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.     

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.     

Dynamic aging of dislocations in materials with a high crystalline relief: Competition between diffusion and impurity entrainment

A model of the dynamic interaction of dislocations with the impurity subsystem of crystals that have a high lattice potential relief (Peierls barriers) has been developed. It is shown that the microscopic structure of migration barriers for impurities near a dislocation core may cause qualitatively different behavior of the impurity atmosphere on a moving dislocation. It is justified that the impurity kinetics during atmosphere formation includes two stages. The first (initial) stage is fast and significantly nonequilibrium; it is followed by the second stage, characterized by a slower approach to equilibrium. The initial stage manifests itself at a sufficiently fast dislocation motion and may lead to an anomalous increase in the driving force (or the yield strength of the material) with an increase in the temperature in some range. Blocking of the dislocation motion by impurities may cause inverse brittle-ductile transition, which is observed in some materials with an increase (rather than the usual decrease) in temperature.     

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.     

Microstructure of high-temperature plastically deformed Zn-doped CdTe; comparison with In-doped GaAs

We investigated the microstructure of CdTe and Cd_0.96Zn_0.04Te samples plastically deformed by uniaxial compression at constant strain rate between 0.6T_m and 0.85T_m (where T_m is the melting temperature, 1365 K). We used scanning electron microscopy (SEM) in the cathodoluminescence (CL) mode to observe dislocation arrangements and transmission electron microscopy (TEM) to measure dislocation dissociation widths. These results are compared to those reported for In doped GaAs, as well as results concerning high temperature plastic deformation of both types of materials. In both cases, isoelectronic doping strongly increases yield stresses and hardening rates. A cell structure appears for larger stresses and strains. The increase of dissociation width in Ga_0.99In_0.01As with respect to GaAs was not observed in Cd_0.96Zn_0.04Te compared to CdTe. On the basis of macroscopic and microscopic studies, a better understanding of the hindering by doping of dislocation multiplication during crystal growth is possible. The involved mechanisms can reduce the as-grown dislocation density in CdTe and GaAs.     

Interaction of intrinsic point defects with dislocation stress fields in hcp zirconium crystal

The crystallographic, energetic, and kinetic characteristics of intrinsic point defects (vacancy-self-interstitial atom) in stable, metastable, and saddle configurations in hcp zirconium crystal have been calculated by the molecular-statics method. The spatial dependences of the interaction energies of intrinsic point defects and stress fields of rectilinear dislocations with Burgers vectors of 1/3[11$ \bar 2 $ \bar 2 0], 1/3 [11$ \bar 2 $ \bar 2 3], and [0001] have been found within the anisotropic linear theory of elasticity. The most likely trajectories of intrinsic point defects in dislocation stress fields (trajectories with minimum energy barriers for motion) have been constructed. Such trajectories result in dislocation only for the interaction of self-interstitial atoms with an edge dislocation that has a Burgers vector of 1/3 [11$ \bar 2 $ \bar 2 3].     

Temperature and Strain Rate Effects on Work-Hardening of KCl Single Crystals

The influence of temperature and strain rate on work hardening of a KCl crystal has been investigated. Using the experimental data on flow stress relaxation, the activation “volume” for dislocation motion and internal counteracting stresses have been calculated in the temperature range of 300 to 40 K. Estimation has been carried out for the variation of a number of mobile dislocations ϱ₂/ϱ₁, when the strain rate is changed by a factor of four.     

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.     

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.     

Correlation between microhardness and dislocation mobility in crystals with a different type of bond

The complex investigations of microhardness and dislocation mobility in temperature interval 77–873 K have been performed on ionic crystals, semimetals and covalent crystals. It is shown that the correlation between the microhardness and dislocation mobility exists as a whole only. The temperature change of microhardness is not often connected with the change of dislocation mobility. An “anomalous” behaviour of semimetals has been established – they have a small microhardness and a low dislocation mobility at a time.     

Formation of space-charge electric field and space charge during the cleaving of Alkali Halide Crystals

The “acoustic probe” method has been used for the measurement of the space-charge and polarization in the crystals LiF, NaF, KCl and NaCl exposed to mechanical action. It has been shown that large mechanical stresses in the bulk of crystals lead to the formation of space-charges and inhomogeneous polarization. The reason for this phenomenon is, probably, the motion of dislocation pile-ups.     

The features of X-ray topographic contrast formation in silicon with dislocation clusters

The features of formation of diffraction images of edge dislocation sets forming clusters (of two, three and more dislocations) as well as small-angle dislocation boundaries (walls) have been studied. A variety of diffraction effects of wave fields created in strongly distorted crystals regions along dislocation lines have been observed. Various intensity interference effects of rescattering and internal reflection of the newly formed and already existing wave fields on thickness distributions of intensity for the case of presence in the same glide plane of edge dislocations with parallel and anti-parallel Burgers vectors were discovered.     

