

Crystallization study of amorphous Fe40Ni40B20 by electrical resistivity measurement

The crystallization in amorphous Fe₄₀Ni₄₀B₂₀ alloy has been studied by electrical resistivity measurement. It shows a three stage compared to the more conventional two-stage crystallization behaviour in many metallic glasses. The Johnson-Mehl-Avrami equation was found to be operative only for 40% transformed crystallized volume for the highest measured isotherm (740 K). The Avrami exponent and activation energy were found to be 1.75 and 53 kcal mol^–1, respectively. The low activation energy in the amorphous alloy has been explained by the structural relaxation model.     

The crystallization of the amorphous alloy Fe40Ni40P14B6

The isothermal transformation behaviour of the metallic glass Fe₄₀Ni₄₀P₁₄B₆ between 320 and 400° C is described. Crystallization occurs by a eutectic mechanism to form Fe-Ni austenite and a body-centred tetragonal phase which is isomorphous with Fe₃P and Ni₃P. The eutectic crystals have a barrel shape such that the c-axis of the tetragonal phase is parallel to the barrel axis. The orientation relationship between the two phases is 1 1 0T 1 1 0 and 0 0 1T 1 1 2. The austenite phase contains (1 1 1) twins.     

Correlation between thermal stability of soft magnetic properties and structural relaxation in Co58Fe5Ni10Si11B16 metallic glass

Effects of isochronal and isothermal annealings on permeability, coercive field, field-induced anisotropy and electrical resistivity in Co₅₈Fe₅Ni₁₀Si₁₁B₁₆ metallic glass pre-annealed at 743 K for 60 min were examined to clarify the correlation between thermal stability of soft magnetic properties and structural relaxation. It was found that both soft magnetic properties and electrical resistivity changed reversibly, depending on annealing temperature, and the values of activation energy for the changes in these properties were very close, lying in the range 1.8 and 2.0 eV. However, although the deterioration of soft magnetic properties was observed in the specimens annealed below the Curie temperature (T _c), the resistivity changes occurred significantly even in the specimens above (T _c), The results strongly suggest that the changes in soft magnetic properties are attributed to an anisotropic chemical short-range ordering (CSRO) among the cobalt, iron and nickel atoms, and the resistivity changes (structural relaxation) are mainly due to an isotropic CSRO.     

Formation mechanism of Fe80Si8B12 metallic glass

In order to gain an understanding of the glass-forming mechanism during the rapid quenching of a metallic alloy, the nucleation and growth process of the crystalline phase which competes with the metallic glass must be investigated. The microstructures of melt-spun Fe₈₀Si₈B₁₂ alloy ribbons with different thicknesses were examined using optical and electron microscopy. The phase competing with the metallic glass is-(Fe, Si) ferrite, nucleated by the homogeneous nucleation. The growth process of-(Fe, Si) dendrites was explained well by Liptonet al.'s theory of dendritic growth in an undercooled alloy melt. It was concluded that the easy glass-forming ability during rapid quenching of the Fe₈₀Si₈B₁₂ alloy is due to (i) the slow growth rate of-(Fe, Si) dendrite, and (ii) the wide gap between the temperatures of the maximum nucleation rate and the maximum growth velocity.     

Amorphous/crystalline Fe55Ni20Cu5P10Si5B5 composite produced by two-component melt–spinning

ABSTRACT

This work presents unique microstructure and thermal properties of the Fe₅₅Ni₂₀Cu₅P₁₀Si₅B₅ alloy produced by novel modification of the melt-spinning method, where two precursor alloys are simultaneously ejected i.e. Fe₄₀Ni₄₀P₁₀Si₅B₅ and Fe₇₀Cu₁₀P₁₀Si₅B₅, forming a composite ribbon. The investigation of melting the precursors by differential scanning calorimetry confirms the liquid miscibility of the Fe₇₀Cu₁₀P₁₀Si₅B₅ alloy. The two-component melt-spun composite obtained from the two alloys is compared to the alloys produced from a homogeneous liquid. The electron microscopy, X-ray diffraction and differential scanning calorimetry show different microstructure of the composite in comparison with the traditionally melt-spun alloys and our results reveal that the composite alloy inherits the thermal properties of the precursors.

This paper is part of a Thematic Issue on The Crystallographic Aspects of Metallic Alloys.     

