Theory of ordering in binary substitution alloy with body-centered cubic lattice

With the framework of a second order approximation of the Kirkwood, a study has been made of ordering in binary substitution alloys with B2 structure. Interatomic interactions in the first and second coordination spheres have been taken into account. It has been shown that an order—disorder phase transformation can be of the first or second kind, depending on the ratio of the ordering energies in the first and second coordination spheres (w and ) and their composition. The calculated configurational thermal capacity in the region of the Kurnakov point agree well with experimental data for /w=0.25.S. M. Kirov Ural Polytechnical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 93–96, September, 1993.     

Surface order in body-centered cubic alloys

Free (100)-surfaces of body-centered cubic binary alloys are studied in a parameter range where the bulk turns from the ordered B2-phase to the disordered A2-phase. A model is chosen that describes iron-aluminium alloys in a fairly realistic way. Mean field treatments and Monte Carlo investigations both show that under certain circumstances the surface remains ordered far above the bulk disordering temperatureT _c, though the surface order parameter and the surface susceptibility exhibit a singularity atT _c with critical exponents characteristic for the ordinary transition. One finds, that if the surface is nonstoechiometric and different layers are not equivalent with respect to perfect bulk ordering, this induces an effective ordering field at the surface. Finally surface potentials are introduced into the model. Realistic values are determined by mean field considerations from experimental data and their effect on the surface is discussed.     

Assembly of body-centered cubic crystals in hard spheres

We investigate the crystallization of monodisperse hard spheres confined by two square patterned substrates (possessing the basic character of the body-centered cubic (bcc) crystal structure) at varying substrate separations via molecular dynamics simulation. Through slowly increasing the density of the system, we find that crystallization under the influence of square patterned substrates can set in at lower densities compared with the homogeneous crystallization. As the substrate separation decreases, the density, where crystallization occurs (i.e., pressure drops), becomes small. Moreover, two distinct regimes are identified in the plane of bcc particle fraction and density for the separation range investigated. For large substrate separations, the bcc particle fraction displays a local maximum as the density is increased, and the resulting formed crystals have a polycrystalline structure. However, and more importantly, another situation emerges for small substrate separations: the capillary effects (stemming from the presence of two substrates) overwhelm the bulk driving forces (stemming from the spontaneous thermal fluctuations in the bulk) during the densification, eventually resulting in the formation of a defect-free bcc crystal (unstable with respect to the bulk hard-sphere crystals) by using two square patterned substrates.     

Uniaxial deformation of nanotwinned nanotubes in body-centered cubic tungsten

We perform large-scale molecular dynamics simulations to delve into tensile and compressive loading of nanotubes containing {112} nanoscale twins in body-centered cubic tungsten, as a function of wall thickness, twin boundary spacing, and strain rate. Solid nanopillars without the interior hollow and/or nanotubes without the nanoscale twins are also investigated as references. Our findings demonstrate that both stress-strain response and deformation behavior of nanotwinned nanotubes and nanopillars exhibit a strong tension-compression asymmetry. The yielding of the nanotwinned nanotubes with thick walls is governed by dislocation nucleation from the twin boundary/surface intersections. With a small wall thickness, however, the failure of the nanotwinned nanotubes is dominated by crack formation and buckling under tensile and compressive loading, respectively. In addition, the strain rate effect, which is more pronounced in compressive loading than in tensile loading, increases with a decreasing twin boundary spacing.     

Statistical theory of slip channels in body-centered cubic metals

Received: 26 July 1996/Accepted: 7 October 1996     

Twinning propensity in nanocrystalline face-centered cubic,body-centered cubic,and hexagonal close-packed metals

The nanotwinned structures in metals exhibit the unique combination of physical properties. The unifying approach is developed that can be applied to nanocrystalline (nc) materials with different crystal structures. It is used to make a bridge between microscopical mechanisms of twin nucleation and macroscopic characteristics of the twinning and calculate them. The grain size range of the nanotwinning propensity, the grain size of its peak, and the requisite external twinning stress are calculated for the nc face-centered cubic metals Al, Cu, Ni, Pd, Au, Ag, for nc body-centered cubic metals Ta, Fe, Nb, Mo, and for hexagonal close-packed nc metals Co, Zr, Mg, Ti.     

Coulomb energy of cubic lattices

The Coulomb energies of simple cubic, face-centered cubic and body-centered cubic point lattices with uniform neutralizing background are given for crystals with up to 16 atoms per unit cell.     

