Microstructure and thermal expansion of copper-based amorphous alloys during structural relaxation

(Cu43Zr48Al9)98Y2 amorphous alloy bar was prepared by the arc melting copper mold absorption casting method,and then,the amorphous alloy was annealed at different temperatures for different times.The influence of heating rate on thermal expansion and thermal stability was studied by thermomechanical analysis(TMA),and the microstructure evolution of the amorphous alloy during structural relaxation and crystallization was studied by XRD and TEM.Results show that the structural evolution behavior of the(Cu43Zr48Al9)98Y2 amorphous alloy can be divided into five different stages(structural relaxation preparation stage,structural relaxation stage,first crystallization stage,second crystallization stage,and grain growth stage).When the heating rate is 20 K/min,the amorphous alloy has the smallest thermal expansion coefficient and the best thermal stability.The width of the supercooled liquid region is 66.42 K.Samples with different relaxation states were prepared by annealing at the heating rate of 20 K/min.The structural evolution of amorphous alloys with different relaxation states is as follows:amorphous→CuZr2+AlCu2Zr7→CuZr2+AlCu2Zr7+CuZr(B2)+CuZr(M)+Cu10Zr7→CuZr2+AlCu2Zr7+CuZr(B2)+CuZr(M).After annealing at 706 K and 726 K(in the supercooled liquid region)for 1.5 h,the amorphous-nanocrystalline composites were obtained.When the annealing temperature is 706 K,the crystallization process of the sample is as follows:amorphous→Cu10Zr7→Cu10Zr7+CuZr,and for the sample at 726 K,it is as follows:amorphous→CuZr2+AlCu2Zr7+Cu10Zr7→Cu10Zr7+CuZr2→CuZr2+CuZr(B2)+Cu10Zr7.

Effect of melt cooling rate on glass transition kinetics and structural relaxation of Vit1 metallic glass

The thin ribbons and the bulk cylindrical rods with diameters of 2 mm and 10 mm of the Vit1 metallic glass(MG)were prepared by the single roller melt spinning method and the copper mold injection casting method,respectively.The cooling rates of the samples during melt solidification were evaluated.The glass transition behaviors of three groups of MG samples with different solidification cooling rates were studied by differential scanning calorimetry(DSC)at different heating rates.The effects of melt cooling rate on the glass transition kinetic parameters such as apparent activation energy(E)and fragility parameter(m)of the Vit1 MG were studied using the Kissinger and the Vogel-Fulcher-Tammann(VFT)equations.Additionally,the structural relaxation enthalpy(ΔHrel)of three groups of MG samples was quantitatively analyzed by DSC through multi-step temperature rise and fall measurements.Results show that the melt cooling rate(R)has a significant effect on the glass transition kinetics and the structural relaxation of the Vit1 MG.As R decreases in the order of magnitude,the glass transition temperature(Tg),E,m,andΔHrel of the Vit1 MG gradually decreases.Furthermore,in the range of the experimental cooling rates,E,m,andΔHrel all have an approximately linear relationship with lgR.

Preparation of polymer nanoparticles,and the effect of nanoconfinement on glass transition,structural relaxation and crystallization

In this review the preparation methods of polymer nanoparticles from chemical microemulsion polymerization to physical methods such as spray-drying,freeze-drying,freeze-extracting,fast evaporation and spreading evaporation have been summarized.The influence of nanoconfinement on glass transition temperature(T_(g))variation from significant or slight decrease,no evident T_(g) deviation,to even T_(g) increase,as well as possible explanations of T_(g) deviations were discussed.The influences of nanoconfinement or entanglement on the other properties such as structural relaxation,crystallization in polymer nanoparticle samples were also reviewed in this article.

Effects of Internal Relaxation under Inplane Strain on the Structural,Electronic and Optical Properties of Perovskite BaZrO3

We present the specific ab-initio calculations that detail the variations of perovskite BaZrO3 caused by in-plane strain. Specifically, the internal relaxation, which was not captured in the widely used biaxial strain model, was included in a complementary manner to lattice relaxation. Density functional theory as well as a hybrid functional method based on a plane wave basis set was employed to calculate the lattice structure, elastic constants, electronic properties and optical properties of perovskite BaZrO3. The lattice parameter c exhibited a clear linear dependence on the imposed in-plane strain, but the Poisson＇s ratio caused by internal relaxation was smaller than the elastic deformation, indicating an ＂inelastic＂ or ＂plastic＂ relaxation manner caused by the introduction of internal relaxation. As a result, the related electronic and optical properties of perovskite BaZrO3 were also strongly affected by the in-plane strain, which revealed an effective way to adjust the properties of perovskite BaZrO3 via internal relaxation.

