Atomic structure and thermal behavior of (Co0.65,Fe0.35)\mathrm{(Co_{0.65},Fe_{0.35})}72Ta8B20\mathrm{_{72}Ta_8B_{20}} metallic glass with excellent soft magnetic properties

New soft magnetic (Co0.65,Fe0.35)72Ta8B20 metallic glass has been synthesized and its atomic structure and thermal stability have been studied using high-energy synchrotron X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. Analysis of the pair correlation functions (PDF) indicates a notable shift in position of the first PDF peak to a lower distance and the decreasing the average coordination number of the first shell, compared to the (Co0.65,Fe0.35)62Ta8B30 glassy alloy, recently produced. DSC analysis shows that the new alloy has a wide supercooled liquid region of 51 K and relatively high activation energy of crystallization of about 445 kJ/mole, calculated according to the Kissinger and Ozawa methods, which demonstrates its high thermal stability. Magnetic measurements show that the new alloy exhibits outstanding soft magnetic properties, i.e., very low coercivity of 1.2 A/m, high saturation magnetization of 93.5 Am2/kg and Curie temperature of 660 K, which are significantly larger than those of (Co0.65,Fe0.35)62Ta8B30 glassy ribbon. The influence of the annealing treatment on evolution of the soft magnetic properties of the (Co0.65,Fe0.35)72Ta8B20 glassy ribbons has been investigated

Influence of annealing on microstructure and magnetic properties of cobalt-based amorphous/nanocrystalline powders synthesized by mechanical alloying

The effects of isochronal annealing on microstructure and magnetic properties of Co40Fe22Ta8B30 powders with a large content of amorphous phase produced by mechanical alloying have been investigated. The differential scanning calorimetery (DSC) results indicate that the synthesized powders exhibit a huge exothermic reaction before the crystallization temperature corresponding to structural relaxation of amorphous phase. Furthermore, the structural evolution of the powders upon isochronal heating has been investigated by in-situ X-ray diffraction (XRD) using high energy synchrotron radiation. The occurrence of an irreversible structural relaxation is confirmed by significant changes in position of the first and second diffuse maxima of the total structure factor S(Q) upon isochronal heating-cooling cycles. Moreover, analysis of the reduced pair distribution functions (PDFs) yields a volume shrinkage of about 1.5% after annealing due to annihilation of the excess free volume generated upon milling. The isochronal annealing significantly affects the magnetic properties of the powders through decreasing the saturation magnetization and coercivity. The correlation between structural relaxation and magnetic properties of the powders is discussed

Fabrication and Characterization of Co40Fe22Ta8−xYxB30Co_{40}Fe_{22}Ta_{8-x}Y_{x}B_{30} (x=0, 2.5, 4, 6, and 8) Metallic Glasses with High Thermal Stability and Good Soft Magnetic Properties

Atomic structure and thermal behavior of $Co_{40}Fe_{22}$ Ta $_{8-x}Y_{x}B_{30}$ (x = 0, 2.5, 4, 6, and 8) metallic glasses with good soft magnetic properties have been investigated by high-energy synchrotron X-ray diffraction and differential scanning calorimeter, respectively. It has been shown that the extension of the supercooled liquid region first increases and reaches a large value of 95 K and subsequently decreases as a function of Y content. Analysis of the structure factors and pair correlation functions in the reciprocal-space and real-space have indicated that the addition of Y noticeably changes the atomic structure and reduces the degree of the medium-range order. Magnetic measurements have implied that the introduction of Y enhances both saturation magnetization and Curie temperatures of the ribbons, while keeping their coercivity very small. The underlying mechanisms for changes in the atomic structure, improving the thermal stability and magnetic properties upon Y addition have been discussed

Inhomogeneous thermal expansion of metallic glasses in atomic-scale studied by in-situ synchrotron X-ray diffraction

