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.     

Energy of a wedge-shaped nanotwin calculated in terms of a dislocation mesoscopic model

The total energy of a wedge-shaped micro- and nanotwin is calculated in terms of a dislocation mesoscopic model. The total energy of the twin is represented as a sum of the elastic energy, energy of interaction between twinning dislocations, and stacking-fault energy of partial dislocations of the wedge-shaped twin. It is found that the evolution of the twin is controlled by the energy of interaction between twinning dislocations: in the case of a microtwin, it is five orders of magnitude higher than the elastic energy and six orders of magnitude higher than the stacking-fault energy. In the case of a nanotwin with the number of twinning dislocations at the twin boundary less than 20, all the three energies listed above are of the same order of magnitude. Therefore, all the components of the total energy contribute to the origination of a wedge-shaped twin. As the length of the twin increases with its width and the number of twinning dislocations at twin boundaries fixed, the total energy modulo grows although the density of twinning dislocations at twin boundaries decreases. This indicates that long-range stress fields due to twinning dislocations play an important part in the evolution of a wedge-shaped twin.     

Direct experimental measurement of the speed-stress relation for dislocations in a plasma crystal

The speed-stress relation for gliding edge dislocations was experimentally measured for the first time. The experimental system used, a two-dimensional plasma crystal, allowed observation of individual dislocations at the "atomistic" level and in real time. At low applied stress dislocations moved subsonically, at higher stress their speed abruptly increased to 1.9 times the speed of shear waves, then slowly grew with stress. There is evidence that immediately after nucleation dislocations can move faster than pressure waves.     

Effect of a magnetic field on the starting stress and mobility of individual dislocations in silicon

The strong effect of a magnetic field on the starting stress and mobility of individual dislocations is discovered in silicon grown by the Czochralski method with a high concentration of dissolved oxygen. It is shown that exposure of dislocations preliminarily introduced into the sample to a magnetic field considerably reduces the starting stresses for the motion of these dislocations. The effect is not observed in samples with a low oxygen concentration. It is assumed that the magnetic field induces singlet-triplet transitions in thermally excited states of silicon-oxygen complexes in the dislocation core, thus stimulating a change in the state (atomic configuration) of oxygen already located at dislocations. As a result, the mean binding energy of oxygen with a dislocation decreases.     

Anisotropy of the oscillation dynamics of a breather on a trap in the electroconvective twist structure of a nematic

The oscillation dynamics of dislocations with a dissociated nucleus in the electroconvective twist structure of a nematic liquid crystal has been studied. The initial state of the roll twist structure is unstable. One of the origins of this is the helical flows of the nematic in the neighboring rolls with the antiparallel axial velocity components. As a result, oscillating or “breathing” defects are formed, which have the properties of breathers and are described by the sine-Gordon equation. It has been demonstrated that the space-time anisotropy of the character of motion of the dislocations in opposite directions orthogonal to the roll structure takes place in such a nonstationary system.     

Transmission electron microscopy study of the interaction between a glide dislocation and a dislocation node

Using in-situ tensile straining in conjunction with stereo imaging in a transmission electron microscope, real-time observations have been made in thin copper foils of the interaction of glide dislocations with a dislocation node. A mechanism is observed by which a dislocation approaching the node effectively bypasses the node by exchanging segments with one of the dislocations constituting the node.     

Effect of a high hydrostatic pressure on the dynamic instability of dislocation motion

The effect of a high hydrostatic pressure on the dislocation dipole vibration frequency and the forces of dynamic drag of dislocations by dislocation dipoles and of dislocation pairs by pinned dislocations is studied. Analytical expressions are obtained for the force of dynamic drag of mobile dislocation pairs by pinned dislocations and for the force of drag of isolated dislocations by dislocation dipoles in hydrostatically compressed crystals. Hydrostatic compression leads to a significant increase in these forces. This effect is most pronounced in alkali-halide crystals, where the drag force increases by a factor of 1.5–2.0.     

Dislocation configurations around a nanoindentation in the surface of a fcc metal

We report a scanning tunnelling microscopy investigation of the emission of dislocations around nanoindentations in the form of dislocation arrangements previously called hillocks , consisting of two pairs of Shockley partial dislocations, each encompassing a stacking fault. The spatial arrangement and size distribution of hillocks around the nanoindentation traces are studied. We show that standard dislocation theory for an isotropic continuum can be used to describe the stability of the hillocks, their size and spatial distribution and the broadening of the corresponding extended dislocations near the surface. A model is proposed in which hillocks originate from the split into dislocations partials of primary perfect dislocation loops punched into the crystal by the scanning tunnelling microscope tip. This model implies the operation of a novel dislocation mechanism involving long-range transport of matter across the surface.     

有倾斜透镜存在时两个刃型位错的相互作用

对存在倾斜透镜时两个刃型位错的相互作用进行了研究.研究表明, 两个离轴刃型位错在一定条件下由于相互作用会消失, 并有一个或两个非正则光涡旋产生, 一个共轴刃型位错和一个离轴刃型位错相互作用时产生一个非正则光涡旋. 当初始场中两个刃型位错相互垂直或者平行时, 出射场中会有一个或者两个刃型位错出现. 改变透镜的倾斜因子不影响出射场中位相奇点的类型和数量, 但位相奇点的横向位置与倾斜因子有线性关系. 两个刃型位错相互作用产生的光涡旋对的三维轨迹是非线性的, 但光涡旋对的中心沿直线传输. 关键词： 位相奇点 刃型位错 非正则光涡旋 倾斜透镜     

Generation and accumulation of dislocations on the silicon surface under the action of pulse-periodic emission from a YAG: Nd laser

Solid-phase damage of the silicon surface due to the generation and accumulation of dislocations is studied. The dislocations are generated under the pulse-periodic action of a YAG: Nd laser. The number of laser pulses that causes surface damage vs. power density and pulse repetition period is derived. A mechanism responsible for the generation and accumulation of the dislocations at the surface is suggested.     

