Magnetic moments and electronic structure of Co-base metallic glasses

Magnetization of Co_80-xT_xB₂₀ glasses (0 ≤ × ≤ 12 at%, T = Fe, Mn, Cr, V) has been measured from 4.2 K to approximately 700 K. The data are well described in terms of the virtual bound states introduced above the center of the d band of the matrix by the light T solutes. Details of the electronic structure, some aspects of which would not be revealed by photoemission, are thereby inferred.     

Correlation between atomic size and elastic properties/glass transition temperature in metallic glasses

<正>As a relative newcomer in the glass family,metallic glass challenges our standpoint on crystalline metals and extends our knowledge concerning the nature of glasses[1].Metallic glasses have attracted considerable attention because of their unique mechanical,physical,and chemical properties[2-4].Newly developed bulk metallic glass can combine toughness and strength beyond the benchmark ranges established by the     

金属玻璃的断裂机理与其断裂韧度的关系

本文选取了三种不同断裂韧度值的金属玻璃Zr41.25Ti13.75Ni10Cu12.5Be22.5,Ce68Al10Cu20Co2和Fe41Co7Cr15Mo14Y2C15B6,通过压缩实验测量了它们的应力-应变关系;样品断裂以后观察了其断口形貌,发现这三种金属玻璃具有不同的断裂模式.经过对这三种金属玻璃做缺口三点弯曲实验,利用数字散斑技术研究了缺口前端应变集中方向弹性应变场的演化过程.根据金属玻璃的屈服准则,阐述了不同断裂韧度值的金属玻璃的断裂机理.     

Local atomic structures of single-component metallic glasses

In this study we examine the structural properties of single-component metallic glasses of aluminum. We use a molecular dynamics simulation based on semi-empirical many-body potential, derived from the embedded atom method (EAM). The radial distribution function (RDF), common neighbors analysis method (CNA), coordination number analysis (CN) and Voronoi tessellation are used to characterize the metal’s local structure during the heating and cooling (quenching). The simulation results reveal that the melting temperature depends on the heating rate. In addition, atomic visualization shows that the structure of aluminum after fast quenching is in a glassy state, confirmed quantitatively by the splitting of the second peak of the radial distribution function, and by the appearance of icosahedral clusters observed via CNA technique. On the other hand, the Wendt-Abraham parameters are calculated to determine the glass transition temperature (T_g), which depends strongly on the cooling rate; it increases while the cooling rate increases. On the basis of CN analysis and Voronoi tessellation, we demonstrate that the transition from the Al liquid to glassy state is mainly due to the formation of distorted and perfect icosahedral clusters.     

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.     

Magnetism and structure of metallic glasses

Hyperfine Interactions - Metallic glasses are a rather new class of materials. They offer exciting possibilities for finding new materials with combinations of properties unknown for crystalline...     

Observation of long-range compositional fluctuations in glasses: implications for atomic and electronic structure

Experimental evidence for long-range compositional fluctuations in glasses is given. The implications for electronic structure and stoichiometry-induced structural variations are analyzed. These fluctuations were discovered by examining the spatial dependence of inner shell near-edge absorption spectra obtained using a 50nm diameter probe. This spectroscopy is sensitive to both angular and distance correlations in bonding. Comparisons with spectra from compositionally equivalent crystals, and multiple-scattering calculations which include core-hole effects are used to analyze the data.     

Correspondences between the classical electrostatic Thomson problem and atomic electronic structure

Correspondences between the Thomson problem and atomic electron shell-filling patterns are observed as systematic non-uniformities in the distribution of potential energy necessary to change configurations of N ≤ 100 electrons into discrete geometries of neighboring N ? 1 systems. These non-uniformities yield electron energy pairs, intra-subshell pattern similarities with empirical ionization energy, and a salient pattern that coincides with size-normalized empirical ionization energies. Spatial symmetry limitations on discrete charges constrained to a spherical volume are conjectured as underlying physical mechanisms responsible for shell-filling patterns in atomic electronic structure and the Periodic Law.     

The influence of efficient atomic packing on the constitution of metallic glasses

Efficient atomic packing is shown to be a fundamental consideration in the formation of metallic glasses. A simple concept of packing efficiency, based on atom packing in the first coordination shell of solute-centred clusters, is proposed and developed. This model leads to the prediction that specific radius ratios, defined as the radius of the solute atom divided by the radius of the solvent atom, are preferred in the constitution of metallic glasses. Analysis of a large number of binary and complex metallic glasses shows that these specific critical radius ratios R* are indeed preferred in known metallic glasses. The predictions of this model extend previous proposals to describe the influence of topology on the formation of metallic glasses. Although this model represents a simple idealization, the strong agreement with published metallic glasses suggests that efficient atomic packing, enabled by solute-centred clusters, forms a fundamental consideration in the constitution of metallic glasses.     

Correlation between supercooled liquid region and crystallization behavior with alloy composition of La-Al-Cu metallic glasses

Bulk metallic glasses (BMGs) with large supercooled liquid region are promising materials for superplastic forming. In this paper, we demonstrate a microstructure-based strategy to pinpoint the composition with the largest supercooled liquid region in La 86 x Al 14 Cu x (x=16 at%-20 at%) metallic glass system. By monitoring the changes in crystallization behavior of the glassy alloys with composition to search for the alloys exhibiting eutectic crystallization, the glassy alloys with the largest supercooled...     

