Kinematics of edge dislocations. I. Involutive distributions of local slip planes

The geometry of continuous distributions of dislocations and secondary point defects created by these distributions is considered. Particularly, the dependence of a distribution of dislocations on the existence of secondary point defects is modeled by treating dislocations as those located in a time-dependent Riemannian material space describing, in a continuous limit, the influence of these point defects on metric properties of a crystal structure. The notions of local glide systems and involutive distributions of local slip planes are introduced in order to describe, in terms of differential geometry, some aspects of the kinematics of the motion of edge dislocations. The analysis leads, among others, to the definition of a class of distributions of dislocations with a distinguished involutive distribution of local slip planes and such that a formula of mesoscale character describing the influence of edge dislocations on the mean curvature of glide surfaces is valid.     

Dislocations and internal length measurement in continuized crystals. I. Riemannian material space

Distributions of dislocations creating point defects are considered. These point defects are described by a metric tensor, which supplements a Burgers field responsible for dislocations. The metric tensor depends on the distribution of dislocations and defines a Riemannian geometry of the material space of a continuized crystal and thus an internal length measurement in this crystal. The dependence of the distribution of dislocations on the existence of point defects created by these dislocations is modeled by treating the Burgers field as a field defined on the Riemannian material space. Field equations, following from geometric identities, are formulated as balance equations on this Riemannian space and their source terms, responsible for interactions of dislocations and point defects, are identified.     

Geometry of crystal structure with defects. II. Non-Euclidean picture

The following point of view is geometrically formulated and its consequences examined: the lattice of a crystalline body with a continuous distribution of dislocations can be locally described as an ideal lattice in non-Euclidean space. The types of distribution of dislocations are described by the classification of three-dimensional real Lie algebras. The influence of point defects and the elastic deformation field on the geometry of the material structure of a crystalline body with dislocations is examined. The case where a crystal with dislocations reacts as a body with internal rotational degrees of freedom is discussed.     

Dislocation dynamics in doped NaCl single crystals determined by pulsed NMR between RT and 300°C

The mean free path of mobile dislocations is measured by determination of the spin-lattice relaxation rate of deforming NaCl single crystals as a function of temperature and of the concentration of Ca⁺⁺ impurities. The latter may influence the magnitude of the mean free path but this depends to a large extent on the point defect configuration. The degree of association and the mobility of the point defects is studied by measuring the spin-lattice relaxation rate without deforming the samples. On the other hand the distribution of dislocations varies also with temperature and this affects the mobility of dislocations too. The work-hardening rate of the crystals is compared with the mean free path as a function of temperature and it is shown that both quantities have extremes under the influence of competitive mechanisms such as an enhanced thermal activation of dislocations at obstacles, an increasing mobility of point defects, and increasing number of intersections of mobile dislocations with dislocation dipoles.     

Modeling dislocation — segment length distributions

The influence of intermolecular interaction between point defects, which are moving pinning points for dislocations, on the length distribution of dislocation segments is discussed. The distribution function is analyzed by Monte Carlo numerical modeling. It is shown that a length distribution can be described in the absence of external stresses by an exponential function that may differ from the Köhler function at high defect concentrations. Intermolecular interaction of defects changes the distribution function, which, with time, also becomes nearly exponential, but with different values of the parameters.     

Influence of random electric fields on luminescence yield and kinetics of insulators

In the present work we investigate theoretically the influence of random electric fields on electron-hole recombination in wide bandgap crystals. Effective Onsager radius and, therefore, electron-hole recombination rate are significantly modified by external electric fields. Electric field distribution functions for point defects and charged dislocations are evaluated analytically. Electron-hole recombination rate decreases with concentration of point defects and dislocations. In simple case of random fields created by charge carriers in highly excited regions the recombination rate is proportional to n 2/3 rather than n , where n is the concentration of excitations. Therefore modification of luminescence kinetics is most pronounced at initial stages of relaxation of highly excited regions.     