Numerous investigations have demonstrated that the elastic strain in metallic glasses subjected to mechanical loading could be inhomogeneous in the atomic-scale and it increases with distance from an average atom and eventually reaches the macroscopic strain at larger inter-atomic distances. We have observed a similar behavior for the thermal strain imposed by heating of Co$_{40}$Fe$_{22}$Ta$_{8}$B$_{30}$ glassy particles below the glass transition temperature by analysis of the scattering data obtained by in-situ high-energy synchrotron X-ray diffraction (XRD). The results imply that the volumetric thermal strains calculated from the shift in position of the principal diffraction maximum and reduced pair correlation function (PDF) peaks are in good agreement for the length scales beyond 0.6 nm, corresponding to the atoms located over the third near-neighbor shell. However, smaller and even negative volumetric thermal strains have been calculated based on theshifts in the positions of the second and first PDF peaks, respectively. The structural changes of Co$_{40}$Fe$_{22}$Ta$_{8}$B$_{30}$ glassy particles are accompanied by decreasing the average coordination number of the first near-neighbor shell, which manifests the occurrence of local changes in the short-range order upon heating. It is believed that the detected length-scale dependence of the volumetricthermal strain is correlated with the local atomic rearrangements taking place in the topologically unstable regions of the glass governed by variations in the atomic-level stresses

Atomic structure, thermal stability and isothermal crystallization kinetics of novel Co-based metallic glasses with excellent soft magnetic properties

In this work, Co$_{66-x}$Fe$_x$Hf$_{6.5}$B$_{27.5}$ (x = 0, 10, 15, 20) metallic glasses (MGs) have been prepared by melt-spinning, and a comprehensive qualitative study including the atomic structure, thermal behavior, crystallization kinetics, and magnetic properties of the MGs is presented. The addition of Fe notably improves the thermal stability of the prepared glasses by increasing the width of the supercooled liquid region (SLR) from 26 K to 55 K. Analysis of the structure factor and the reduced-pair distribution function (PDF) reveal subtle variations in short-range ordering (SRO) and an increase of the atomic packing density of the glassy phase by 20 at% Fe addition. Notable rises in saturation polarization, Js, (from 0.21 T to 0.67 T), Curie point, Tc, (from 315 K to 530 K) and coercivity, Hc, (from 0.5 A/m to 2.5 A/m) are found after Fe alloying. The isothermal crystallization kinetics determined by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach demonstrate that the novel Co$_{46}$Fe$_{20}$Hf$_{6.5}$B$_{27.5}$ MG exhibits a remarkably higher activation energy and a significantly longer incubation time (about 16 min) before crystallization compared to existing Fe-free (x = 0) as well as most other Fe/Co-based MGs. Slight increases in Js (up to 5%) and Tc (about 4.7%), and a remarkable drop in Hc (up to 80%) are achieved for the Co$_{46}$Fe$_{20}$Hf$_{6.5}$B$_{27.5}$ MG upon isothermal annealing for a period of 3600 s. The attractive features such as high thermal stability, low coercivity of 0.5 A/m, and higher Js (up 0.7 T) compared to many Co-based glasses, make C$_{46}$Fe$_{20}$Hf$_{6.5}$B$_{27.5}$ MG a promising soft magnetic material for future applications in power electronics, electric machines, and fabrication of large-size glassy samples by hot consolidation of MG particles

Thermal and soft magnetic properties of Co40Fe22Ta8B30Co_{40}Fe_{22}Ta_{8}B_{30} glassy particles: In-situ X-ray diffraction and magnetometry studies

The structural evolution of Co40Fe22 Ta 8B30 glassy particles has been studied by in-situ high-energy synchrotron X-ray diffraction (XRD) upon isochronal annealing. The changes in position, intensity, and full width at half maximum (FWHM) of the first and second diffuse maxima of the XRD patterns suggest the occurrence of irreversible structural relaxation upon the first heating up to a temperature close to the glass transition temperature Tg . The variations in reduced pair correlation functions upon annealing are discussed in the frame of the topological fluctuation theory for structural relaxation. Isochronal annealing of the Co40Fe22 Ta 8B30 glassy particles improves their soft magnetic properties through decreasing the coercivity and increasing the magnetic susceptibility, saturation magnetization, and Curie temperature

Thermal behavior, structural relaxation and magnetic study of a new Hf-microalloyed Co-based glassy alloy with high thermal stability

