Dislocations in yttrium orthoaluminate single crystals

The dislocations present in single crystals of yttrium orthoaluminate grown by the Czochralski technique have been studied by means of etch pits and electron microscopy. They are shown to be predominantly edge in character with Burgers vectors which are principallya [1 0 0] andb [0 1 0]. In most instances, the dislocations form simple tilt boundaries. A few dislocations of a more random nature occur in association with twin boundaries. Dislocation formation is shown to be a characteristic of c-axis growth related to stresses caused by the anisotropic contraction of the material;b-axis crystals can be grown in a dislocation-free form.     

Dislocations and the plasticity of ionic crystals

Abstract

Some recent developments in the dislocation theory of plastic deformation in ionic crystals are reviewed. It has been found that the Peierls force operates on slip planes of type {110} and {00l}, that interactions between dislocations and point defects are different on these planes, that point defects are produced by jogged dislocations, and that cross-slip is responsible for polycrystalline ductility. There are still many interesting questions to be answered in the field of ionic crystals, which provides an excellent testing ground for dislocation theory.

MST/268     

Dislocations and twins in Czochralski-grown gallium phosphide single crystals

An etching and electron microscopy study has been carried out on {111} oriented slices of gallium phosphide (GaP) taken from Czochralski-grown ingots. The correlation of the etched structure with substructural defects as revealed by transmission electron microscopy has been determined, It has been found that GaP grown by this technique has a high dislocation density and exhibits polygonisation and mechanical twinning. It is found that there is a one-to-one correspondence between surface etch pits and grown-in and mechanically induced dislocations. Stacking faults are also revealed in the etching studies and are shown to be twins in the matrix of the {111} 112 type. It is proposed that the twinning occurs due to the severe growth conditions and may arise by dissociation of dislocations in the sub-boundaries.     

Dislocations of ZnO single crystals examined by X-ray topography and photoluminescence

Dislocations generated by indentation and subsequent annealing at elevated temperatures up to 800 °C in high quality (0001) wafers of ZnO single crystals were investigated by transmission X-ray topography and photoluminescence. Damages induced by indentation in ZnO wafers were guessed to lead to dislocated regions from X-ray topographic images. PL intensities of 3.36 eV near-band edge peak and 2.4 eV deep emission band in ZnO decreased drastically, with increasing in annealing temperature up to 800 °C, irrespective of dislocated or non-dislocated regions. The development of a new emission band 2.8–3.0 eV was found in non-damaged ZnO through annealing at 700 and 800 °C, which suggests that dislocations suppress the development of the new peak.     

Dislocations produced in magnesium oxide crystals due to contact pressures developed by softer cones

An experimental investigation shows that dislocations, encompassing a predictable dislocated volume, are produced in MgO single crystals subjected to circular contact pressures due to cones of other solids which may be an order of magnitude softer. The mean contact pressure necessary to produce localized plastic flow, without fracture, is shown to be directly related to the critical resolved shear stress of the MgO. Dislocation etching is used to investigate the influence of cone hardness and normal loading on the nature of the deformed zone in the harder crystal and the results are discussed in terms of the deformation characteristics of the softer cones and the crystal.     

Dislocations in semiconductors

Abstract

Glide in elemental semiconductors or III–V compounds takes place by dissociated 1/2〈110〉 dislocations on {111} glide planes. Structural models and valence force calculations suggest that the cores of the basic partial dislocation may be reconstructed. Kinks may be reconstructed or have dangling bonds. The former will be associated with fairly shallow levels, the latter with deep acceptor and donor levels in the bandgap. Deep–level transient spectroscopy, electrical, optical, and electron paramagnetic resonance data give information on energy levels and concentrations of dangling bonds and deep levels, but the identification with particular sites on the dislocations is very difficult. The electronic states also lead to the dislocations acting as recombination centres, and give rise to photoplastic effects. There are strong interactions with impurities, which have a profound effect on the electronic properties. Dislocation velocities at relatively low temperatures are controlled by the Peierls force, and motion occurs by the generation and motion of double kinks. There is a pronounced dependence of dislocation velocity on doping, for both elemental semiconductors and III–V compounds. A recent theory attributes the doping effect to a dependence of the concentration of charged dangling bond kinks on the Fermi level, and of the kink velocity on the charge state. The doping effect is also reflected in mechanical properties, for example the variation of yield stress with temperature, rosette diameters, and radial cracking behaviour around hardness indentations, and values of hardness. In III–V compounds different mobilities of α– and β–dislocations give rise to anisotropies in hardness, rosette configurations, and associated cracking, and for {111} indentations different hardness values, slip patterns, and cracking behaviour result on the group III and V crystal faces.

