Direct observation of dislocations in GaP crystals

The bi-refringent method is applied to investigate dislocations in GaP single crystals. The results are compared to those obtained by X-ray topography, electroluminescence, and electron beam induced conductivity. It is shown that the bi-refringent method reveals slip lines, lattice stresses, as well as single dislocations, with a high contrast depending on the orientation of the sample with respect to the polarization plane of the incident light. The method enables the resolution of single dislocations in crystals with high dislocation densities and the distinction between decorated and undecorated dislocations. Furthermore, dislocations in an epitaxial layer can be observed separately from those in the substrate material.     

Tracing of dislocations in gypsum single crystals

An attempt has been made to trace the trajectory of dislocations of isolated as well as matched pairs of (010) basal cleavages of gypsum when etched with analar grade nitric acid and 0.1n potassium hydroxide. The dislocations are shown to be oblique, parallel, continuous lines passing through the body of the crystal. The lines are cut into smaller fragments on cleaving the crystal into thinner flakes. The implications of this are discussed.     

Tracking of dislocations in gel-grown gypsum single crystals

Sparingly soluble calcium sulphate dihydrate (gypsum) crystals are grown by the gel method, derived from the diffusion of calcium chloride into the set gel containing ammonium sulphate. By etching (010) matched cleavages (of gypsum grown by above method) with analar grade nitric acid as well as successively etching a thin flake, it has been established that the tracks of dislocations initially make an inclination to the cleavage face and continue almost parallel to the cleavage face, contrary to the earlier findings on natural gypsum [1] where the dislocations are shown to be oblique, parallel lines passing through the body of the crystal. The change in the tracking of dislocations in gel-grown gypsum has been attributed to the role of gel inclusion, as well as a faster growth rate. The implications are discussed.     

Edge dislocations with large burgers vector in sphalerite crystals

The X-ray diffraction topography technique based on the Borrmann effect was used to study the dislocated structure of Czochralski-grown indium-doped GaAs(001) crystals. Among the growth dislocations aligned in the [001] ingot growth direction, there are dislocations with large Burgers vectors (b=a〈100〉) and { 100} slip planes.     

Wave beams in crystals with dislocations and quadratic nonlinearity

Nonlinear elastic waves in dislocated crystals are considered in terms of equations with constant coefficients, which involve a quadratic nonlinearity. A nonlinear modulation equation with a cubic nonlinearity is obtained for the first harmonic amplitude, a dispersion equation is derived, and the absorption coefficient is determined. The stability and focusing (self-focusing) of a wave beam are studied.     

Transmission electron microscopic observation of dislocations in x-phthalocyanine crystals

Transmission electron microscopy has been utilized to directly reveal the defects that are present in thin single crystals of x-phthalocyanine polymorph. The bright field images are characteristic of dislocation arrays while the associated diffraction patterns indicate that the parent monoclinic structure of x-phthalocyanine may undergo a stress-induced phase transformation into a daughter orthorhombic structure. The transformation is akin to a martensitic process as a result of the operation of an invariant plane strain. The dislocation arrays observed have been interpreted in terms of slip dislocations.     

Transmission electron microscopy of dislocations in nickel oxide single crystals

The dislocation structure of NiO single crystals used in diffusion studies has been examined by transmission electron microscopy. The crystals contain dislocations (the dislocation density being 4 × 10¹² m^–2) that are probably a result of the growth process. The dislocations have a Burgers vector ofa/2 110. On annealing at temperatures above 1400° C the density is reduced to 7 × 10¹¹ m^–2, most of the dislocations forming low angle boundary arrays. The dislocation density was found to be much greater in the vicinity (within 1m) of a mechanically polished surface.     

Long-term dynamic stability of discrete dislocations in graphene at finite temperature

We present an assessment of the finite-temperature dynamical stability of discrete dislocations in graphene. In order to ascertain stability, we insert discrete dislocation quadrupole configurations into molecular dynamics calculations as initial conditions. In calculations we use Sandia National Laboratories Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) and the Adaptive Intermolecular Reactive Empirical Bond-Order (AIREBO) potential. The analysis shows that the core structures predicted by discrete dislocation theory are dynamically stable up to temperatures of 2,500 K, though they tend to relax somewhat in the course of molecular dynamics. In addition, we find that discrete dislocation theory accurately predicts energies, though it exhibits a slight overly-stiff bias.     

Deformation mechanisms in polymer crystals

The deformation of polydiacetylene single crystals has been investigated by scanning electron microscopy. Three types of deformation twins have been identified. The twinning planes are (0 1 2), (¯2 1 2), and (2 1 2), types not previously reported in polymer crystals. Deformation features in polydiacetylene single crystals observed by other workers are interpreted in terms of these three twins. The possibility of similar types of twins occurring in other polymer crystals is discussed. The twins in the polydiacetylene single crystals and those found in other non-polymeric materials are compared.     

