Electromagnetic emission under uniaxial compression of ice: II. Analysis of the relationship between an electromagnetic signal and the dynamics of charged dislocation pile-ups

The nature of the electrical activity of mesoscopic defects in I_h ice and the relationship of this activity with the kinetics of plastic deformation and fracture is discussed. The theoretical time dependences of the shear caused by active dislocation pile-ups are compared with the shapes of actual electromagnetic signals generated in different stages of the plastic deformation of I_h ice. Good agreement with the model of free expansion of a conservative dislocation pile-up and the model of nucleation and propagation of a slip band is established.     

Electromagnetic emission under uniaxial compression of ice: III. Dynamics and statistics of dislocation avalanches and cracks

Statistical analysis of the plastic deformation steps and fracture of polycrystalline ice has been performed. It is established that an increase in deformation leads to gradual evolution of the statistics of amplitudes of mesoscopic deformation jumps from random (with a Poisson distribution of dislocation avalanche amplitudes) to “critical” (with a power-law distribution), which indicates occurrence of long-range correlations of the dislocation mesodynamics of deformed polycrystalline ice. The state of self-organized criticality at subcritical ice fracture has been revealed from the power-law statistics of the amplitudes of electric pulses and pauses between them, flicker-noise structure, and almost monofractal character of signals.     

Regularities of plastic deformation in indentation on the (111) plane of NaCl single crystals

Dislocation structures arising in indentation on the (111) face of NaCl single crystals are studied by etch-pit technique. The deformation temperature varied in the interval from 77 to 520 °K. The effect of radiation defects introduced by γ-irradiation on the shape of dislocation rosette was investigated. Three types of dislocation rosettes depending on the experimental conditions were observed. A possible explanation of this phenomenon is proposed. It is shown that the polarity of plastic deformation observed under indentation is an universal phenomenon which is revealed in the temperature range from 77 to 470 °K. The phenomenon of dislocation rosette inversion was first observed.     

Elektronenmikroskopische Untersuchung der Verformungsmikrostruktur eines ferritischen Stahles

The relationship between dislocation structure and the plastic deformation of a low carbon steel at room temperature was investigated. The analysis of the results shows that the dislocation structure is a very important parameter for the deformation microstructure. It was found a connexion between the plastic deformation and the density of dislocation. This connexion is described with a dislocation model.     

Connection between electrical and deformation characteristics of PbS crystals

The influence of plastic deformation on temperature dependence of carrier concentration n and mobility μ in PbS crystals has been studied. Comparison of the results obtained with selective etching patterns from original and deformed crystals makes it possible to conclude that during the plastic deformation there takes place a dissolution of precipitates by passing of dislocations through them. As a results, rows of point defects appear along the path of dislocation movement which may be the cause of a change in n and μ due to deformation. A change in the electron scattering mechanism below 110 K has been discovered.     

Electromagnetic emission under uniaxial compression of ice: I. Identification of nonstationary processes of structural relaxation by electromagnetic signals

Discrete electromagnetic emission caused by a jump of plastic deformation and fracture of single-crystal and polycrystalline ice is revealed and investigated. An album of electromagnetic signals is compiled, which makes it possible, on the basis of the electromagnetic-signal shape, to identify and study the kinetics of mesoscopic events of structural relaxation that are related to the dynamics of dislocation pile-ups and cracks.     

Antistructure disorder and its relation to dislocations in III–V semiconductors

Intimate relationship between antistructure defects and dislocations occurs in GaAs which manifests itself as: (i) appearing of spatial correlation between grown-in dislocations and the EL2 defects (component of the latter are As_Ga antisites), (ii) similar suppression of the concentration of EL2 and dislocation density due to the doping with donor impurities, (iii) generation of As_Ga antisites during the plastic deformation of a crystal. Al these effects can be understood in terms of the dislocation-mediated generation of As_Ga antisites via absorption of As_i interstitials at dislocation jogs. Large anion (cation) precipitates appearing at dislocations are pointed out to be important for both antisite and dislocation generation under certain conditions.     