Thermal effects and evolution of the defect concentration based on shear modulus relaxation data in a Zr-based metallic glass

A relationship between thermal effects and relaxation of the high-frequency shear modulus upon heat treatment of bulk Zr48(Cu5/6Ag1/6)44Al8 metallic glass is found.This relationship is attributed to the relaxation of a interstitial-type defect system frozen-in from the melt upon glass production.Calorimetric data show that thermal effects occurring on heating include heat release below the glass transition temperature,heat absorption above it and heat release caused by crystallization.The equation derived within the Interstitialcy theory can be used to calculate the shear modulus relaxation using the calorimetric data.The obtained results are used to trace the defect concentration as functions of temperature and thermal prehistory.

Universal basis underlying temperature, pressure and size induced dynamical evolution in metallic glass-forming liquids

The dramatic temperature-dependence of liquids dynamics has attracted considerable scientific interests and efforts in the past decades, but the physics of which remains elusive. In addition to temperature, some other parameters, such as pressure, loading and size, can also tune the liquid dynamics and induce glass transition, which makes the situation more complicated. Here, we performed molecular dynamics simulations for Ni_(50)Zr_(50) bulk liquid and nanodroplet to study the dynamics evolution in the complex multivariate phase space, especially along the isotherm with the change of pressure or droplet size. It is found that the short-time Debye–Waller factor universally determines the long-time relaxation dynamics no matter how the temperature, pressure or size changes. The basic correlation even holds at the local atomic scale. This finding provides general understanding of the microscopic mechanism of dynamic arrest and dynamic heterogeneity.

Structure Relaxation of Mg_(65)Cu_(25)Gd_(10) Metallic Glass and Its Effect on Strength

To identify the re-arrangement of constituent atoms of an amorphous Mg65Cu25Gd10 alloy happened with annealing, structure relaxation of the alloy was investigated as a function of an- nealing time at 373 K through extended X-ray absorption fine structure （EXAFS） analysis procedures. To understand the effect of structure relaxation on strength, compression tests were conducted for both the as-cast and the annealed Mg65Cu25Gd10 samples. It is found that short range order around Cu and Gd atoms exhibits different variation trends with increasing annealing time at 373 K, though the structure of the alloy still remains to be amorphous. Based on the fact that the strength of the alloy first exhibits a reduction and then a recovery with annealing time, it is suggested that the enhancement of short range order around Cu should be responsible for the strength reduction, while the enhancement of short range order around Gd should be responsible for the strength recovery.

Numerical Simulation of Stress Field in Physical Tempering Process of Flat Soda Lime Silica Glass

Based on the analysis of different theory for glass tempering process,the“structural theory”with stress relaxation and structural relaxation effects was selected to investigate the tempering of flat glass quantificationally.The geometrical model with small size and non-homogeneous mesh were considered to build the finite element models according to the characteristics of stress field.The tempering process of flat glass with12 mm thickness was calculated with the verified finite element model.The transient and permanent stress of the central area,edge and corner end of the flat glass are obtained and analyzed.From the calculation results of basic case,the transient tensile stress at the upper surface of the central area,the center point of edge,the edge of edge,the edge of corner were 14.30,18.94,40.76 and 34.75 MPa,respectively.The transient tensile stress at these points were dangerous to promote the glass to break during the tempering.In addition,the point at the diagonal line of symmetry plane in the thickness direction,which is 14 mm from corner,has the maximum permanent tensile stress about 70.01 MPa in the flat glass after tempering.Thus,it is indicated that the corner is the weakest region in the tempered glass.

Molecular dynamics simulation of relationship between local structure and dynamics during glass transition of Mg_7Zn_3 alloy

The rapid solidification process of Mg7Zn3 alloy was simulated by the molecular dynamics method. The relationship between the local structure and the dynamics during the liquid-glass transition was deeply investigated. It was found that the Mg-centered FK polyhedron and the Zn-centered icosahedron play a critical role in the formation of Mg7Zn3 metallic glass. The self-diffusion coefficients of Mg and Zn atoms deviate from the Arrhenius law near the melting temperature and then satisfy the power law. According to the time correlation functions of mean-square displacement, incoherent intermediate scattering function and non-Gaussian parameter, it was found that the β-relaxation in Mg7Zn3 supercooled liquid becomes more and more evident with decreasing temperature, and the α-relaxation time rapidly increases in the VFT law. Moreover, the smaller Zn atom has a faster relaxation behavior than the Mg atom. Some local atomic structures with short-range order have lower mobility, and they play a critical role in the appearance of cage effect in theβ-relaxation regime. The dynamics deviates from the Arrhenius law just at the temperature as the number of local atomic structures begins to rapidly increase. The dynamic glass transition temperature （Tc） is close to the glass transition point in structure （TgStr）.

