Method for calculating coefficients of concentrational variations in lattice constants and the distribution of impurity atoms between sublattices in intermetallic compounds

A method is proposed for calculating coefficients of concentrational variations in lattice constants in solid solutions, based on first-principles determination of the total energy of solid solutions and the continual approximation in the solutions theory, allowing for the deformation interaction of impurity atoms due to distortions of the solvent crystal lattice.     

Microscale deformations in a two-dimensional lattice: Structural transitions and bifurcations at critical shear

A two-dimensional model of a crystal lattice with an essentially nonlinear interaction between atoms for arbitrary mutual displacements under shear conditions is considered. The energy of deformation contains a periodic term and gradient terms. The equilibrium equation in the sine-Helmholtz form (with two characteristic coherence lengths) is solved exactly. It is shown that a large uniform shear deformation is unstable and is accompanied by periodic modulations emerging in the plane of the layer and leading to nonuniform extensions and compressions of atomic chains, i.e., to a change in the long-range order. The chain slip by a lattice half-period or a larger value changes the long-range order as well. The obtained relations connecting the unit cell size and the amplitude of the displacement field determine the conditions for the existence of a modulated structure. A bifurcation transition from a purely elastic deformation of the lattice to its elastoplastic deformation, as well as a transition to its disordered (amorphous-type) state, is observed.     

Molecular Dynamic Simulation of High Speed Deformation Effect on Microstructure Change in Thermal Heated Metals

In the present paper computer simulation of high-speed deformation (shock wave propagation) by molecular dynamic method is performed in thin copper sample, having the form of rectangular parallelepiped (10 a ‐ 10 a ‐ 20 a , where a is the lattice constant) with 8000 atoms. On the surfaces Z 0 =0 and Z max =20 a the mirror boundary conditions with rigid walls and the periodic boundary conditions along X and Y directions corresponding to short sides of deformed crystal are used, which allows to investigate the reflection of shock wave from the surfaces in Z direction. The changes of microstructure have been investigated up to 12 ps. The numerical calculations of microstructure changes have been performed here taking into account the effect of thermal heating of crystal lattice before shock wave front. The numerical results show that comparing with the propagation of shock waves under room temperature in thermal heated structure additional displaced atoms (vacancies and interstitials) are produced. The obtained results show that the production of point defects during high-speed deformation is determined by the thermal softening of microstructure and generation rate of point defects very strong increases with an increasing of high speed deformation rate. The peculiarities of microstructure changes in deformed copper are analyzed here at the different initial temperatures and various high-speed deformations (average ion velocities behind shock wave).     

Plane wave topography on crystals with step-like impurity distributions

In this paper a qualitative and quantitative study is presented of the contrast formation in the plane wave topography of quartz plates with step-like impurity atoms distributions which are connected with different lattice parameters. Contrast distributions are calculated by means of a simple local application of the dynamical theory of the X-ray diffraction from the perfect crystal and using a simple mathematical model of the lattice deformation according to the isotropic theory of elasticity. A surprisingly good agreement between calculations and experimental results can be stated.The authors thank Dr. V. Holý for helpful discussions.     

辐照损伤对钨晶格热导率影响的分子动力学研究

钨是最具应用前景的面向等离子体候选材料,但核聚变堆内强烈的辐照环境会使钨的近表面区域产生辐照损伤,进而影响其关键的导热性能.本文构建了包含辐照损伤相关缺陷的晶体钨模型,并采用非平衡分子动力学的方法定量研究了这些缺陷对钨导热性能的影响.结果表明,随中子辐射能量的增加,晶体内部留下的Frenkel缺陷数目增多进而导致钨的晶格热导率降低;间隙原子比空位更易于向晶界偏聚,且钨中的间隙钨原子与空位相比,使晶格热导率下降程度更大.纳米级氦气泡导致晶格热导率的显著降低,气孔率为2.1%时晶格热导率降至完美晶体的约25%.这些不同的缺陷造成不同程度的周围晶格扭曲,增加了声子散射几率,是导致晶格热导率下降的根源.     

