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.     

Mechanoluminescence in Centrosymmetric Crystals

The paper reports that the mechanoluminescence (ML) is not an inherent property of only the non-centrosymmetric crystals. The ML may appear in number of centrosymmetric crystals due to variety of processes. The ML of 82 centrosymmetric crystals are reported and different models are proposed for the ML excitation. The models proposed are: space charge electrification model, triboelectrification model, phase transformation model, gas adsorption model, chemical reaction model, thermal population model, molecular deformation model, cleavage electrification model, defective piezoelectric phase model, dislocation defect stripping model, dislocation unpinning model, dislocation annihilation model, charged dislocation model and incandecent light emission model. It is shown that on the basis of the proposed model, intense mechanoluminescent materials with desired nature and characteristics may be prepared.     

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.     

TEM-Untersuchungen von Gitterfehlern in ZnSiP2-Einkristallen

Results are dealt with concerning TEM investigations of lattice defects in ZnSiP₂ single crystals. After the crystal growth dislocations or stacking faults were found in a few cases only. More frequently twins were present in the microstructure. The crystallographic elements of twinning are {112} 〈111〉. After plastic deformation by bending at 900 °C local dislocation arrangements with high defect density (N_v ≈ 10⁶…︁ 10⁷ cm⁻²) were observed. By means of the diffraction contrast one Burgers vector b = 1/2 〈111〉 could be identified. In some cases the crystals also contained wide deformation stacking faults, which were limited by partial dislocations. The density of twins was not increased under the conditions of deformation reported here. As it can be concluded from investigations of Oettel et al. and from the results of the twin analysis, slip and generation of stacking faults take place on {112}-planes in ZnSiP₂ crystals. Crystallographic considerations on both processes are dealt with.     

Autowave model of crystal plasticity: Macro- and microdefects

A new approach to the problem of the plastic flow of solid crystals is proposed. This approach is based on studying the macroscopic localization patterns of plastic deformation, which can be considered as different types of autowave processes of defect self-organization. An unambiguous correspondence between the localization patterns and stages of plastic flow in single crystals and polycrystals is established. The propagation velocity of localized plasticity autowaves is inversely proportional to the strain-hardening coefficient, and the dispersion relation is quadratic. A new model is proposed to describe the development of plastic flow localization.     

Twinning in hydrothermally grown synthetic quartz as revealed by etch structures

Etch structures obtained by HF etching on habit and cleaved rhombohedral surfaces of synthetic quartz grown by hydrothermal crystallization technique, indicative of twinning in them are described, illustrated and discussed. The twin traces appear in the form of lines of dis-continuity in [1121] and [1211] directions. Etch pits within the parallelogram-shaped bound areas are oppositely oriented with respect to those on the rhombohedral surface of the main crystal. The form of twinning observed is found to be of parallel lattice type and the surface within and outside the bound areas are coplanar in most of the cases. The rhombohedral faces of the twinned crystals are found to be geometrically related to each other as if one part is derived from the other by a rotation of 180° about [1210] direction. Discontinuation in the crystal lattice along the twin trace is evidenced by obstructions in the normal development of an etch pit. Deep penetration of twin traces and parallelogram-shaped regions, enclosing etch pits of opposite orientation (twinned areas), well within the crystal structure is demonstrated by comparison of etch patterns on matched rhombohedral cleavages and the results on their successive etching. The type of twinning displayed by the crystals is discussed in the light of models suggested. Causes for the generation of twinning in synthetic quartz are explained to lie in the availability of twinned nuclei in the seed plates used for hydrothermal crystallization of quartz.     

Block (mosaic) structure as an indicator for cooling of crystals grown from a melt

The specific features of the formation of the block (mosaic) structure with low-angle misorientation of the block boundaries are investigated using fluorite and germanium crystals as an example. These structural features are considered an indicator of conditions under which plastic deformation and polygonization occur during cooling of the crystals grown from melts.     

Dynamics of twin layer broadening in indium

The mobility of plane-parallel twin layer boundaries in indium crystals has been studied in the 235–390 K temperature range under stresses, τ, of (0.25–20) g/mm² (static load) and (35–335) g/mm² (crest values under impact load). For τ/G = (0.2–0.6) × 10⁻⁵ (G is the shear modulus), the process of twin layer broadening has been shown to be thermally activated; the process parameters and their stress dependence have been found. – Possible mechanisms of twin layer broadening have been analysed, and it has been concluded that a pole mechanism is inadequate to treat the results obtained. The twin broadening is considered to be due to nucleation and motion of twinning dislocations along boundaries in each subsequent twinning plane; it has been shown that the thermal activation parameters measured while broadening a twin layer, can be inconsistent with the elementary acts of broadening process (nucleation or motion of twinning dislocations). A deep gap between data on stress dependence of the activation energy of twin layer broadening for indium and calcite crystals and the Sumino theory is explained by the determining influence of the real imperfect structure of specimens on the process studied.     

Studies on twin boundaries in calcite crystals

Natural single crystals of calcite have been cleaved along (100) planes and cleavage faces have been etched in 2% and 3% citric acid solutions. Etching produces twin boundaries oriented in 〈010〉 directions. The etch pits on the two sides of the twin boundary are oppositely oriented. It has been conjectured that the rows of pits might have been formed due to etching of dislocations on twin boundaries. One to one correspondence of twin boundaries has been established on matched cleavage faces. This is further confirmed by studying the induced twin regions produced on a (100) cleavage plane by indenting that plane itself. The implications are discussed.     

