非晶态合金Fe83B17和Ni64B36结构的研究

本文叙述了用偏振中子衍射方法对Fe₈₃B₁₇非晶合金和中子衍射方法对Ni₆₄B₃₆非晶合金的结构研究。由实验数据计算了每种合金的偏干涉函数(PSF)S_ij(Q),和偏简约径向分布函数G_ij(r)。并得到原子短程结构上的各项参数。在此基础上,讨论了这一类非晶态合金的结构模型,计算了其一种模型简单单元的中子衍射散射强度,并和实验所得的结果作了比较。 关键词：     

Design of high T g Zr-based metallic glasses using atomistic simulation and experiment

In this paper, we systematically investigate local atomic structures of Zr_100?x Al _x (0???x???72) alloys using molecular dynamics simulations. Radial distribution functions of Zr-Al configurations at 300 K indicate that Zr-Al metallic glasses form only when the Al atomic concentration is larger than 32%. Voronoi polyhedral analysis shows that Zr₄₀Al₆₀ has the highest fraction of ?0,0,12,0? icosahedra around Al atoms, which are characteristic of amorphous microstructures. Variations of thermal expansion coefficient and heat capacity of Zr_100?x Al _x (40???x???72) metallic glasses as a function of temperature from 1100 to 800?K reveal that Zr₄₀Al₆₀ has the highest transition temperature of 1008?K. To confirm the simulation results, Zr-Al metallic glasses were fabricated using co-sputtering deposition; differential scanning calorimetry testing suggests the highest crystallisation-onset temperature of above 920?K is within Zr_100?x Al _x where 43?<?x?<?61. The experimental finding is in a good agreement with the simulation predictions.     

Temperature dependence of the kinetic energy in the Zr<Subscript>40</Subscript>Be<Subscript>60</Subscript> amorphous alloy

The average kinetic energy 〈E(T)〉 of the atomic nucleus for each element of the amorphous alloy Zr₄₀Be₆₀ in the temperature range 10–300 K has been measured for the first time using VESUVIO spectrometer (ISIS). The experimental values of 〈E(T)〉 have been compared to the partial ZrBe spectra refined by a recursion method based on the data obtained with thermal neutron scattering. The satisfactory agreement has been reached with the calculations using partial spectra based on thermal neutron spectra obtained with recursion method. In addition, the experimental data have been compared to the Debye model. The measurements at different temperatures (10, 200, and 300 K) will provide an opportunity to evaluate the significance of anharmonicity in the dynamics of metallic glasses.     

Local and average atomic order of ion-conducting Bi0.775La0.225O1.5 studied by neutron scattering and reverse Monte Carlo simulations

Bi_0.775La_0.225O_1.5 has been investigated by means of neutron scattering in combination with Rietveld refinement and reverse Monte Carlo (RMC) simulations to study the average and local atomic order of the material. At low temperature the crystallographic analysis shows a clear split of anionic positions similar to that reported for Sr doped bismuth oxides. Although this split appears to be absent in the average structure at high temperature, an even larger split of Bi–O local distances is found in the pair distribution function calculated from the RMC models. This latter feature can be indicative of a tendency to have similar oxygen environments for all Bi atoms at high temperature, as a precursor of the fluorite structure that is the stable phase at even higher temperatures.     

Zr48Cu45Al7大块金属玻璃的原子结构研究

应用同步辐射X射线衍射(XRD)和广延X射线吸收精细结构边方法(EXAFS),结合反蒙特卡罗(RMC)拟合、Voronoi分形技术等对Zr₅₀Cu₅₀二元和Zr₄₈Cu₄₅Al₇三元金属玻璃材料的微观结构进行了系统的研究.结果表明:ZrCuAl三元金属玻璃中Al原子与Zr原子、Cu原子之间存在强相互作用,表现为键长的明显缩短,导致其微观结构中的Voronoi团簇体积普遍小于Zr_{50关键词： 大块金属玻璃 原子结构 玻璃形成能力 同步辐射技术}     