体心立方金属W薄膜晶体取向的膜厚尺寸效应及其表面映射

体心立方W膜(110)织构系数T₁₁₀的变化存在非单调的厚度尺寸效应，这依赖于薄膜中晶粒形核和长大时表面能和应变能的相互作用，薄膜表面结构演变反映了两者的竞争过程.应用小波变换结合分形几何描述薄膜表面结构各向异性行为，用此法构建了薄膜织构系数T₁₁₀与表面结构各向异性的关系，表明薄膜晶体取向存在表面映射. 关键词： 金属薄膜 晶体取向 膜厚 表面形貌     

Thermo-physical properties of body-centered cubic iron-magnesium alloys under extreme conditions

Using density functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the thermo-physical properties of body-centered cubic (bcc) iron-magnesium alloys, containing 5 and 10 atomic % Mg, under extreme conditions, at high pressure and high temperature. The temperature effect is taken into account via the Fermi-Dirac distribution of the electrons. We find that at high pressures pure bcc iron is dynamically unstable at any temperature, having a negative tetragonal shear modulus (C^′). Magnesium alloying significantly increases C^′ of Fe, and bcc Fe-Mg alloys become dynamically stable at high temperature. The electronic structure origin of the stabilization effect of Mg is discussed in detail. We show that the thermo-physical properties of a bcc Fe-Mg alloy with 5% Mg agree well with those of the Earth’s inner core as provided by seismic observations.     

Dislocation nucleation from symmetric tilt grain boundaries in body-centered cubic vanadium

We perform molecular dynamics (MD) simulations with two interatomic potentials to study dislocation nucleation from six symmetric tilt grain boundaries (GB) using bicrystal models in body-centered cubic vanadium. The influences of the misorientation angle are explored in the context of activated slip systems, critical resolved shear stress (CRSS), and GB energy. It is found that for four GBs, the activated slip systems are not those with the highest Schmid factor, i.e., the Schmid law breaks down. For all misorientation angles, the bicrystal is associated with a lower CRSS than their single crystalline counterparts. Moreover, the GB energy decreases in compressive loading at the yield point with respect to the undeformed configuration, in contrast to tensile loading.     

Inhomogeneous atomic Bose-Fermi mixtures in cubic lattices

We determine the ground state properties of inhomogeneous mixtures of bosons and fermions in cubic lattices and parabolic confining potentials. For finite hopping we determine the domain boundaries between Mott-insulator plateaux and hopping-dominated regions for lattices of arbitrary dimension within mean-field and perturbation theory. The results are compared with a new numerical method that is based on a Gutzwiller variational approach for the bosons and an exact treatment for the fermions. The findings can be applied as a guideline for future experiments with trapped atomic Bose-Fermi mixtures in optical lattices.     

Green's functions for body-centred cubic lattices

The presented paper contains the tables of Green's functions for bcc lattices for outband frequencies 1·0/ _m 1·6. The central-force model is used, the interaction with 8 nearest and 6 next-nearest neighbours is considered and the number of different Green's functions is fairly decreased by symmetry. Numerical difficulties arising by computing Green's functions are discussed. The derivation of symmetry relations for a dynamical matrix is generalized for the matrix of Green's functions.     

Self-correlation functions for impurity diffusion in cubic lattices

Hyperfine Interactions - Radial parts of the spatial self-correlation functions for a long-range diffusion of an isolated impurity via the monovacancy mechanism have been calculated for the...     

Molecular dynamics investigation of the structural stability of body-centered cubic zirconium nanofilms

The influence of the film thickness and temperature on the phase stability of body-centered cubic (BCC) zirconium in infinite films with different crystallographic orientations has been investigated using the molecular dynamics method with a many-body interatomic interaction potential obtained within the embedded atom model. The calculations have been performed for BCC zirconium films with thicknesses ranging from 2 to 13 nm and with low Miller indices (001), (110), and (111). It has been shown that the BCC(001) zirconium nanofilms with thicknesses up to 6.1 nm, which are formed in the temperature range from 500 to 1300 K, undergo a reorientational phase transition through an intermediate metastable face-centered cubic (FCC) phase with the subsequent transformation into the hexagonal close-packed (HCP) structure (BCC(001)-FCC-BCC??(110)-HCP). When the temperature of initialization of the films is 500 K and below, the BCC-FCC transformation is observed and the FCC phase remains stable. The (110) films are characterized by a strong dependence of the temperature of the BCC-HCP phase transition on the film thickness up to values of 5.8 nm. In the (111) films, the amorphization of the initial BCC phase with the subsequent formation of the BCC phase with a twin structure is observed.     

Dispersion relation for two-dimensional simple cubic lattices

The linear wave equation for the simple cubic lattice is given in this paper. The dispersion relations of both longitudinal and transverse waves are given analytically for the acoustic mode and the optical mode, respectively.      

Localized vibrations of point defects in body-centred cubic lattices

The paper gives an exact calculation of the localized frequencies of substitutional defects in a body-centred cubic lattice by the method of Green's functions and compares it with the approximate calculation carried out after [14]. The exact calculation is based on newly computed Green's functions of a b.c.c. lattice [18]. It is shown how by means of group theory the symmetry of the system can be used both in an approximate and in the exact calculation. Some symmetry relations between Green's functions, which limit the number of functions necessary for numerical calculations, are derived.     

体心立方晶格的布里渊区形状不是正十二面体

介绍了可能的五种正多面体,指出正十二面体是晶体结构及其布里渊区不可能具有的正多面体形状.体心立方晶格的布里渊区是菱形十二面体,不是正十二面体.     

Dispersion relation for two-dimensional simple cubic lattices

The linear wave equation for the simple cubic lattice is given in this paper. The dispersion relations of both longitudinal and transverse waves are given analytically for the acoustic mode and the optical mode, respectively.