Possible dynamical mechanism of dislocation drag in metals in the easy slip stage

The slip of an edge dislocation through a system of parallel immobile dislocation dipoles oriented parallel to it has been investigated. A new mechanism of dislocation drag (irreversible transformation of the kinetic energy of a moving dislocation into the energy of natural vibrations of a pair of edge dislocations (forming a dipole), excited by the elastic field of the moving dislocation) is proposed and analyzed. The dynamic drag force of moving dislocation caused by this mechanism is calculated. It is shown that this force is inversely proportional to the slip velocity of mobile dislocations.     

Microplasticity of CsI crystals

The study of the mechanisms of plastic deformation of CsI crystals has found the participation of not only the main {110} 〈100〉 slip system but also of the secondary one {110} 〈110〉. Besides that, production and motion of point defects (or small prismatic loops) take place. The gliding on secondary slip system and the deformation accounted for by the generation and motion of point defects is facilitated at low temperatures and high deformation rates. The character of the motion and multiplication of dislocations in the main slip system is investigated. From the temperature and stress dependence of the mobility of isolated dislocations quantitative data on the thermally activated motion of edge dislocations on the main slip system have been obtained. It is shown that, as in the case of other alkali halides, the thermally activated motion of edge dislocations in CsI crystals on {110} 〈100〉 system is limited by their interaction with local obstacles.     

Luminescence during release of pressure in X-ray irradiated alkali halide crystals

Emission of light is found during application as well as release of uniaxial pressure in X-ray irradiated alkali halide crystals. As unpinning of dislocations can not take place, however, dislocations can move in backward directions during release of the pressure; the emission of light during release of pressure gives a direct support that the interaction of moving dislocations with F-centers is responsible for the emission of light during deformation of colored alkali halide crystals.     

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.     

In-situ HVEM Observations of Dislocation Processes during High Temperature Deformation of Silicon Crystals

The characteristics in the plastic deformation of silicon crystals are first reviewed. Such characteristics have been interpreted quantitatively on the basis of some models on the velocity and the multiplication of dislocations during deformation. The results of the in-situ observations of silicon crystals deformed at elevated temperatures in a HVEM are presented. The slowness and the smoothness in the dislocation motion, the dynamic pile-up as a general mode of the collective motion of dislocations, the formation processes of multiplication centers of dislocations observed during the deformation all support the validity of the models adopted. Dislocation dipoles and Lomer-Cottrell sessiles are observed not to act as strong obstacles which play important roles in the work hardening of the crystals.     

In-situ Observations on Dynamic Processes Associated with Stacking Faults and Deformation Twins in Silicon Crystals

Stacking faults and thin deformation twins are observed to develop during the in-situ deformation of silicon crystals at elevated temperatures in a HVEM. Both the nucleation and the motion of partial dislocations take place under the applied stress in the regions of the stress concentration. Twinning dislocations moving on twin boundaries are extremely flexible. The spontaneous nucleation of loops of twinning dislocation on twin boundaries is observed rather frequently. No pole mechanism is observed to be operating. Stacking faults and deformation twins interact with glide dislocations moving along the intersecting planes by various ways.     

In situ observation of formation of misfit dislocations in pseudomorphic monolayer overgrowth of metals and non-metals

Epitaxial overgrowth in cases of deposit and substrate combinations of PbSe/PbS, Au/Pd and Pd/Au were observed in situ in an electron microscope. It was found that the growth proceeds with the very characteristic features of monolayer overgrowth as implied by the theory of Frank and Van der Merwe; no indication of nucleation was observed and the formation of misfit dislocations was noticed over the whole area of films at certain critical thicknesses of the overgrowths. Points were noted which distinguish the monolayer overgrowth from the case where misfit dislocations are introduced in nucleated islands, to which Matthews has counted the growth processes in these combinations. The importance of climb motion as well as slip motion was stressed as the mechanism of introduction of misfit dislocations. Some results of electron diffraction and electron microscopy of films prepared in separate evaporation units were also quoted. These were to examine the relation between misfit dislocation density and mean lattice parameter and the effect of degree of vacuum on the growth mode and to indicate that the same growth mode takes place for some other combinations of IV–VI compounds, SnTe/PbSe, PbTe/PbS and PbTe/PbSe.     