Imaging from atomic structure to electronic structure

This paper discusses the possibility of retrieving the electron distribution (with highlighted valence electron distribution information) of materials from recorded HREM images. This process can be achieved by solving two inverse problems: reconstruction of the exit wave and reconstruction of the electron distribution from exit waves. The first inverse problem can be solved using a focal series reconstruction method. We show that the second inverse problem can be solved by combining a series of exit waves recorded at different thickness conditions. This process is designed based on an improved understanding of the dynamical scattering process. It also explains the fundamental difficulty of obtaining the valence electron distribution information and the basis of our solution.     

Relevance of Fe atomic volumes for the magnetic properties of Fe-rich metallic glasses

The atomic volume V_a-Fe that can be assigned to Fe atoms in Fe–metalloid (Fe–MD) and Fe–early transition metal (Fe–TE) glasses was deduced in a previous paper (I. Bakonyi, Acta Materialia 53 (2005) 2509) from an analysis of available density data for such amorphous alloys. In the present paper, based on a similarity of the amorphous and face-centered cubic (fcc) structures, the distinctly different magnetic behaviors of these two families of amorphous alloys are discussed in terms of the relative position of V_a-Fe and the critical volume V_fcc?-Fe≈11.7 Å³/atom separating the so-called low-spin (LS) and high-spin (HS) state of fcc-Fe. For Fe–MD systems, V_a-Fe is found to be definitely larger than V_fcc*-Fe whereas for Fe-TE systems V_a-Fe is fairly close to V_fcc*-Fe. Since in topologically disordered alloys a distribution of atomic volumes is inherently present, in Fe–MD glasses the Fe atoms can be assumed to exhibit exclusively the HS state whereas in Fe–TE amorphous alloys a comparable fraction of Fe atoms can be either in the LS or the HS state. According to previous theoretical band structure calculations, an antiferromagnetic state can also be stable just around V_fcc*-Fe. The simultaneous presence of Fe atoms with such a rich variety of magnetic states due to the specific position of the average of the atomic volume distribution can well explain the complex magnetic behavior observed in Fe-rich Fe–TE metallic glasses such as, e.g., in amorphous Fe–Zr alloys around 90 at% Fe.     

Correlation of atomic cluster symmetry and glass-forming ability of metallic glass

Local structures play a crucial role in glass formation and properties. In addition to topological short-range order, the geometric property of site symmetry is another important but less known characteristic of local structures. It is shown that the observed sharp increase of glass forming ability of Ce70-xAl10Cu20Cox upon Co addition is correlated with a dramatic increase of Al site symmetry, as reflected by decreasing quadrupole frequency measured by 27Al NMR. The result is consistent with the structure model of Al-centered icosahedral clusters as the predominant structural building blocks.     

The diamond surface: atomic and electronic structure

Experimental studies of the diamond surface with primary emphasis on the (111) surface are presented. Aspects of the diamond surface which are addressed include (1) the electronic structure, (2) the atomic structure, and (3) the effect of termination of the lattice by foreign atoms. Limited studies of graphite are discussed for comparison with the diamond results. Experimental results from valence band and core level photoemission spectroscopy (PES), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) are used to study and characterize both the clean and hydrogenated surface. In addition, the interaction of hydrogen with the diamond surface is examined using results from vibrational high resolution low energy electron loss spectroscopy and photon stimulated ion desorption (PSID) yield at photon energies just above the carbon k-edge. Both EELS and PSID verify that the mechanically polished 1 × 1 surface is hydrogen terminated and also that the reconstructed 2×2/2×1 surface is hydrogen free. We apply this basic knowledge of the clean diamond surface and of diamond-hydrogen systematics to understanding of the fundamental growth characteristics of diamond films.     

How the SiC substrate impacts graphene's atomic and electronic structure

Graphene, the two‐dimensional form of carbon presents outstanding electronic and transport properties. This gives hope for the development of applications in nanoelectronics. However, for industrial purpose, graphene has to be supported by a substrate. We focus here on the graphene‐on‐SiC system to discuss how the SiC substrate interacts with the graphene layer and to show the effect of the interface on graphene atomic and electronic structures.

     

金属玻璃温度依赖的拉压屈服不对称研究

通过引入静水应力对自由体积演化的影响, 研究了金属玻璃在不同温度下的拉压屈服行为. 结果表明, 在拉伸和压缩载荷下, 屈服强度均满足(T/T_g)^1/2的温度依赖关系; 同时, 在不同温度下, 材料的压力敏感系数保持为常值0.1. 随着温度的升高, 压力对自由体积的影响逐渐降低, 从而导致材料的拉压屈服不对称性逐渐趋于不显著. 在高温下, 显著的结构弛豫减缓了自由体积增长速率从而抑制材料迅速屈服. 这些结果将有助于更深入的认识金属玻璃屈服及其拉压不对称性的内在机理.     

金属玻璃的稳定性

玻璃态材料自诞生以来一直受到自身稳定性的困扰。为了保障玻璃的服役安全性,人们一直努力提高玻璃的耐久性和稳定性。玻璃能否经年累月地保持长期稳定？影响其稳定性的物理机制和根源是什么？这些都是非晶态物理领域长期关注的重要难题。根据以往对非晶合金稳定性的研究结果,文章将简单介绍玻璃亚稳性的起源,然后介绍玻璃如何能够保持长期稳定,如何提高玻璃的热力学和动力学稳定性以制备出超稳玻璃,最后介绍影响玻璃稳定性的材料因素,用于指导有效地获得高稳定性玻璃。对玻璃稳定性的认识和理解,既能够保障玻璃材料服役过程中的安全性,又对认识过冷液体和玻璃转变等基本物理问题非常关键。