Interactions between dislocations and twins in deformed titanium aluminide crystals

Using scanning, transmission electron microscopy and aberration-corrected scanning transmission electron microscopy, we have studied the interactions between dislocations and twins in impact deformed polysynthetic twinned TiAl crystal. The 1/3?<?1$\overline{1}\overline{1}$] and 1/6?<?211] step dislocations on coherent twin boundaries reveal the interactions of glissile 1/2?<?101> dislocations with the coherent twin boundaries. An abnormal stacking fault was found adjacent to the coherent twin boundary. It has the same stacking sequence but different atom species in the [$\overline{1}$10] direction with an additional displacement of 1/4[$\overline{1}$10] in two neighboring {111} layers, and is likely induced by the slip of a 1/12[112] (i.e. 1/4[$\overline{1}$10] + 1/6[2$\overline{1}$1]) dislocation.     

Evidence of edge and screw dislocations in gypsum single crystals

A row of closely packed equispaced etch pits are observed on etching (010) basal cleavages of gypsum with analar grade nitric acid and 0·1 N potassium hydroxide solution. It is established that these rows of pits are true to the whole bulk of the crystal, by successive etching and etching of match pairs, thin flakes as well as using different etchants. These rows of pits reveal low angle grain boundaries consisting of equispaced edge dislocations in the crystal. Irregular arrays of etch pits observed exactly correspond to its match face. Studying these rows on higher resolution, it is observed that they may reveal tilt or twist boundaries consisting of both edge and screw dislocations, probably created due to the impingement of the mis-oriented guest crystal causing stresses to the growing crystal. The implications are discussed.     

The relationship between dislocations and molecular structure in aromatic crystals

Topographical studies of cleaved anthraquinone crystals provide evidence for twinning on (20¯1) in accordance with the predictions of the sense of angle (SA) mechanism. Supplementary studies on etched, as-grown and cleaved crystals of anthraquinone and p-terphenyl, and a comparison of the results with the available data on anthracene, reveal that the dislocation arrangements in the three isostructural solids can be interpreted in terms of the relative dimensions of the unit cells. In anthraquinone, the (100) [010] slip system is found to be of greater significance than the (001) [010] system, which is dominant in anthracene, whereas the near orthorhombic nature of the p-terphenyl structure results in slip being favoured on (1¯10). Twinning and glide are common to (20¯1) of anthraquinone, whereas glide alone has been observed on (20¯1) of anthracene and (201) of p-terphenyl. Possible partial dislocations and associated stacking faults are discussed.     

Temperature dependence of friction force acting on dislocations in silicon crystals

The characteristics of the dislocation motion in silicon crystals during annealing have been investigated by means ofin situ X-ray topographic observations in the temperature range from 1173 to 1273 K. When annealed at elevated temperatures, the small displacements in position of the dislocation segments took place so as to balance the friction force acting on these segments with the elastic interaction force between dislocations. From the analysis of (i) repulsive interaction between parallel screw dislocations with the same sign, and of (ii) configurational changes of dislocation half-loops, the friction force acting on dislocations can be determined at each annealing temperature by estimating the repulsive interaction force between parallel dislocations, and the force due to line tension for curved dislocation. On the basis of the analysis of the temperature dependence of friction force, the activation energy for dislocation motion is evaluated to be 2.4 eV for a screw dislocation and 2.2 eV for a 60° dislocation.     

Creation and motion of dislocations and fracture in metal and silicon crystals

By making a step on one surface ( ) of a rectangular small paralellepiped copper crystal, dislocations could be created by the molecular dynamic method. The dislocation created was not a complete edge dislocation but a pair of Heidenreich-Shockley partial dislocations. Each time a dislocation was created, the stress on the surface was released. Small copper crystals having a notch were pulled (until fracture), compressed and buckled by use of the molecular dynamic method. An embedded atom potential was used to represent the interaction between atoms. Dislocations were created near the tip of the notch. A very sharp yield stress was observed. The results of high speed deformations of pure silicon small crystals using the molecular dynamics are presented. The results suggest that plastic deformation may be possible for the silicon with a high speed deformation even at room temperature. Another small size single crystal, the same size and the same surfaces, was compressed using molecular dynamic method. The surfaces are {110}, {112} and {111}. The compressed direction was [111]. It was found that silicon crystals are possible to be compressed with a high speed deformation. This may suggest that silicon may be plastically deformed with high speed deformation.     

Domain instability in an ensemble of dislocations during plastic deformation of crystals

An analysis is made of the nonlinear dynamics of perturbations of the dislocation density and elastic field using a proposed evolution model which takes into account the negative velocity sensitivity of the deforming stresses. As a result of the evolution of domain instability, it is observed that periodic and isolated solutions (solitons) exist for the initial variables. Pis’ma Zh. Tekh. Fiz. 24, 42–48 (April 26, 1998)