Isotherms of thermal conductivity in PbTe-MnTe solid solutions

The temperature dependences of the thermal conductivity λ of PbTe-MnTe solid solutions (0–4 mol % MnTe) are measured in the range 170–670 K. The data obtained are used in constructing the isotherms of the lattice thermal conductivity λ_l and in estimating the effective cross section for phonon scattering by Mn impurity atoms. It is found that all the isotherms exhibit an anomalous increase in λ_l in the concentration range 1.25–2.0 mol % MnTe, which disagrees with the usually observed decrease in λ_l with an increase in the impurity concentration. It is assumed that the anomalous increase in λ_l manifests itself after attainment of the percolation threshold when a continuous chain of overlapping deformation fields produced by individual atoms (an infinite cluster) is formed in the crystal. In the crystal lattice, stresses are partly compensated and phonon scattering decreases. The assumption is made that the effect observed has a universal character.     

On the calculation of rms atom displacements in a crystal lattice

A new general method is presented for calculating rms displacements and velocities of an arbitrary system of atoms performing harmonic oscillations. For illustrating purposes, the rms displacements of atoms in a particular crystal lattice model are calculated, including foreign atoms in subsurface regions of the crystal.     

Topological study of the chemisorption behavior of carbon monoxide on the (112) surface of Ni---Cu alloys

Chemisorption properties of bimetallic Ni---Cu systems are studied. Ni atoms in a Cu lattice are strongly negatively charged as compared with a pure Ni crystal. The insertion of Cu atoms into a Ni lattice causes the opposite effect. The chemisorption properties are practically not changed by insertion of other metal atoms into the bulk. However, the substitution of metal atoms on the surface leads to a substantial change in chemisorption properties which cannot be explained on the basis of the behavior of pure monometallic species.     

The zero-point energy in the molecular hydrogen crystal

We propose a modified Einstein approximation to describe zero-point energy vibrations in a quantum crystal. Our aim was to develop a computationally cheap tool suitable for lattice structure optimisation. As in the classical Einstein model the representative atom vibrates in an effective potential due to the surrounding atoms of the crystal; the atoms however are not strictly placed at the positions corresponding to the crystal potential energy minima but their positions are described by the quantum mechanical density distributions. The effective potential computed that way is suitable for the application in solid para-hydrogen in contrast to the normal (unmodified) Einstein approximation. We compute the cohesive energy of the para-hydrogen crystal and perform lattice structure optimisation. The hexagonal closed packed is more stable than the fcc closed packed lattice and the lattice constants obtained are in very good agreement with the experimental values.     

Inner-shell vacancy production in collisions of channeled ions with lattice atoms

The theory of inner-shell vacancy production in collisions of channeled ions with lattice atoms of a crystal is developed. Thermal vibrations of crystal atoms are taken into account. The effect of selective vacancy production in inner electron shells of ions is predicted.     

AFM针尖纳米切削晶向和切削方向影响的分子动力学

"建立了AFM针尖切削单晶铜的三维分子动力学模型,研究了工件材料不同晶向和刀具切削方向对切削过程中工件材料变形的影响．采用EAM势计算工件原子之间的作用,采用Morse势计算刀具原子之间的作用．模拟结果表明工件材料晶向和切削方向对纳米切削过程有显著影响．沿[110]方向切削比[100]方向切削产生的切屑结合更紧密,切削工件材料(110)晶向比切削工件材料(100)晶向产生的切屑体积更小,工件材料变形区域更小．研究了工件材料晶向和切削方向组合的不同纳米切削过程中系统势能变化情况．"     

Substructural phase transitions during intense plastic deformation of low-carbon ferrite-perlite steel

We have studied the evolution of the defect structure and phase composition of low-carbon ferrite-perlite steel subjected to intense plastic deformation using diffraction electron microscopy. It has been shown that a high degree of deformation is accompanied by disruption of the perlite columns. We have found and described two perlite decay mechanisms: decay of the carbide plates by a path of their granulation due to dislocation slip and dissolution of cementite arising from the outflow of carbon atoms from the carbide phase into ferrite crystal lattice defects. We have described the phenomenon of morphological reconstruction of the cementite-phase particles (a transition from layers to spheres) under plastic deformation conditions. Tomsk State Architectural and Construction University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 63–71, March, 1998.     

The mobility of a dislocation in the Frenkel-Kontorova model

The stability of equilibrium configurations of atoms, which represent a dislocation, is studied on a one-dimensional static model of a crystal. The potentialW, which represents the influence of the surrounding crystal lattice on the atoms of the model, is composed of parabolic sections. An expression is derived for Peierls stress and cases are found when this stress is equal to zero.     