X-ray investigations of the defect structure of aluminium after plastic deformation

With plastic deformation of polycrystalline metals deformation textures are build up. Our experimental results show that the dislocation density increases, even with small plastic deformation. Simultaneously a decrease in the value of coherence length was revealed. The formation of a distinctly marked sub-structure, which is accompanied by an increase of the dislocation density, is essentially completed when plastic strain reaches 30 per cent. Further increase of the degree of deformation is followed by a change of orientation of the subgrains. When plastic strain is 60 per cent the deformation texture is almost completely accomplished.     

Magnetoplastic effects in crystals in the context of spin-dependent chemical kinetics

Possible mechanisms of a number of magnetoplastic effects in magnetically disordered crystals are analyzed in the context of the chemical kinetics of reactions occurring in a system of structural defects upon plastic deformation. Particular attention is given to spin-dependent reactions between real-structure elements containing paramagnetic centers (dislocation cores, impurity centers, electronic excitations, and so on). It is shown that reactions of several types may occur between these elements, and, in some cases, the relative deformation rate can be related to the rate constants of intracrystalline spin-dependent reactions.     

Climb mechanisms and critical temperature in off-stoichiometric V3Si

A dislocation climb model is proposed for both vanadium and silicon rich V₃Si single crystals. Just the type of constitutional point defects is needed solely which dominates in the sample under consideration. The climb mechanism involves a conversion of one defect type into another and so may be responsible for the changes in critical temperature of the superconductor as observed after plastic deformation at elevated temperatures.     

High-temperature Localization of Plastic Deformation in Lithium Fluoride Single Crystals

Experimental results on the superlocalization of plastic deformation at high strains and high temperatures in LiF single crystals are described. The physical conditions of transition to a localized plastic flow in the temperature range of 603 K to 1073 K T (0.53–0.94T_mp) using constant strain rate compression tests under strain rates from 10⁻³s⁻¹ to 10⁻²s⁻¹ are found. The results indicate that the deformation mechanism involves dislocation climb, controlled by diffusion. The connection of high temperature flow instability with an excessive concentration of point defects (strain vacancies) in zone of shear has been confirmed.     

Effect of Impurity Concentration and Plastic Deformation on Dislocation Density of KCl Crystals

Using the etch pit technique both the distribution and density of dislocations in undeformed and dynamically deformed nominally pure and Sr²⁺ or Pb²⁺ doped KCl crystals are examined. Additionally, the role of specimen geometry in the plastic deformation within the I deformation stage is determined. For all the crystals examined the relationship between screw dislocation density and plastic flow stress obeys the formula τ ⋍ ϱ_sc.^1/2.     

A Molecular Dynamics Simulation of an ODIC Phase

Abstract

This paper is the second of a series devoted to a molecular dynamics study of a two dimensional ODIC crystal which can be viewed as the schematization of the real 3d plastic NaCN. In this paper we are mainly concerned with some static and dynamical properties of the model namely the translational lattice phonons, the correlations between neighbouring molecules orientations, the collective behaviour of the reorientational processes and the influence of the rotation-translation coupling effect. This last effect being strongly dependent on the characteristic times of the rotational dynamics compared to the inverse frequencies of the translational lattice modes, we have rund two different molecular dynamics experiments with two different values of the moment of inertia of the dumbbells which in our model figure the CN^? ions. The relevant collective correlation functions have then been computed and the outcoming informations are analysed and compared with the results of the presently available theories of Y. Y amada and of K. H. Michel which both assume a linear orientation-translation coupling.     

Formation of Dislocations and Twins As a Result of Uniaxial Compression of Magnesium Single Crystals: Molecular Dynamics Simulation

An atomistic simulation of the deformation of ideal magnesium crystal along the \([11\bar 20]\) crystallographic axis has been performed. The evolution of structural defects under load at T = 300–350 K is considered in detail. It is established that the nucleation of dislocations in an ideal crystal occurs when the stress reaches a level of 0.1G (G is the shear modulus). The acting deformation modes are found to be prismatic slip of a dislocations and \(\{ 10\bar 13\} \) twinning. The formation of dislocation networks and dislocation sites in the twinning plane is observed. Some reactions are proposed to describe the dislocation evolution in the \((\bar 3034)\) plane.     