Structural Defects in Type I Diamonds

Natural gem diamonds of Type I single crystals were observed with an ultra high voltage transmission electron microscope in order to characterize structural defects such as dislocation, dislocation loops, and platelets. The results of analyses indicated that edge dislocations with Burgers vector 1/2 [011 1] and 60° dislocations which were inclined at an angle of 60° to 〈011〉 directions were present in the diamonds. Dislocation loops were concluded to be in interstitical character. It was speculated that an impurity associated with the platelets was silicon and dislocation loops were formed by dislocation motion.     

HVEM In-situ Deformation Investigations on Cyclically Deformed Nickel Single Crystal

In-situ deformation investigations were carried out on thin foils of a nickel single crystal which was cyclically pre-deformed. The motion of dislocations does not start in the whole specimen simultaneously but begins at localized places in the dislocation-poor regions. The plastic deformation may be characterized by a collective movement of dislocations. Measurements of the effective stress show its characteristic dependence on the position in the direction of the Burgers vector. There are clear indications of structural changes in the dense regions of dislocations at strains exceeding the cyclic total strain amplitude.     

On the delay of thermally activated motion of pyramidal dislocations in zinc

The effect of shear stress and temperature on the delay time, t_i, of thermally activated motion of dislocations in the {112 2} 〈112 3〉 slip system in Zn single crystals is studied. The delay is associated with a higher density of crystal defects at the initial path of dislocation motion.     

Acoustic study of elastic properties,dislocation damping and plasticity of crystals

The results of acoustic studies of elasticity and dislocation inelasticity of crystals are presented. Regularities of elastic anisotropy variation with ionicity of bonds in A_NB_8-N crystals have been considered. Amplitude dependences of ultrasound damping supply the information on dislocation structure and dislocation — to point defect interaction. Oscillation of dislocations during internal friction measurements are compared to their translation motion at the onset of plastic flow.     

Dynamic interaction of dislocations with impurity subsystem in crystalline materials

The entrainment of impurities by moving dislocations results in the accumulation of impurities in dislocation cores, which eventually significantly modifies the dynamic properties of dislocations. In the framework of the kink mechanism, the possible modes of motion are found self-consistently and the conditions for dislocation immobilization are determined. The dependence of the immobilization stress (the parameter that is most important for ??defect engineering?? in semiconductors) on the material parameters and experimental conditions is calculated.     

The influence of elastic anisotropy on interaction of dislocations with point defects in BCC metals

With the use of the anisotropic theory of elasticity the energies and the force-distance profiles for the interaction of various types of straight dislocations with point defects of intrusion (atoms of N, C or O) and of replacement in BCC metals with a different degree of elastic anisotropy (Nb, V, Ta, α-Fe) have been calculated. The calculated force-distance profiles in most cases are not centrally symmetric, that leads up to a gliding asymmetry at the direct and reverse directions of dislocation motion. The crystal elastic anisotropy can change the relative mobility of dislocations of various types and accounts for a number of experimentally observed peculiarities of BCC metals stress-strain properties.     

Zur Integration der Grundgleichungen für bewegte Eigenspannungsquellen in kubischen Kristallen

Solutions of the equations of dislocation theory are given being based on a mathematical method which decouples the system of differential equations for the displacement vector. The solutions regard elastic anisotropy of crystalline matter not only in statics but also in dynamics. — The method succeeds in the case of plane strain in cubic or hexagonal crystals, allowing most general plastic distorsions as well as processes of arbitrary timedependence. Decoupling is the basis for using complex variable methods which may be applied when dislocations move in a uniform way.     

Growth mechanisms,polytypism, and real structure of kaolinite microcrystals

The mechanisms of growth of kaolinite microcrystals (0.1–5.0 μm in size) at deposits related to the cluvial weathering crust, as well as to the low-temperature and medium-temperature hydrothermal processes of transformations of minerals in different rocks in Russia, Kazakhstan, Ukraine, Czechia, Vietnam, India, Cuba, and Madagascar, are investigated using transmission electron microscopy and vacuum decoration with gold. It is established that kaolinite microcrystals grow according to two mechanisms: the mechanism of periodic formation of two-dimensional nuclei and the mechanism of spiral growth. The spiral growth of kaolinite microcrystals is dominant and occurs on steps of screw dislocations that differ in sign and magnitude of the Burgers vector along the c axis. The layered growth of kaolinite originates from a widespread source in the form of a step between polar (+ and ?) dislocations, i.e., a growth analogue of the Frank-Read dislocation source. The density of growth screw dislocations varies over a wide range and can be as high as ～10⁹ cm^?2. Layered stepped kaolinite growth pyramids for all mechanisms of growth on the (001) face of kaolinite exhibit the main features of the triclinic 1Tc and real structures of this mineral.     