Enthalpy relaxation behaviour of (Fe,Co, Ni)75Si10B15 amorphous alloys upon low temperature annealing

The effect of composition on the anneal-induced enthalpy relaxation for (Fe, Co, Ni)₇₅Si₁₀B₁₅ amorphous alloys was examined calorimetrically. Upon heating the sample annealed at temperatures well belowT _g (T _g<T _g–150 K), an excess endothermic reaction (enthalpy relaxation) occurs above the annealing temperature. The endothermic specific heat, C _p, evolves in a continuous manner with annealing time. The change in the magnitude of C _p withT _a is divided into two stages: a low-temperature stage which peaks at aboutT _g–150 K and a high-temperature peak just belowT _g. The activation energy,E _m, increases with the peak temperature of C _p,T _m, from 2.29 to 3.15 eV for the low-temperature peak and from 4.40 to 4.96 eV for the high-temperature peak. The low-temperature endothermic reaction is attributed to local and medium range atomic rearrangements and the high-temperature reaction to the long-range cooperative atomic regroupings. The magnitude of the low-temperature C _p is most pronounced for (Fe-Co)₇₅Si₁₀B₁₅, followed by (Fe-Ni)₇₅Si₁₀B₁₅, (Co-Ni)₇₅Si₁₀B₁₅, Fe₇₅Si₁₀B₁₅, Co₇₅Si₁₀B₁₅ and then Ni₇₅Si₁₀B₁₅. The reason for such a significant compositional effect of the enthalpy relaxation was investigated, based on the concept of distribution in relaxation time, which had been previously derived from the percolation theory, and was interpreted as due to the difference in the degree of local structure and compositional disorder in the as-quenched state; i.e. the higher the degree of short-range disorder (the larger the fraction of short relaxation time) the larger is the magnitude of the enthalpy relaxation (H _{, endo}). There existed a strong correlation between (H _{, endo}) or C _p and anneal-induced embrittlement tendency; the larger the (H _{, endo}) and C _p the larger is the embrittlement tendency. Such a strong correlation was interpreted based on the assumption that the internal strain which generated in the transition from as-quenched disordered state to a relaxed equilibrium state causes an enhancement of embrittlement tendency.     

Isochronal crystallization kinetics of Fe40Ni40B20 amorphous alloy

The kinetics of crystallization of amorphous Fe₄₀Ni₄₀B₂₀ alloy upon isochronal annealing was investigated applying power-compensating differential scanning calorimetry with heating rates of (5, 10, 20, 30, 40) K/min. The corresponding microstructural evolution was studied by means of X-ray diffraction, focused ion beam and scanning electron microscopy, and transmission electron microscopy. Crystallization of Fe₄₀Ni₄₀B₂₀ alloy upon isochronal annealing takes place by formation of nano-scaled grains consisting of a face-centered cubic solid solution phase (Fe,Ni) and an orthorhombic compound phase (Fe,Ni)₃B. Kinetic analysis was performed by application of a modular model of phase transformation kinetics, fitted to all experimental transformation-rate curves simultaneously. The crystallization reaction can be described by nucleation with a continuous nucleation rate incorporating a nucleation index a and by growth in three dimensions according to a linear growth law. The kinetics of transformation and the resulting microstructure observed upon isochronal annealing clearly differ from those upon isothermal annealing investigated in a previous study, reflecting different mechanisms operating upon isochronal and isothermal crystallization.     

The influence of composition on the formation and stability of Ni-Si-B metallic glasses

The influence of composition on the formation and stability of Ni-based glassy alloys containing Si and B has been investigated systematically. Depending on the SiB ratio certain compositions, with total metalloid contents in the range 17 to 49 at% (a wider range than has hitherto been reported for metal-metalloid glasses), have been vitrified by melt-spinning to an average thickness of 17 m. For metalloid concentrations greater than 36 to 40 at% the amorphous phase is brittle in the as-quenched state. The highest crystallization temperatures, T _x, occur at 32 to 38 at% metalloid, the actual value of T _x again depending on the SiB ratio. It is shown that, over a wide composition range, the glass-forming boundary corresponds closely with a value for the reduced crystallization temperature isometric of 0.52. This value corresponds to a critical cooling rate for glass formation of about 10⁶ K sec^–1, predicted from kinetic theories and assuming that T _x is a good estimate of the glass transition temperature, T _g. This agrees quite closely with the cooling rate for approximately 17 m thick tape predicted from thermal studies of the melt-spinning process. Hence, T _x/T _liq usefully describes the glass-forming ability (GFA) for much of the composition range studied, although preliminary results suggest that near the centre of the glass-forming range the GFA may be over-estimated. Substitution of Si by other metalloid elements from Groups IIIb–Vb in Ni₇₈Si₁₀B₁₂ glassy alloy generally decreases the thermal-stability.     

Crystallization of the amorphous alloy Fe40Ni40B20: a ferromagnetic resonance study

The ferromagnetic resonance (FMR) technique was used to study the crystallization of the metallic glass Fe₄₀Ni₄₀B₂₀ (Vitrovac 0040). The line intensity of the amorphous phase was measured for several isothermal annealing times in the temperature range 360 to 375° C. The transformed fraction, as derived from FMR data, satisfies the Johnson-Mehl-Avrami equation with the exponentn between 1.49 and 1.70. The activation energy for crystallization is estimated from the times to 25% to 75% transformation to be 364 kJ mol^–1.