X-ray topographic investigation of growth defects in natural calcium fluoride crystals

Calcium fluoride crystals have been cleaved along {111} faces and the freshly cleaved faces have been chemically etched in 2.6 N nitric acid solution. The etched faces have been optically studied. One-to-one correspondence of dislocation etch pits have been established on the matched cleavage faces and on the opposite sides of thin flakes. The same crystal has been studied using X-ray topographic technique. By using 022 - and 02 2 reflections, stereopair projection topographs were studied and compared with the optical micrographs. The close resemblance between dislocation etch pits and dislocation out-crop images has been established. The orientation of the Burgers vectors of the dislocation lines has been identified using AgKα radiation with 111 -, 11 1- and 1 11 reflections. It has been confirmed that Burgers vectors of dislocation lines lie parallel to 〈110〉 directions. The implications are discussed.     

Defects in LEC-grown Indium Phosphide Crystals Doped with Tin

Crystalline defects in Sn-doped LEC indium phosphide have been revealed by chemical etching and analyzed by TEM. Grown-in dislocations, various kinds of defect clusters and colonies of microdefects were found. The symmetrical defect clusters are shown to equate mostly with larger dislocation loops exhibiting shear components and/or other dislocation arrangements generated by a stress source which is positioned in the centre of the dislocation cluster. Those centres are often formed by a plate-like agglomeration composited of tiny inclusions and very small faulted dislocation loops. Such planarly arranged accumulations of microdefects lie on {111} planes. The direct vicinity of single threading grown-in dislocations is always enriched with tiny perfect dislocation loops and precipitates. Additionally, very large isolated interstitial-type perfect dislocation loops with b = a₀/2 〈110〉 have been found by TEM experiments. Mostly, the {110} habit plane of such loops is decorated with an high number of small dislocation loops and precipitates as a consequence of dislocation climb.     

Dislocation density in pure crystals grown from melts

Physical reasons are considered which are responsible for dislocation formation in crystals grown by different ways, i. e. by Czochralski's, Stepanov's, Kyropoulos's, Verneuil's, Bridgman's, Stockbarger's methods and floating zone technique. Equations are presented allowing to evaluate the dislocation density in pure crystals prior to testing. The conditions for obtaining dislocation free crystals have been considered. The equations for dislocation density evalution have been compared with the experimental results of different authors. Thermal stresses have been proved to be the main source of dislocation formation, and the equations corresponding to this mechanism in most cases enable to predict the dislocation density with the error being a factor of 1–3.     

Asymmetry of isolated dislocation mobility in Silicon single crystals

The main quantitative characteristics of asymmetry of isolated dislocation mobility in Si single crystals manifested in an essential difference (up to an order of magnitude) of the velocities of broadening and narrowing dislocation half-loops have been investigated. Transition from broadening to narrowing of the dislocation half-loop is found to result in the nonmonotonic temperature dependence of the 60° segments velocities as well as in changing the stress dependence of the velocity. The high temperature annealing of the sample befor the reversal of the dislocation glide is found to cause the asymmetry disappearance. The analysis of the results obtained is performed. It is concluded that the phenomenon observed may be due to a rearrangement of the point defect structure during the course of the dislocation glide.     

Effect of organic impurities on the surface morphology and growth mechanism of KBP crystals (C8H5O4K)

The surface morphology of the (010) face of potassium biphthalate (KBP) crystals grown from aqueous solutions under the supersaturation ranging within 0.029–0.04 has been studied by the methods of optical and electron microscopies. It was revealed that the (010) surface has polygonal growth macrohills of the dislocation nature, small hillocks developing by the mechanism of successive two-dimensional nucleation, and numerous two-dimensional nuclei. The density of small hillocks (10⁴–10⁵ cm^?2) exceeds the dislocation density in KBP crystals by one to two orders of magnitude. It is shown that at low supersaturations, the (010) face grows simultaneously by the dislocation mechanism and the mechanism of successive two-dimensional nucleation. It is also established that the tangential velocity of growth-step motion on the (010) face increases in the presence of organic impurities. This effect can be used as one of the factors increasing the growth rates of crystal faces at low impurity concentrations (the so-called catalytic effect of impurities).     

Storage of dislocations during plastic deformation of polycrystalline copper-manganese solid solutions

The regularities of dislocation storage during the deformation of homogeneous polycrystalline solid solutions with different degrees of solid-solution hardening have been studied and described. The effect of alloy concentration and test temperature is considered. The role of different dislocation density components (average scalar dislocation density, excess density, and dislocation density in walls and cells) is selected. Particular attention has been paid to the parameters of cellular substructure measured at different test temperatures and alloying concentrations. The important role of the solid-solution hardening in the regularities of dislocation storage is established.     

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.