Effects of the dislocation collective behaviour in plasticity of single crystals

An analysis has been carried out of the motion of a pair of sign-similar edge dislocations under a constant shear stress in a simplest nonuniform field of internal stresses τ_i with a constant gradient. The influence is shown of the dislocation interaction and τ_i on the activation „volume”︁ of the average velocity of a dislocation pair, which is measured by the flow stress relaxation. The experimentally revealed anomalies of the activation „volume”︁ measured by the flow stress relaxation are regarded as corroborating the analysis and as manifestations of the collective effects of the elastic interaction of sign-similar mobile dislocations.     

The dependence of anomalous temperature peak of a deforming stress on microstructural parameters

The temperature dependence of a deforming stress (including both low-temperature decreasing branch and an anomalous high-temperature peak) has been calculated on the basis of one mechanism. The theory developed takes into account spontaneous thermally activated processes of dislocation locking and unlocking and describes the transition from conventional dislocation gliding to the jumpwise motion of dislocations with the change of the temperature (in full accordance with the in situelectron microscopy observations). The dependences of the plasticity characteristics (flow stress, strain-rate sensitivity, etc.) on the microstructural parameters such as free dislocation path and the energy barriers that should be overcome by dislocations for transformation of their cores from the “gliding” to the “sessile states” and back have also been determined.     

Macrosteps dynamics and the growth of crystals and epitaxial layers

Step pattern stability of the vicinal surfaces during growth was analyzed using various surface kinetics models. It was shown that standard analysis of the vicinal surfaces provides no indication on the possible step coalescence and therefore could not be used to elucidate macrostep creation during growth. A scenario of the instability, leading go macrostep creation, was based on the dynamics of the step train, i.e. the step structure consisting of the high (train) and low (inter-train) density of the steps. The critical is step motion at the rear of the train which potentially leads to the step coalescence i.e. creation of the double and multiple step. The result of the analysis shows that the decisive factor for the step coalescence is the step density ratio in and out of the train. The ratio lower than 2 prevents double step formation irrespective of the kinetics. For higher ratio the coalesce depends on step kinetics: fast incorporation from lower terrace stabilizes the single steps, fast incorporation from upper leads to step coalescence. The double step is slower than the single steps, so the single steps behind catch up creating multistep and finally macrostep structure. The final surface structure consists of the macrosteps and superterraces, i.e. relatively flat vicinal segments. The macrostep alimentation from lower superterrace leads to emission of the single steps which move forward. Thus the single step motion is dominant crystal growth mode in the presence of the macrosteps. These steps finally are absorbed by the next macrostep. The absorption and emission of single steps sustain the macrostep existence, i.e. the macrostep fate is determined the single step dynamics. The condition for single step emission was derived. In addition, the macrosteps are prone to creation of the overhangs which results from surface dynamics coupling to impingement from the mother phase. The angular preferential access of the bulk material to the macrostep edge, leads to the overhang instability and creation of inclusions and dislocations.     

Studies of helical dislocations,polygonization and pile-ups of dislocations in magnesium orthosilicate single crystals

(010) cleavages of magnesium orthosilicate crystals have been etched in concentrated hydrochloric acid and in its vapour at room temperature and also in the melts of sodium and potassium hydroxides at 400 °. Evidences of helical dislocations with their axes along [100] and [001] directions as revealed by etching is reported. Observations of polygonizations and pile-ups of dislocations are described and the implications are discussed.     

Strain accommodation and interfacial structure of AlN interlayers in GaN

The strain accommodation mechanisms at AlN interlayers in GaN, grown by radio‐frequency plasma assisted molecular beam epitaxy, are studied using transmission electron microscopy techniques and atomistic modelling. Interlayers of various thicknesses grown within GaN epilayers deposited on both sapphire and silicon substrates have been employed. Interlayers of thickness below 6 nm do not exhibit line defects although local roughness of the upper interlayer interface is observed as a result of the Al adatom kinetics and higher interfacial energy compared to the lower interface. Above 6 nm, introduction of a ‐type misfit and threading dislocations constitutes the principal relaxation mechanism. Due to strain partitioning between AlN and GaN, threading dislocations adopt inclined zig‐zag lines thus contributing to the relief of alternating compressive‐tensile elastic strain across the AlN/GaN heterostructure. The observed dislocation configurations are consistent with a model of independent motion by climb or ancillary glide in response to their localized three‐dimensional strain environment. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)