In the present work, the influence of a minor Hf addition on the atomic structure, crystallization behavior, thermal stability and magnetic properties of a Co-based metallic glass was studied. Thermal analysis indicates that the thermal stability of new glassy ribbons microalloyed with 2.5 at.% Hf is notably enhanced through increasing the incubation time prior to devitrification and enlarging the width of the supercooled liquid region from 72 K to 96 K. Magnetic studies reveal that the new glass exhibits an excellent soft magnetic behavior, i.e., a very low coercivity of 0.26 A/m in the relaxed state, and a comparable saturation magnetization as the Hf-free ribbon. Structural relaxation of the Hf-containing alloy upon isothermal annealing below the glass transition temperature, $T_g$, was investigated by differential scanning calorimetry (DSC) and high-energy synchrotron X-ray diffraction (XRD). The evolution of the recovered enthalpy with annealing time can be expressed by the Kohlrausch-Williams-Watts (KWW) exponential function with a Kohlrausch exponent of 0.88, indicating a broad spectrum of relaxation times. Analysis of the reduced pair correlation functions, G(r), reveals volume shrinkage upon annealing caused by elimination of liquid-like sites including free-volume and anti-free-volume according to the shift in the positions of the G(r) maxima in the medium-range scale. The influence of structural relaxation on the variation of the Curie temperature and the coercivity of the new Hf-microalloyed glassy ribbon is discussed. A faster evolution of the Curie temperature with annealing time compared to the coercivity indicates a preferential dependence of the former on the chemical short-range order (SRO)

Microstructural characterization and amorphous phase formation in Co40Fe22Ta8B30 powders produced by mechanical alloying

In this work, microstructural evolution and amorphous phase formation in Co40Fe22Ta8B30 alloy produced by mechanical alloying (MA) of the elemental powder mixture under argon gas atmosphere was investigated. Milling time had a profound effect on the phase transformation, microstructure, morphological evolution and thermal behavior of the powders. These effects were studied by the X-ray powder diffraction (XRD) in reflection mode using Cu Kα and in transmission configuration using synchrotron radiation, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results showed that at the early stage of the milling, microstructure consisted of nanocrystalline bcc-(Fe, Co) phases and unreacted tantalum. Further milling, produced an amorphous phase, which became a dominant phase with a fraction of 96 wt% after 200 h milling. The DSC profile of 200 h milled powders demonstrated a huge and broad exothermic hump due to the structural relaxation, followed by a single exothermic peak, indicating the crystallization of the amorphous phase. Further XRD studies in transmission mode by synchrotron radiation revealed that the crystalline products were (Co, Fe)20.82Ta2.18B6, (Co, Fe)21 Ta2 B6, and (Co, Fe)3B2. The amorphization mechanisms were discussed in terms of severe grain refinement, atomic size effect, the concept of local topological instability and the heat of mixing of the reactants

Thermal and soft magnetic properties of Co 40

The structural evolution of Co40Fe22 Ta 8B30 glassy particles has been studied by in-situ high-energy synchrotron X-ray diffraction (XRD) upon isochronal annealing. The changes in position, intensity, and full width at half maximum (FWHM) of the first and second diffuse maxima of the XRD patterns suggest the occurrence of irreversible structural relaxation upon the first heating up to a temperature close to the glass transition temperature Tg . The variations in reduced pair correlation functions upon annealing are discussed in the frame of the topological fluctuation theory for structural relaxation. Isochronal annealing of the Co40Fe22 Ta 8B30 glassy particles improves their soft magnetic properties through decreasing the coercivity and increasing the magnetic susceptibility, saturation magnetization, and Curie temperature

Fabrication and characterization of Co 40

Atomic structure and thermal behavior of $Co_{40}Fe_{22}$ Ta $_{8-x}Y_{x}B_{30}$ (x = 0, 2.5, 4, 6, and 8) metallic glasses with good soft magnetic properties have been investigated by high-energy synchrotron X-ray diffraction and differential scanning calorimeter, respectively. It has been shown that the extension of the supercooled liquid region first increases and reaches a large value of 95 K and subsequently decreases as a function of Y content. Analysis of the structure factors and pair correlation functions in the reciprocal-space and real-space have indicated that the addition of Y noticeably changes the atomic structure and reduces the degree of the medium-range order. Magnetic measurements have implied that the introduction of Y enhances both saturation magnetization and Curie temperatures of the ribbons, while keeping their coercivity very small. The underlying mechanisms for changes in the atomic structure, improving the thermal stability and magnetic properties upon Y addition have been discussed