Plastic deformation and fracture in LiF Bicrystals with asymmetric tilt boundary

The peculiarities of plastic deformation and fracture in isoaxial LiF bicrystals with misorientation angle α = 10° and tilt boundary of various asymmetry were experimentally studied by the single slip method. It was found that a number of effects at the boundary, attacked by mixed dislocations, is possible: retardation of dislocations near the boundary and pile-ups formation, dislocation multiplication, accumulation of grain boundary dislocations with differential Burgers vector. Predominance of each or that effect in the process of crack initiation at the definite misorientation angle depends on the mutual orientation of slip planes in the neighbouring grains and on the edge and screw component part in the dislocations, attacking the boundary. The main role, apparently, belongs to dislocation pile-ups. Cracks are always nucleated in the boundary, but in the case of strong asymmetry they stretch along cubic planes.     

Shear bands evolution in bulk metallic glass with extended plasticity

Shear band development and evolution of their spacing under bending in a Zr-based bulk metallic glass with extended plasticity has been monitored as a function of bending angle from the onset of plastic deformation to fracture. We find that sliding of existing shear bands is an important mechanism accounting for the plastic deformation of the plastic bulk metallic glass. The results are beneficial to understanding deformation in metallic glasses.     

Influence of chemical composition on dislocation structure and its change by plastic deformation of V3Si single crystals

The dislocation structure and its change by plastic deformation of V₃Si single crystals has been studied by an etch technique. The solution hardening effect established elsewhere is interpreted in terms of the observed dependence of the dislocation density ϱ(r) and its increment with the plastic deformation on the chemical composition within the range of homogeneity.     

Deformation Modes and Structure Evolution in Laser‐Shock‐Loaded Molybdenum Single Crystals of High Purity

This paper reports on the microstructural changes occurring within molybdenum single crystals after shock treatment with an excimer laser‐system in a confined ablation mode with different number of impacts. Using different complementary investigation methods (optical microscopy, scanning electron microscopy and transmission electron microscopy) it is found that slip and twinning are active modes of deformation during the shock‐induced plastic deformation. After laser treatment with a single laser pulse slip bands on {112} planes containing several microscopic twins are the dominating microstructural feature, whereas further laser‐shock‐processing leads to the formation of a homogeneous arrangement of screw dislocations, tangles and loops. Deformation modes and microstructure in laser shocked samples prove to be quite similar to those of explosive shocked specimens although the laser‐induced peak pressure is about an order of magnitude lower than that during explosive loading.It is shown that the laser shock‐induced hardening increases with increasing number of impacts in the range from 1 to 6 impacts. This shock‐induced strengthening is correlated with the overall dislocation density.     

Zur Bestimmung der Konzentration von Leerstellen und Versetzungen plastisch deformierter Metalle mittels Positronenannihilation

The use of positron annihilation to estimate the concentration of crystal defects is described and a model for the estimation of defect concentrations is given in case that more than one kind of defects are present. This model is based on the recovery of one kind of defects to another in steps. From angular correlation measurements of cold-worked and recovered Pd have been concluded that it is possible to distinguish between vacancies and dislocations in the experiment. Using a value of μ = 2 · 10¹⁵ s⁻¹ for the trapping rate per unit defect concentration, the data for 7.5% thickness reduction yields values in the expected order of magnitude for dislocation density and vacancy concentration. The study of the change of the angular correlation shape by plastic deformation shows that the narrowing of the angular correlation is caused by the reduction of both, indensity and width of the gaussian part which originates in annihilation with core electrons.     

Dynamic Aspects of Dislocation Behaviour Near Low-Angle Boundary in Molybdenum Single Crystals

The dislocation behaviour was inventigated in specimens of monocrystals of molybdenium during in — situ stretching in the direction [001] near low — angle twist boundary which was close to be parallel to the direction of the external force. Low-angle boundary is easily penetrable for the fast moving nonscrew dislocations and acts as an effective stopper for screw dislocations which are quite parallel to the forming boundary dislocations. The mixed tipe dislocations emission by the low-angle boundary was noticed. On the stage when plastic deformation is performed mainly by the motion of screw dislocations the “relay-race”-like transmission of the dislocations motion through the boundary was observed.