Bulk Glassy Alloys: Historical Development and Current Research

This paper reviews the development of current research in bulk glassy alloys by focusing on the trigger point for the synthesis of the first bulk glassy alloys by the conventional mold casting method. This review covers the background, discovery, characteristics, and applications of bulk glassy alloys, as well as recent topics regarding them. Applications of bulk glassy alloys have been expanding, particularly for Fe-based bulk glassy alloys, due to their unique properties, high glass-forming ability, and low cost. In the near future, the engineering importance of bulk glassy alloys is expected to increase steadily, and continuous interest in these novel metallic materials for basic science research is anticipated.

First-principles study of bulk and (001) surface of TiC

The structural and electronic properties of bulk and (001) plane of TiC were investigated by the first-principles total-energy pseudopotential method based on density functional theory.The calculated bulk properties indicate that bonding nature in TiC is a combination of ionicity,covalency and metallicity,in which the Ti-C covalent bonding is the predominate one.The calculated results of structural relaxation and surface energy for TiC(001) slab indicate that slab with 7 layers shows bulk-like characteristic interiors,and the changes of slab occur on the outmost three layers,which shows that the relaxation only influences the top three layers.Meanwhile,the strong Ti-C covalent bonding can be found in the distribution of charge density on the (110) and (001) planes.Ti-C covalent bonding is enhanced by the charge depletion and accumulation in the vacuum and the interlayer region between top two atomic layers.

First-principles study of TiC(110) surface

The structural and electronic properties of TiC（110） surfaces are calculated using the first-principles total-energy plane-wave pseudopotential method based on density functional theory. The calculated results of structural relaxation and surface energy for TiC（110） slab indicate that slab with 7 layers shows bulk-like characteristic interiors, and the changes of slab occur on the outmost three layers, which shows that the relaxation only influences the top three layers. Meanwhile, the strong Ti—C covalent bonding can be found in the distribution of charge density on the （100） plane. The interlayer Ti—C chemical bonds are reinforced and the outermost interlayer distance is reduced as a result of the charge depletion in the vacuum and the charge accumulations in the interlayer region between the first and second layers. The surface energy of TiC（110） is calculated to be 3.53 J/m2.

Compression Behaviour of Ni77P23 Amorphous Alloy up to 30.5 GPa

The compression behaviour of Ni77P23 amorphous alloy is investigated at room temperature in a diamond-anvil cell instrument using in-situ high pressure energy dispersive x-ray diffraction with a synchrotron radiation source. The equation of state is determined by fitting the experimental data according to the Birch-Murnaghan equation. It is found that the structure of Ni77P23 amorphous alloy is stable under pressures up to 30.5 GPa. Within the pressure range from zero to the experimental one, the pressure-induced structural relaxation is reversible.

Investigation on RQ Embrittlement Mechanism of FeCuNbSiB Soft Magnetic Alloy

The XRD, TEM, PAT, Curie temperature and internal friction methods were used to study systematically the embrittlement mechanism of rapidly quenched ( RQ ) nanocrystalline soft magnetic alloy Fe73.5Cu1Nb3Si13.5B9.The test results confirmed that the RQ embrittlement mechanism of amorphous alloy FeCuNbSiB was not related to crystallization but that was related to structural relaxation. Furthermore, the structural relaxation temperature of amorphous alloy FeCuNbSiB was much lower than that of used commonly amorphous alloy Fe78B13Si9. It meant that the RQ embrittlement is easier to happen for nanocrystalline alloy FeCuNbSiB than amorphous alloy Fe-B-Si.

Influencing Factors of Reheating Shrinkage Rate of Glass Substrate on LTPS Process

Different heating treatments with the variation of heating rates,holding temperatures and holding time were used to simulate the LTPS procedure.The experimental results show that the reheating shrinkage rates of glass substrates are rarely changed with increasing the heating rate,but strongly enhanced by raising the holding temperature and time,which shows that the reheating shrinkage of glass is closely related to heat treatment and structural relaxation.The production process of glass is critical to the reheating shrinkage of glass.