Study of the influence of vacancies and vacancy complexes on the yield limit of Al subjected to strain aging

Mechanical properties of metals are most sensitive to the presence of point defects. The influence of point defects on the kinetics of plastic deformation is highly diversified: the point defects can be the main carriers of plastic deformation (diffusion creep, crowdion plasticity, etc.), can imitate the velocity of nonconservative motion of dislocations, and can serve as centers of pinning of dislocations. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 18–20, March, 2008.     

GaN薄膜的热导率模型研究

准确预测GaN半导体材料的热导率对GaN基功率电子器件的热设计具有重要意义.本文基于第一性原理计算和经典Debye-Callaway模型,通过分析和完善Debye-Callaway模型中关于声子散射率的子模型,建立了用于预测温度、同位素、点缺陷、位错、薄膜厚度、应力等因素影响的GaN薄膜热导率的理论模型.具体来说,对声...     

Influence of hydrostatic compression on the sliding of a pair of edge dislocations in metals with a high impurity concentration

The dynamic drag of dislocations by point defects in hydrostatically compressed metals is theoretically investigated. Account is taken of the influence of a high pressure on dislocation-dislocation and dislocation-point defect interactions. It is shown that the pressure dependence of the force causing dynamic drag of dislocations is determined by competition between these interactions.     

Numerical calculation of the rate of strain of interstitial solid solutions under irradiation. I. Model of radiation creep

A model of radiation creep of interstitial solid solutions is developed on the basis of the combined motion of dislocations, including their glide and climb past obstacles. The obstacles considered are forest dislocations and pileups of radiation-induced point defects. A computational formula for the rate of strain is derived which describes creep at high stresses, when the gliding dislocations overcome some of the barriers by force, and a method is described for determining the average distance traversed by a dislocation in the glide plane under the influence of the stress until it is stopped by barriers. The results are compared with those of other authors. It is shown that the formula obtained for the rate of strain goes over in particular cases to those given by the previously known SIPA, Gittus-Mansur, and glide-climb models of radiation creep. Zh. Tekh. Fiz. 69, 64–71 (January 1999)     

Recombination activity of dislocations on (0 0 0 1) introduced in wurtzite ZnO at elevated temperatures

The activity for non-radiative recombination at dislocations on (0 0 0 1) basal planes was examined in wurtzite ZnO bulk single crystals. In panchromatic cathodoluminescence intensity maps, the dislocations did not exhibit apparent contrast when they were introduced at elevated temperatures of 923–1073 K, while the dislocations introduced at low temperatures (below 623 K) were observed as dark bands. It was suggested that the dislocations formed complexes involving point defects, via the thermal migration of point defects at elevated temperatures, resulting in the suppression of the recombination activity. The complexes did not influence the existing emission lines in pre-dislocated crystals.     

Evolution of the concentration of point defects at the tip of a crack

The evolution of the distribution of interstitial impurity atoms in the plastic zone around the tip of a tension crack is analyzed. The transport of point defects is determined by: 1) the hydrostatic component of the elastic stress at the crack tip, created by the superposition of the elastic fields of the crack and dislocations; 2) the elastic field of moving dislocations (“sweeping out” of interstitial impurity atoms); 3) the dislocation-driven transport of point defects present in the dislocation cores. The contributions of each mechanism of transport of point defects to the crack tip are calculated over the entire time from the start of loading of a sample containing a crack until an equilibrium distribution of plastic deformation is established after the cessation of loading. Numerical calculations are carried out for interstitial hydrogen atoms dissolved in an α-Fe crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 1580–1585 (September 1997)     

On the Geometry of Diffusion Processes in Continuized Dislocated Crystals

Nonequilibrium diffusion processes of point defects in continuized dislocated crystals are considered. A generalized, stochastically motivated gauge procedure to introduce the geometry of a material space describing the influence of dislocations on the free diffusion process, is used. The dependence of diffusing processes and their steady states on the curvature of the material space of edge dislocations, on the scalar density of these dislocations, and on the interaction energy between dislocations and a diffusing atom, is analysed. An equation defining the interaction energy is deduced, using statistical arguments, from the material space geometry.     

