On the structure of Ge–As–Te–Cu glasses

International audienceShort range order in glassy 0.9(Ge0.1As0.15Te0.75)–0.1Cu (GATC1) and 0.9(Ge0.05As0.55Te0.4)–0.1Cu (GATC2) was studied by neutron- and X-ray diffraction as well as EXAFS (extended X-ray absorption fine structure) measurements at the K-edges of all components. The reverse Monte Carlo simulation technique was used to create models consistent with all experimental datasets. It was found that Cu binds predominantly to Te in GATC1 while Cu–As and Cu–Cu bonding is also significant in GATC2. Ge and As atoms have 4 and 3 Ge/As/Te neighbors in both compositions. In GATC1 the formation of ‘extra’ Te–Te bonds can be observed, similarly to GeTe4–AgI glasse

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

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 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

Short-Range Order in Ge-As-Te Glasses

The structure of Te-rich (75–80 at.% Te) and Te-poor (40 at.% Te) Ge–As–Te glasses has been investigated by diffraction and extended X-ray absorption fine structure (EXAFS) measurements. Large-scale structural models have been created by fitting simultaneously diffraction and EXAFS datasets by the reverse Monte Carlo simulation technique. It is found that As–As bonds improve the fit quality in the case of Te-rich glasses while no Ge–Ge bonding is necessary in these compositions. In the Te-poor glasses, Te–Te homopolar bonds are also observed while Ge binds preferentially to Te rather than to As. Ge–As and Ge–Te coordination numbers do not change significantly with increasing Ge content

Signature of local stress states in the deformation behavior of metallic glasses

The design of ductile heterogeneous metallic glasses (MGs) with enhanced deformability by purposely controlling theshear-band dynamics via modulation of the atomic-scale structures and local stress states remains a significantchallenge. Here, we correlate the changes in the local atomic structure when cooling to cryogenic temperature withthe observed improved shear stability. The enhanced atomic-level structural and elastic heterogeneities related to thenonaffine thermal contraction of the short-range order (SRO) and medium-range order (MRO) change thecharacteristics of the activation process of the shear transformation zones (STZs). The experimental observationscorroborated by Eshelby inclusion analysis and molecular dynamics simulations disclose the correlation between thestructuralfluctuations and the change in the stressfield around the STZ. The variations in the inclination axes of theSTZs alter their percolation mechanism, affect the shear-band dynamics and kinetics, and consequently delay shearfailure. These results expand the understanding of the correlation between the atomic-level structure and elementaryplastic events in monolithic MGs and thereby pave the way for the design of new ductile metallic alloys