Photo-induced Phenomena in GeS_4 Glasses

GeS4 bulk glasses were prepared by the melt-quench technique and the samples were irradiated by 532-nm linearly polarized light. After the laser treatment, the photo-induced changes of the samples were investigated by UV-1601 speetrophotometer and optical second-order nonlinear tester. The results show that the transmittance of the samples around 532 nm obviously decreases and Bragg reflector forms, which is due to the production of photon-generated carriers. With the increase of laser pulse energy or the extension of irradiation duration, the Bragg reflector increases and gradually tends to be stable. These can be ascribed to the excitation- capture process of the carriers. After irradiation, the relaxation phenomenon results from the release of part of the absorbed energy in the glass matrix. And the fitting equation of the relaxation process is consistent with a conventional Kohlrausch stretched exponential function. The origin of the second harmonic generation （SHG） is because of the dipole reorientation caused by the photo-induced anisotropy in the glass.

Density Functional Theory for the Investigation of Catalytic Activity of X@Cu_(12)(X = Cu, Ni, Co, or Fe) for N_2O Decomposition

We have investigated the reaction mechanism for N20 decomposition on Cu13 via density functional theory. It is found that N20 decomposition on the cluster is more prone to be along the Eley-Rideal （ER） pathway in comparison with the Langmuir-Hinshelwood （LH） channel. There exists structural relaxation for Cu13 cluster in the reaction, which may influence the catalytic activity of cluster for the subsequent N2O decomposition. The core atom in the Cu13 cluster is substituted with the Fe, Co, or Ni to enhance structural stability and prevent from the obvious configuration relaxation in the reaction. Note that these bimetallic clusters are of icosahedra as the Cu13. They have activities for N2O dissociation along ER pathway and the heteroatorn in the cluster can prevent configuration from relaxation. Finally, the Ni@Cu12 cluster can be as a superior catalyst in a complete catalytic cycle via comparison in this study.

Uncovering the internal structure of five-fold twinned nanowires through 3D electron diffraction mapping

Five-fold twinned nanostructures are intrinsically strained or relaxed by extended defects to satisfy the space-filling requirement.Although both of metallic and semiconductor five-fold twinned nanostructures show inhomogeneity in their cross-sectional strain distribution,the evident strain concentration at twin boundaries in the semiconductor systems has been found in contrast to the metallic systems.Naturally,a problem is raised how the chemical bonding characteristics of various five-fold twinned nanosystems affects their strain-relieving defect structures.Here using three-dimensional(3D)electron diffraction mapping methodology,the intrinsic strain and the strain-relieving defects in a pentagonal Ag nanowire and a star-shaped boron carbide nanowire,both of them have basically equal radial twin-plane width about 30 nm,are nondestructively characterized.The non-uniform strain and defect distribution between the five single crystalline segments are found in both of the five-fold twinned nanowires.Diffraction intensity fine structure analysis for the boron carbide five-fold twinned nanowire indicates the presence of high-density of planar defects which are responsible for the accommodation of the intrinsic angular excess.However,for the Ag five-fold twinned nanowire,the star-disclination strain field is still present,although is partially relieved by the formation of localized stacking fault layers accompanied by partial dislocations.Energetic analysis suggests that the variety in the strain-relaxation ways for the two types of five-fold twinned nanowires could be ascribed to the large difference in shear modulus between the soft noble metal Ag and the superhard covalent compound boron carbide.

Temperature-frequency dependence and mechanism of dielectric properties for γ-Y_2Si_2O_7

This paper reports that single-phase γ-Y2Si2O7 is prepared via a sufficient blending and cold-pressed sintering technique from Y2O3 powder and SiO2 nanopowder. It studies the dielectric properties of γ-Y2Si2O7 as a function of the temperature and frequency. The γ-Y2Si2O7 exhibits low dielectric loss and non-Debye relaxation behaviour from 25 to 1400℃ in the range of 7.3 18 GHz. The mechanism for polarization relaxation of the as-prepared γ-Y2Si2O7 differing from that of SiO2 is explained. Such particular dielectric properties could potentially make specific attraction for extensive practical applications.

INFLUENCE OF A MISORIENTATION ANGLE ON AN ENERGR OF THE SYMMETRIC GRAIN BOUNDARY IN FCC METALS

In the present work the research of grain boundary (CB) energy versus angle of misorientation in fcc metals Al, Cu, An and Ni was carried out. An axis of CB misorientation is a direction [100], angle of misorientation makes from 2皍p in 23*. The interatomic interaction was opproximated by Morse' s pair semi-empirical potential. Two variants of relaxation technique were used: (1) rigid relax- ation with the change of atom quantity per a GB (vacancy relaxation ) and (2) full atomic relaxation by a molecular static method. The obtained orientation dependence has a good agreement with experi- ment. There are cusps on a curve in the range of special GB angles.The comparison of obtained curves with calculated ones in model Van der Merwe was carried out. Dependencies obtained in our investiga- tions are not smooth and have an oscillatory character. The oscillations reflect a discrete structure of a lattice.