On a dynamic effect in the collective motion of dislocations

The interaction between fast moving dislocations and vibrational modes, localized on point defects is investigated. By means of quantum mechanical calculations the existence of a force between the dislocations due to their interaction with the modes is established in the case of the coordinated motion of two or more dislocations. This force might play an important role in the case of edge dislocations, moving faster than the velocity of Rayleigh waves. As known, such dislocations of the same mechanical sign attract one another (in contrast to the static case), leading to fracture. The aforementioned force due to point defects might prevent dislocations from joining and reduce the probability of fracture formation.     

Effect of a weak magnetic field on the state of structural defects and the plasticity of ionic crystals

The aim of this work is to examine the influence of a weak (on the energy scale) magnetic field on the state of dislocations and point defects in ionic crystals. It is found that complex point defects existing in a metastable state are sensitive to a magnetic field B∼1 T. The contributions are identified, and the kinetics of various types of reactions within the structural defects and between them leading to plastification of the crystals in a magnetic field are determined. The effect of light on the sensitivity of the point defects to a magnetic field is described, and the spectral characteristics of this effect are determined. A resonant effect of the combined action of a weak constant magnetic field and a high-frequency magnetic field on the dislocation mobility is found to occur when these fields satisfy the conditions of electron paramagnetic resonance. Zh. éksp. Teor. Fiz. 115, 605–623 (February 1999)     

Theory of magnetic cold-work peak in ferromagnetic materials

The magnetic after effect due to dislocations is investigated for three types of configurations: i) randomly distributed immobile point defects, ii) Cottrell clouds, iii) strongly bound defects in the dislocation core. It is shown that only the case of the mobile Cottrell cloud leads to a simple Debye process, whereas the other cases must be described by a more general relaxation function. The role of point defects in the core of dislocations is studied in some details. In this case the relaxation time is found to be determined mainly by the binding energy of the impurity atoms to the dislocation core.     

Application of the model of an edge dislocation in the form of a uniformly charged filament for description of light diffraction from alkali halide and covalent crystals

The interaction of an optical electromagnetic wave with a system of parallel charged edge dislocations in alkali halide and covalent crystals is studied. The optical activity of dislocations is associated with the existence of shielding clouds of mobile point defects with a radius of the order of the wavelength of incident light. Expressions relating the amplitude of the diffracted wave to the parameters of crystal defects (the density of point defects and the density and linear charge of dislocations) are obtained. Methods for the experimental estimation of defect parameters from the characteristics of the diffraction pattern are proposed.     

The differential geometry of elementary point and line defects in Bravais crystals

Since line defects (dislocations) and point defects (vacancies, self-interstitials, point stacking faults) in Bravais crystals can mutually convert, only theories which comprise these two sorts of defects can be closed in the sense of general field theory. Since the pioneering work of Kondo and of Bilby, Bullough, and Smith it is clear that differential geometry is the appropriate mathematical tool to formulate a field theory of defects in ordered structures. This is done here on the example of the Bravais crystal, where the above-mentioned defects are the only elementary point and line defects. It is shown that point defects can be described by a step-counting procedure which makes it possible to include also point stacking faults as elementary point defects. The results comprise two equations with the appropriate interpretation of the mathematical symbols. The point defects are step-counting defects and are essentially described by a metric tensorg, which supplements the torsion responsible for the dislocations. The proposed theory is meant to form a framework for defect phenomena, in a similar way that Maxwell's theory is a framework for the electromagnetic world.     

Collective overcoming of point defects by dislocations in the dynamic region

A mechanism of collective overcoming of point defects by dislocations during the over-barrier slip has been proposed. It has been shown that the interaction between dislocations promotes the overcoming of point defects at a high dislocation density.