On crystal shear,lattice rotation and constraint stress in (1 1 0) channel die compression: rate-independent and viscoplastic analyses and predictions compared

Rate-independent crystal plasticity theory and a classic viscoplastic power-law are investigated, contrasted and compared for finite deformation analysis of fcc crystals in channel die compression, including full consideration of lattice straining. Both experiment-based anisotropic and isotropic (Taylor) hardenings are evaluated in rate-independent theory; and an unlimited range of power-law exponent n is considered in viscoplasticity. The focus is on predictions of lateral constraint stress, lattice rotation and crystal shear, and their comparison with experiment. General elastic-plastic equations (for both theories) are given for the range of unstable lattice orientations in (1?1?0) compression (‘range I’) and evaluated before and after a finite rotation of the lattice about the load axis. Equations also are given and evaluated for the ‘Brass’ orientation. It is shown that the theories can be in close agreement at the onset of finite deformation in range I, but that viscoplasticity gives results (for any n) after finite rotation that are in sharp contrast to rate-independent theory. The latter’s predictions for crystal shear and lattice rotation are in good to very good agreement with finite deformation experiments on aluminium and copper. The inclusion of lattice elasticity is found to have a negligible effect in range I. In contrast, for finite deformation in the stable Brass orientation, elastic-viscoplastic theory can be made to agree very closely with rate-independent theory and with experiment.     

Nonlinear picosecond acoustics of low-temperature paramagnetic crystals

The interaction of transverse picosecond deformation pulses with a system of S=1/2 spins, forming a paramagnetic crystal, is investigated theoretically. The influence of acoustic dispersion, owing to the lattice structure, and the anharmonicity of atomic vibrations on the formation of stationary soliton-like video pulses, interacting with the spins of paramagnetic atoms, is studied for the example of a cubic crystal. The phenomenon of total reflection of deformation pulses from the paramagnetic crystal is predicted. The amplification and self-compression of nonresonant pulses in nonequilibrium paramagnetics is studied, and it is shown that generation of higher-order (up to tenth-order) harmonics of the frequency of the initial signal is possible. It is concluded that for appropriate crystal parameters, a limiting solition with a duration of one period of the oscillations, corresponding to spatial size of the order of the lattice constant of the crystal, is possible at the final stage of amplification.V. V. Kuibyshev Tomsk State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 94–113, July, 1993.     

Correlation of temperature anomalies of the hyperfine field and thermal expansion of the crystal lattice

Consideration is given to the effect of thermal expansion of crystal lattice upon the partial contributions to the magnetic hyperfine field for non-magnetic atoms in metallic ferromagnets. The existence of correlation between the temperature anomalies of a hyperfine field and the thermal expansion of the crystal lattice is shown in a simple and illustrative way.     

BSO晶体的高温拉曼光谱与高温结构特征

阐述了BSO晶体在常温下的结构特征,通过测量晶体在常温下的拉曼光谱,对晶格主要振动模式进行了指认,测量晶体升温至熔化过程中不同温度下的拉曼光谱,分析研究了其微观结构在升温过程中的变化以及在高温状态下的结构特征。     

Theory of a surface phase transformation induced by impurity atoms in binary alloys containing interstitial impurity atoms

Mutual influence of the surface segregation of atoms on lattice sites and in interstitials is studied in the self-consistent field approximation. The effect of occurrence of a local surface phase transformation is predicted, which is caused by a local loss of stability of the statistical distribution of atoms of components on the surface due to the presence of interstitial impurity atoms. A theory is constructed for an induced phase transformation for [001] plane of a single crystal of a binary alloy with an fcc lattice whose octahedral interstitials are occupied by interstitial atoms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 50–55, September, 1984.     

STUDY OF DOPING EFFECT ON THE Bi2Sr2Ca2Cu3Oy SUPERCONDUCTOR

We have systematically investigated the doping effect on the Bi₂Sr₂Ca₂Cu₃O_y superconductive material. After entering into the crystal lattice, Pb and/or Sb atoms cause not only variations of the temperature condition during the sample preparation process, but also variations of structures and superconductivity. By analyzing the experimental results, we have come to the conclusion that the Pb and/or Sb atoms enter into the Bi-O planes, preferably occupy the sites of Bi ions, regulate the crystal structures and the distribution of oxygen ions in Bi-O planes and nearby lattice sites, thus benefiting the formation of the high-T_c phase and improving the superconductivity.