Dynamic interaction of edge dislocations with point defects and prismatic dislocation loops at high-strain-rate deformation of crystals

The motion of an ensemble of edge dislocations at high-strain-rate deformation of a crystal with a high concentration of prismatic dislocation loops and point defects has been analyzed. It has been shown that, under certain conditions, the drag of an edge dislocation by prismatic dislocation loops has the character of dry friction, and the magnitude of the drag force of the dislocation is determined by the relationship between the concentration of prismatic dislocation loops and the density of mobile dislocations. An increase in the density of mobile dislocations leads to an enhancement of their collective interaction, thus facilitating the overcoming of prismatic dislocation loops by edge dislocations. The total drag force of an edge dislocation is a nonmonotonic function of the concentration of point defects, which, under certain conditions, has a minimum.     

In situ investigation of the effect of a magnetic field on the mobility of dislocations in deformed KCl:Ca single crystals

This paper presents the results of an experimental investigation into the mobility of edge dislocations in KCl:Ca single crystals and the effect of a static magnetic field of 0.3 T on the dislocation mobility. The experiments on the effect of a magnetic field on the dislocation mobility were carried out with the use of a high-resolution (1 ms) method that permits in situ measurements of the sample dipole moment induced by the motion of charged dislocations as the crystal is being deformed. It is found that the starting stress is reduced in a magnetic field and the activation volume for overcoming of point defects by the dislocations is increased. It is further found that the magnetic field increases the rate of motion of the dislocations at the initial stage of deformation (to the point of dislocation multiplication) but has no effect on the mobility in the multiplication stage. Fiz. Tverd. Tela (St. Petersburg) 39, 630–633 (April 1997)     

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.     

On the isotropy of continuized dislocated crystals. I. The isotropic lattice distortion

The geometrical theory of continuous distributions of dislocations traditionally neglects the dependence of a distribution of dislocations on the existence of point defects created by this distribution (e.g., due to intersections of dislocation lines). In this paper the influence of such point defects on metric properties of the continuized dislocated Bravais crystalline structure is assumed to be isotropic. The influence of the point defects on the distribution of dislocations is then modeled by treating dislocations as those located in a conformally flat space. This approach leads (among others) to new results concerning the geometry of glide surfaces.     

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.     

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.     

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.     

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.     

Dynamics of kinks on dislocations in SiGe single crystals

The method of intermittent pulse loading is used for obtaining the dependences of the mean free path of individual dislocations in SiGe single crystals with various concentration of Ge (0–5.5 at. %) on the duration of loading pulses and time intervals between them. It is found that these dependences change qualitatively upon an increase in the Ge concentration. It is shown that the motion of dislocations in SiGe crystals under small shear stresses is characterized by a nonlinear drift of kinks and the formation of superkinks. A theory of the motion of dislocations under the action of intermittent pulse loading under the conditions of heterogeneous kink dynamics is developed. Extended quasi-one-dimensional defects repeating the shape of a part of a segment of a moving dislocation are discovered in SiGe crystals containing 0.96 at. % Ge. The mechanism of formation of such defects as the result of the shedding of a part of the impurity atmosphere by a dislocation segment during overcoming of a local obstacle is proposed.     

Point Defect Production Under High Internal Stress Without Dislocations in Ni and Cu

Kiritani et al. have observed a large number of small vacancy clusters without dislocations at the tip of torn portions of fcc metals such as Au, Ag, Cu and Ni. Small vacancy clusters, rather than dislocation cell structures, have also been observed after high-speed compressive deformation, suggesting the possibility of plastic deformation without dislocations. In this paper, in order to investigate the mechanism of deformation without dislocations, change in formation energy of point defects under high internal stress was estimated by computer simulation. Elastic deformation up to - 20% strain was found to provide a remarkable lowering of formation energy of point defects. For example, when Ni is subjected to elastic strain, the formation energy of an interstitial atom decreases to 40% that without strain and the formation energy of a vacancy decreases to 51% that without strain. The number of point defects formed under thermal equilibrium during deformation was evaluated. The number was judged to be insufficient for explaining the formation of vacancy clusters as observed in experiments.     

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.     

Magnetoplastic effect in silicon: A search for new methods of controlling structure-sensitive properties of elemental semiconductors

The effect of static magnetic fields up to 1 T on the state of impurity point defects and the mobility of surface dislocation segments in doped (0.01–1.00 Ω cm) silicon has been considered. Long-lived (∼100 h) changes in the state of point defects have been revealed from the mobility of dislocations introduced after the magnetic-field treatment. The concentration dependence of the magnetoplastic effect in p-type silicon has been studied. A threshold impurity concentration of 10¹⁵ cm⁻³ has been found, below which the magneto-plastic effect has not been observed. The influence of magnetic-field pretreatment on the expectation times and activation barriers for dislocation depinning from stoppers and the effect of thermal preannealing on the magnetoplasticity in Si have been considered.     

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.     

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.     

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.     

Melting of a dust crystal with defects

The effect of defects on the melting of a two-layer dust Wigner crystal in the electrode layer of a radio-frequency gas discharge is investigated by Langevin molecular dynamics. Two types of defects have been included in the consideration: (a) point defects and dislocations and (b) particles levitating above and below a two-layer crystal (so-called strong defects). It is shown that local melting of a two-layer crystal found experimentally is explained by the occurrence of strong defects located above the crystal.     

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.     

The effect of defects on phonon scattering in YIG

Effect of different form of defects on phonon scattering in YIG have been studied. It has been found that defects in form of continuous lines behave like internal crystallite boundaries while weak sheet of defects scatter phonon like dislocations.     

Amplitude dependence of the internal friction and Young’s modulus defect of polycrystalline indium

The dependences of the internal friction and the Young’s modulus defect of polycrystalline indium on the oscillatory strain amplitude have been studied over a wide range of temperatures (7–320 K) and oscillatory strain amplitudes (10⁻⁷−3.5 × 10⁻⁴) at oscillatory loading frequencies of about 100 kHz. It has been revealed that the amplitude dependences of the internal friction and the Young’s modulus defect include stages associated with the interaction of dislocations with point defects and the interdislocation interaction. The temperature range characterized by the formation of point-defect atmospheres (the Cottrell atmospheres) near dislocations in indium has been determined.     

循环应变波形对铝疲劳过程中超声衰减的影响

研究了高纯多晶铝在三角和正弦两种波形的应变控制下的疲劳过程中的超声衰减.结果表明，在小应变振幅下，影响超声衰减变化地主要是位错与点缺陷间的相互作用，波形的变化能改变点缺陷的分布，因而影响超声衰减的变化；而在大应变振幅下，位错大量增殖，对超声衰减变化起主导作用的是位错与位错间的相互作用，波形对位错增殖和位错间相互作用影响甚小. 关键词：