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.     

Interaction of dislocation pile-up with a low-angle tilt boundary: a discrete dislocation dynamics study

Abstract

The pile-up of dislocations between two low-angle tilt boundaries (LATB) in an fcc crystal was simulated using three-dimensional discrete dislocation dynamics. The LATB was constructed using glissile edge dislocations stacked on each other. The dislocations in the pile-up were chosen such that their reactions with the dislocations in the LATB resulted in glissile junctions. Parallel pairs of dislocations were inserted to a maximum allowable value estimated from theoretical expressions. A resolved shear stress was applied and increased in steps so as to move the dislocations in the pile-up towards the boundaries. The shear stress required to break the lead dislocation from the wall was determined for varying spacings between the two boundaries. The shear stress and boundary spacing followed the Hall–Petch type relation. Dislocation pile-ups without a LATB were also simulated. The spacing of the dislocations in the pile-up with LATB was found to be closer (ie higher dislocation density) than that without LATB. It was shown through analytical expressions that LATB exerts an attractive force on the dislocations in the pile-up thereby creating a denser pile-up.     

Simulation of the formation of a glide band

The initial stage in the development of a glide band in NaCl crystals is simulated. The process of double cross slip of segments of screw dislocations was examined according to the scheme proposed by Wiedersich. A quantitative estimate of the parameters characterizing the development of the glide band is made: structures and rate of expansion of the band, average slip distance of the dislocations, dislocation density, and others. The degree to which the different parameters affect the development of the glide band is established: ratio of the edge and screw dislocation velocities, as well as the lattice friction stress of dislocations.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 9, pp. 82–96, September, 1981.     

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.     

Section images of dislocations normal to the surface of a 6H-SiC single crystal

Section topographs of edge and screw dislocations with an axis along [0001] in 6H-SiC are taken and interpreted, and the image formation is explained for this case. The contrast induced by various arrangements of dislocations within the Borrmann triangle is experimentally studied. The sign of the Burgers vector of an edge or screw dislocation normal to the crystal surface is shown to be unambiguously determined from the section-topograph image of this dislocation. The sign of the Burgers vector of a screw dislocation can also be determined from its image taken with Lang projection topography. The contribution of a long-range strain field to the section images of edge and screw dislocations normal to the crystal surface is revealed. The experimental contrasts recorded using section topography and Borrmann-effect-based topography are compared.     

Asymptotic expressions for the nearest and furthest dislocations in a pile-up against a grain boundary

In 1965, Armstrong and Head explored the problem of a pile-up of screw dislocations against a grain boundary. They used numerical methods to determine the positions of the dislocations in the pile-up and they were able to fit approximate formulae for the locations of the first and last dislocations. These formulae were used to gain insights into the Hall–Petch relationship. More recently, Voskoboinikov et al. used asymptotic techniques to study the equivalent problem of a pile-up of a large number of screw dislocations against a bimetallic interface. In this paper, we extend the work of Voskoboinikov et al. to construct systematic asymptotic expressions for the formulae proposed by Armstrong and Head. The further extension of these techniques to more general pile-ups is also outlined. As a result of this work, we show that a pile-up against a grain boundary can become equivalent to a pile-up against a locked dislocation in the case where the mismatch across the boundary is small.     

Towards a further understanding of dislocation pileups in the presence of stress gradients

This study is an essential complement and extension to the stress-gradient concept recently proposed by Hirth. An analytic method is presented for studying the behaviour of double-ended dislocation pileup in the presence of various stress gradients by solving a singular integral equation based on the continuous approximation of dislocations. Four special cases of double-ended pileup in the presence of stress gradients are discussed in detail. The corresponding dislocation distribution, the length of pileup, the total number of dislocations within the pileup and the force on the leading dislocations at the pileup ends are derived, respectively. It is shown that both the number of dislocations and the force on the leading dislocation in a pileup are sensitive to the relative magnitude of stress near the dislocation source and both are less than that in constant stress case. Of particular importance, it is indicated that the small-scaled materials subjected to a stress involving a gradient would be stronger than that under a constant stress. Applied to wire torsion and foil bending, the stress gradient model predicts an increase in the initial yielding, which is in reasonable agreement with the recent experimental data. The proposed stress gradient concept may provide a new physical insight into the size-dependent plasticity phenomena at small length scale.     

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)     

Screening of the elastic field in a dislocation ensemble

The phenomenon of screening of the elastic field of screw dislocations is investigated on the basis of a system of self-consistent field equations for a dislocation ensemble. Expressions are derived for the effective dislocation interaction potential, the screening length, and the average elastic field energy associated with the correlation interaction of dislocations. An expression for the correlation dislocation flux is formulated in the slightly inhomogeneous case. Fiz. Tverd. Tela (St. Petersburg) 39, 1575–1579 (September 1997)     

Edge stacking dislocations in two-dimensional bilayers with a small lattice mismatch

Incomplete stacking dislocations are predicted to form at edges of the shorter upper layer in two-dimensional hexagonal bilayers upon stretching the longer bottom layer. A concept of the edge Burgers vector is introduced to describe such dislocations by analogy with the Burgers vector of standard bulk dislocations. Analytical solutions for the structure and energy of edge stacking dislocations in bilayer graphene are obtained depending on the magnitude of elongation and angles between the edge Burgers vector, direction of elongation and edge. The barrier for penetration of stacking dislocations inside the bilayer is estimated. The possibilities to measure the barrier to relative motion of graphene layers and strain of graphene on a substrate by observation of edge stacking dislocations are discussed.