Atomic structures of Zr-based metallic glasses

The atomic structures of Zr-Ni and Zr-Ti-Al-Cu-Ni metallic glasses were investigated by using classical molecular dynamic (MD), reverse Monte Carlo (RMC), ab initio MD (AIMD) simulations and high resolution transmission electron microscopy (HRTEM) techniques. We focused on the short-range order (SRO) and medium-range order (MRO) in the glassy structure. It is shown that there are icosahedral, FCC-and BCC-type SROs in the Zr-based metallic glasses. A structural model, characterized by imperfect ordered packing (IOP), was proposed based on the MD simulation and confirmed by the HRTEM observation. Furthermore, the evolution from IOP to nanocrystal during the crystallization of metallic glasses was also explored. It is found that the growth from IOP to nanocrystal proceeds through three distinct stages: the formation of quasi-ordered structure with one-dimensional (1D) periodicity, then 2D periodicity, and finally the formation of 3D nanocrystals. It is also noted that these three growth steps are crosslinked. Supported by the National Natural Science Foundation of China (Grant Nos. 50431030 and 50471097), the National Basic Research Program of China (Grant No. 2007CB613901), and the Programme of Introducing Talents of Discipline to Universities (Grant No. B07003)     

Local atomic structures in Zr-Ni metallic glasses

Local atomic structures of Zr100−xNix (x=33.3, 36, 50 at%) binary metallic glasses were investigated by means of extended X-ray absorption fine structure (EXAFS) probe. Structural parameters show that the Zr-Ni bond length, RZr-Ni, keeps a constant value of 2.62 Å, regardless of alloy compositions. This result implies that there is a strongly chemical interaction between Zr and Ni atoms, leading to significant chemical short-range orders (CSROs) in the present metallic glasses. Further analysis indicates that the SRO structures in these metallic glasses are extremely similar with those in their crystalline counterparts. It is interesting to note that the CSROs in the eutectic Zr₆₄Ni₃₆ metallic glass consist of Zr₂Ni-like and ZrNi-like CSROs.     

A network problem: Modelling alkali-silicate glasses with RMC

Structural models of four potassium-silicate glasses, (K₂O)_x(SiO₂)_1-x with x=0, 0.15, 0.25 and 0.45, have been created using the reverse Monte Carlo (RMC) method. The current use of RMC for creating topologically constrained models of amorphous materials is described in detail. The models are based on the simultaneous combination of neutron diffraction and ²⁹Si magic angle spinning NMR data, the latter for the first time. We show that the predictions of Modified Random Network theory are consistent with the data. It is found that models whose topology is determined by short-range chemical bonding constraints and the macroscopic density are able to reproduce all details of the structure factor, including its complex form at low momentum transfer. It is not necessary to ‘build in’ specific ring structures or other complex units beforehand; these arise naturally as a consequence of the constraints of density and chemical bonding.     

Structure of Na2S–GeS2 glasses studied by using neutron and X-ray diffraction and reverse Monte Carlo modeling

Neutron and X-ray diffraction measurements were performed to investigate the structure of Na₂S–GeS₂ glasses synthesized by mechanical alloying. The Ge–S coordination numbers calculated from the total correlation functions show that GeS₄ tetrahedra form the basic framework structure of Na₂S–GeS₂ glasses. In addition, a three-dimensional structural model of the (Na₂S)₅₀(GeS₂)₅₀ glass was obtained by using reverse Monte Carlo (RMC) simulation on neutron and X-ray diffraction data, and it was compared with the RMC model previously obtained for a (Li₂S)₅₀(GeS₂)₅₀ glass. The results show that the Ge–S framework structures in the two glasses are almost the same in the short and intermediate ranges; that is, the Ge–S framework structures are formed mainly by the connection of corner-sharing GeS₄ tetrahedra. Many of the Na ions are coordinated by four S atoms, which are non-bridging S atoms and/or bridging S atoms.     

High pressure electrical resistivity studies on Ga-Te glasses

Abstract

Bulk, semiconducting Ga_xTe_100-x (17 ≤ x ≤ 25) glasses are prepared by the meltingh quenching method and electrical resistivity measurements are carried out at high pressures up to 8 GPa and low temperatures down to 77 K in a Bridgman anvil system. It is found that all the Ga_xTe_100-x, glasses exhibit metallization under pressure, with a continuous decrease in electrical resistivity and activation energy for conduction. Further, the high pressure metallic phases of Ga_xTe_100-x samples are found to be crystalline.     