Transmission electron microscopy observations of twin boundaries and sub-boundary networks in bulk CdZnTe crystals

The microstructure characteristics of the twin boundaries and sub-boundary networks in bulk cadmium zinc telluride (CdZnTe) crystals have been studied by transmission electron microscopy (TEM). Three types of twin boundaries were identified and characterized, which are (i) single straight twin boundary, (ii) tilt twin boundary, and (iii) twin boundary with steps. Boundary dislocations at tilt twin boundary and high dislocation density around steps were observed. The origin of the boundary dislocations is ascribed to the lattice misfit between two tilt grains. The formation of the twin boundary with steps is suggested to be the interaction between the twin boundary and dislocations. Honeycomb-like sub-boundary defects were also observed. The probable reason for the formation of the sub-boundary networks is discussed.     

The electron-dislocation interaction and electronic effects at low-temperature deformation

The effect of the electronic state of crystals on the processes of their plastic flow is considered. Presented are the results of the observation and study of the effects of the electrondislocation interaction at a low-temperature deformation of metals.     

Reducing Inversion Twinning in Single Crystal Growth of GaPO4

Gallium orthophosphate (GaPO₄) single crystals were grown by the reverse temperature gradient method from phosphoric acid solutions under hydrothermal conditions. Twins after (110) were studied by etching faces having been cut perpendicular to one of the twofold axes. Based on the determination of the twin boundary position as well as on the knowledge of the growth rates of different crystallographic forms, a few faces have been chosen to be quite promising for growing high‐quality GaPO₄ single crystals if they are offered at the referring seed crystal. From the characterization of the grown crystals conditions have been found, which may lead to the reduction of the inversion twin number during the growth process.     

X-Ray Topographic Investigation of Dendritic Silicon Crystals

Using Lang and double-crystal X-ray topographic methods the dislocation structure of dendritic silicon crystals have been investigated. It is shown that the surface layers of these crystals have a more perfect structure than their bulk volume. The twin lamella is a dislocation-free formation and there are dislocation-free zones ∼ 1,5 mm in width in the volume of crystals.     

Growth of defect-free Sr1−xBaxNb2O6 (SBN) crystals

SBN single crystals are grown by the Czochralski technique. Growth defects (striations, dislocations, twin lamellae) are investigated as depending on growth conditions. Crystals almost free from growth defects and applicable to optical devices are prepared by an optimized growth technology.     

Included and occluded grains in iron single crystals

The paper presents some experimental results concerning the occurrence of stray grains in high-purity iron single crystals grown by the strain-anneal method. The experimental results indicate that the stray grains are either the unabsorbed grains from the polycrystalline matrix with twin orientation or true annealing twins. The iron crystals with a small number of stray grains can be prepared when a steep temperature gradient is used during growth.     

Mechanism of spatial ordering of dislocations during plastic deformation of crystals

Mechanisms of spatial ordering of dislocations during plastic deformation of crystals are considered. The system of evolution equations, which take into account the effects of elastic and correlated interactions of screw dislocations, is derived. The study is performed with due regard for the dynamics of spatial fluctuations of the dislocation density.     

The influence of magnetic effects on the mechanical properties and real structure of nonmagnetic crystals

This review for the first time systematizes the results of our investigations into the influence of magnetic effects on the mechanical properties and the real structure of nonmagnetic crystals. It is found that the preliminary magnetic treatment of alkali halide crystals leads to a decrease in their solubility and a change in the microhardness and yield stress. The magnetic field strongly affects the macroplasticity of LiF, NaCl, and PbS crystals under deformation in a magnetic field. This is accompanied by a change in the shape of stress-strain curves, a shortening of deformation stages, a change in the hardening coefficients, and a decrease in the yield stress. It is revealed that the magnetic effects exhibit threshold behavior. The yield stress is measured as a function of the magnetic induction and the strain rate. It is established that the magnetic and electric fields have a joint effect on the kinetics of plastic deformation. A kinematic model of the macroscopic magnetoplastic effect is proposed.     

Hydrothermal etching of flux grown phenakite (Be2SiO4) single crystals

The etching studies were made on flux grown phenakite (Be₂SiO₄) single crystals to elucidate the twinning phenomenon and the growth mechanism. The sharp and distinct etch pits and the twin boundaries were successfully obtained after the hydrothermal etching. From the etching and the X-ray precession experiments, it was conjectured that the phenakite had an inversion twin along the c axis. The twinning should be originated during the earlier stage of growth where the dendritic and acicular crystals were grown and thereafter the twinned crystals grew to the prismatic crystals during the latter stage with decreasing the temperature.     

Crystallographic aspects of macroinhomogeneous plastic flow in single crystals of metals

Various crystallographic aspects of the distribution of strain-localization zones have been studied in single crystals of metals subjected to tensile stresses at different orientations of the tension axes and characterized by different mechanisms of plastic flow (slip of dislocations and martensite transformations). It is shown that the crystallographic orientations of the strain-localization zones (interpreted as the patterns of plastic-flow self-organization) are preserved within the whole deformation process. Some characteristic features of the dynamics of the strain sites are considered.     

Über das elektrolytische Wachstum und die Auflösung von Silbereinkristallen unter potentiostatischen Bedingungen. Einfluß der Zwillingsgrenzen

The rates of growth and dissolution of silver single crystals have been investigated under potentiostatic conditions. The results show that dissolution is more rapid than growth. That is explained with the effect of additional active centres in case of dissolution which have no noticeable influence on crystal growth rate. It is shown that twin boundaries have great influence on crystal growth rate. In the case of dissolution their effect is negligible and cannot be noticed. The asymmetry of the processes of growth and dissolution is stronger expressed in case of low values of potential, especially when crystals are of the normal type. The influence of twin boundaries on crystal growth rate increases with overvoltage.