A fractal interpretation of size effects on the strength of metallic glasses

Yielding strength of metallic glasses in the uniaxial tensile and compressive tests is scale-dependent, which is attributed to the self-similar distribution of atomic cluster and free volume in the work. In contrast with the Weibull statistical theory previously employed in scaling phenomena of metallic glasses, fractal scaling laws are for the first time applied to describe the size effect inherent to the material disorder. Especially, the Multifractal Scaling Law (MFSL) originally proposed for quasi-brittle materials is used to interpret some experimental data in the literature. The best-fitted parameters (_fy$f_{\mathrm{y}}$ and l_ch) from the MFSL are in good consistency with the bulk yielding strength and the shear band size of metallic glasses observed in the alternative approaches or experiments. The fractal size effect laws provide insight into not only the scaling phenomena, but also further engineering strength predictions and designs.     

Brillouin scattering from ferromagnetic metallic glasses

We report the first measurements of thermally excited surface acoustic phonons, and bulk and surface magnons from metallic glasses using Brillouin scattering. In particular we present data on the compositions Fe₄₀Ni₄₀P₁₄B₆ and Fe₈₀B₂₀. The velocity of the surface phonons was found to agree within 1.5% with the value calculated from elastic continuum theory using known bulk elastic constants. The observed field dependence of the bulk magnon frequency can be fit within experimental error to the classical relation for the k = 0 mode by including alldemagnetizing corrections. The values for the magnetization obtained from both the surface and bulk magnons agree with the bulk values determined previously with a magnetometer. The measured field dependences of the surface magnon frequency allow us to determine the g-factors, yielding 2.08±0.02 for FeB and 2.03±0.03 for FeNiPB.     

Modification of magnetic anisotropy in metallic glasses using high-energy ion beam irradiation

Heavy ion irradiation in the electronic stopping power region induces macroscopic dimensional change in metallic glasses and introduces magnetic anisotropy in some magnetic materials. The present work is on the irradiation study of ferromagnetic metallic glasses, where both dimensional change and modification of magnetic anisotropy are expected. Magnetic anisotropy was measured using Mössbauer spectroscopy of virgin and irradiated Fe₄₀Ni₄₀B₂₀ and Fe₄₀Ni₃₈Mo₄B₁₈ metallic glass ribbons. 90 MeV ¹²⁷I beam was used for the irradiations. Irradiation doses were 5×10¹³ and 7.5×10¹³ ions/cm². The relative intensity ratios D ₂₃ of the second and third lines of the Mössbauer spectra were measured to determine the magnetic anisotropy. The virgin samples of both the materials display in-plane magnetic anisotropy, i.e., the spins are oriented parallel to the ribbon plane. Irradiation is found to cause reduction in magnetic anisotropy. Near-complete randomization of magnetic moments is observed at high irradiation doses. Correlation is found between the residual stresses introduced by ion irradiation and the change in magnetic anisotropy.     

Disordered solids: Universal behavior of structure, dynamics, and transport phenomena

Disordered materials (glasses and amorphous substances, melts, polymers, biological media, etc.) are an important class of objects. Despite the chaos usually associated with their structure, glasses and amorphous substances of various kinds (semiconducting, dielectric, metallic) possess a universal spatial scale length ∼1 nm, an order parameter, which can be as important theoretically as the unit cell for crystals. The disorder in disordered substances is not absolute; the periodicity positions of atomic inherent in crystals is maintained within a few coordination spheres and is then somehow destroyed. The way in which the order breaks down makes it possible to distinguish the glasses from amorphous materials in terms of the form of the structural correlation function. The inhomogeneities in question are not exotic, unique formations or analogs of defects in crystals, but are the fragments out of which amorphous substances and glasses are entirely constructed. The spatial inhomogeneity of disordered substances having a characteristic scale length of ∼1 nm leads to some universal characteristics in their vibrational properties, changes the relaxation mechanism for electronic excitation, and determines the specific features of charge transport. Fiz. Tverd. Tela (St. Petersburg) 41, 805–808 (May 1999)     

Spin-dependent transport properties and Seebeck effects for a crossed graphene superlattice <Emphasis Type="Italic">p-n</Emphasis> junction with armchair edge

Using the nonequilibrium Green’s function method combined with the tight-binding Hamiltonian, we theoretically investigate the spin-dependent transmission probability and spin Seebeck coefficient of a crossed armchair-edge graphene nanoribbon (AGNR) superlattice p-n junction under a perpendicular magnetic field with a ferromagnetic insulator, where junction widths W₁ of 40 and 41 are considered to exemplify the effect of semiconducting and metallic AGNRs, respectively. A pristine AGNR system is metallic when the transverse layer m = 3j + 2 with a positive integer j and an insulator otherwise. When stubs are present, a semiconducting AGNR junction with width W₁ = 40 always shows metallic behavior regardless of the potential drop magnitude, magnetization strength, stub length, and perpendicular magnetic field strength. However, metallic or semiconducting behavior can be obtained from a metallic AGNR junction with W₁ = 41 by adjusting these physical parameters. Furthermore, a metal-to-semiconductor transition can be obtained for both superlattice p-n junctions by adjusting the number of periods of the superlattice. In addition, the spin-dependent Seebeck coefficient and spin Seebeck coefficient of the two systems are of the same order of magnitude owing to the appearance of a transmission gap, and the maximum absolute value of the spin Seebeck coefficient reaches 370 µV/K when the optimized parameters are used. The calculated results offer new possibilities for designing electronic or heat-spintronic nanodevices based on the graphene superlattice p-n junction.     

Phonon scattering by electrons and low energy excitations in a metallic glass

Measurements of the thermal conductivity of a superconducting metallic glass Zr₇₀Cu₃₀ in zero field and in a magnetic field exceeding the upper critical field allow a quantitative determination of the strength of the phonon-electron scattering for the first time in amorphous metals. The temperature dependence of the residual phonon scattering by localized low energy excitations is similar to that found in insulating glasses.     

Positron studies of defect structures in metallic glasses

A variety of metallic glasses (Cu_xZr_100?x with x = 30, 40 and 50, Ni₂₄Zr₇₆ and RE₆₇ Co₃₃, RE = Nd, Sm, Gd, Dy, Er) were studied using 2γ angular correlation techniques. Significant positron trapping is observed in all the glassy alloys investigated. The defect structure changes drastically during crystallization. For the RE-Co glasses the results suggest the existence of vacancy-like defects in the amorphous alloys.     

Application of the model of delocalized atoms to metallic glasses

The parameters of the model of delocalized atoms applied to metallic glasses have been calculated using the data on empirical constants of the Vogel–Fulcher–Tammann equation (for the temperature dependence of viscosity). It has been shown that these materials obey the same glass-formation criterion as amorphous organic polymers and inorganic glasses. This fact qualitatively confirms the universality of the main regularities of the liquid–glass transition process for all amorphous materials regardless of their origin. The energy of the delocalization of an atom in metallic glasses, Δε _e ≈ 20–25 kJ/mol, coincides with the results obtained for oxide inorganic glasses. It is substantially lower than the activation energies for a viscous flow and for ion diffusion. The delocalization of an atom (its displacement from the equilibrium position) for amorphous metallic alloys is a low-energy small-scale process similar to that for other glass-like systems.     

Fe-B-Y-Nb bulk metallic glasses in relation to clusters

Bulk metallic glass formations in the Fe-B-Y-Nb quaternary alloy system were investigated by using the cluster line rule in combination with the minor alloying principle. The Fe-B-Y ternary system was selected as the basic system and the intersections of cluster lines were taken as the basic ternary compositions. The basic compositions were further alloyed with minor amounts of Nb. After 3–5 at.% Nb was added, the basic composition Fe_68.6B_25.7Y_5.7, which was developed from the most densely packed cluster Fe₈B₃, formed 3 mm bulk metallic glasses. These quaternary bulk metallic glasses (Fe_68.6B_25.7Y_5.7)_100−x Nb _x (x = 3–5 at.%) are expressed approximately with a unified simple composition formula: (Fe₈B₃)₁(Y, Nb)₁. The (Fe_68.6B_25.7Y_5.7)₉₇Nb₃ bulk metallic glass has the largest glass forming ability with the following characteristic parameters T _g = 907 K, T _x = 1006 K, T _g/T _l = 0.644, γ = 0.434, and longness t = 22 mm. The combination of the cluster line rule and the minor-alloying principle is a promising new route towards the quantitative composition design of multi-component metallic glasses. Supported by the National Natural Science Foundation of China (Grant Nos. 50671018, 50631010 and 50401020) and the National Basic Research Program of China (Grant No. 2007CB613902)