MST/269     

Dislocations in ceramics

Abstract

The type and nature of dislocations in oxide ceramics are reviewed and compared. Slip systems in these materials tend to be limited and difficult to activate, and Burgers vectors are often large. Thus, dislocation dissociation has been an important theme of research and climb dissociation has been frequently observed. Elasticity theory suggests that climb dissociation should be expected whenever diffusion can occur, and may be widespread in other materials deformed at elevated temperatures. Low temperature behaviour remains an area where much work needs to be done, particularly to assess the role of the Peierls barrier on the dislocation-core configuration.

MST/270     

A geometric model of decohesion in solid continua

A modeling framework for material separation is proposed within the broad class of cohesive-type models. A key feature of the proposed framework is that the cohesive tractions are governed by a separate boundary-value problem defined on the separation surface, rather than by a traction-separation rule or other constitutive construct. The traction BVP incorporates a boundary condition intended to enforce continuity of the strain field at the separation front, thus rendering the material state bounded and continuous at the front. The phenomenology of material separation is governed by a rupture function, which can depend arbitrarily on the bulk material state. The rupture function, which is evaluated on the separation front, delineates the conditions for, and direction of, material separation. A linearly-elastic analytical solution using the new theory is provided to illustrate the functioning of the theory, including the regularizing effect of the cohesive traction.     

Dislocations in manganese-doped InSb

Electron probe X-ray microanalysis data for quenched InSb〈Mn〉 samples demonstrate that most of the manganese goes to doping of dislocations in the semiconductor lattice. The manganese-doped dislocations in InSb determine the magnetic and electrical properties of the material at room temperature and above. According to magnetic measurements, this is accompanied by the formation of several magnetic phases. Codoping of InSb with manganese and zinc with the aim of neutralizing one of the magnetic phases allowed us to obtain a ferromagnetic semiconductor with a Curie temperature of 320 K.     

Dislocations in crystalline helium

It is suggested that it may be possible to use ions to detect and study dislocations in crystalline He. An experiment similar to that of Rayfield and Reif is suggested and discussed.Work partially supported by the U.S. Atomic Energy Commission under contract No. AT(11-1) 1569.     

Dislocations and gauge invariance

The paper deals with gauge invariance applied to dislocations in their field theory formulation. By comparison with electromagnetism, the role of distortion and velocity fields as potentials for dislocation density and dislocation current is shown. The gauge transformation involving one vector field for those potentials is given and the equilibrium equation is recognized to be a guage condition. The constitutive laws are shown to form a basis of this gauge condition.     

Dislocations and Pinning Points

Early interactions with Jaques Friedel are recalled. Some aspects of his early work on dislocations are related to current theoretical challenges. Some new challenges based on configurations revealed by atomistic simulations are discussed.     

Dislocations in energetic materials

An assessment has been made of the primary dislocation slip systems in the explosives pentaerythritol tetranitrate (PETN) and cyclotrimethylene trinitramine (RDX) using a combination of dislocation etching and microhardness indentation techniques. Hardness measurements were made on all major habit faces as a function of temperature and load. These showed that, within the attainable temperature range (PETN 293 to 353 K, RDX 293 to 373 K) no change in hardness occurred which could be associated with the development of deformation mechanisms additional to those operating at room temperature. The hardness values of both materials were consistent with values obtained in some previous measurements (PETN 17 kg mm^–2, RDX 39 kg mm^–2). Solvent etching with acetone (5 sec at 274 K) proved to be an excellent method for revealing the emergent ends of growth and mechanically induced dislocations in PETN. Etching of microhardness indentations confirmed that observable slip traces comprised dislocations. These migrated up to 160m (20g load) from the indentation point on both {110} and {101} faces. The alignments defined a {110} primary slip plane. Parallel experiments with RDX yielded evidence of highly localized slip around the indentation mark (90m, 50g load). Two alignments of etch pits were noticeable. The better defined of these lay at the intersection of the (010) plane with the habit faces. The second could not be defined absolutely but most probably corresponds to the intersection of either the (011) or (012) plane with the surfaces. Consideration of the Burgers vectors of dislocations which are likely to glide in these planes lead us to speculate that the primary slip systems are, PETN {110} [001], and RDX (010) [001]. Such an assignment would be consistent with the relative hardness of the two materials.     

Dislocations in plastically deformed apatite

Dislocations were introduced at room temperature by a Vickers indentation into a biological ceramic, apatite. Two types of apatite, a single crystal of fluorapatite and a sintered polycrystal of hydroxyapatite, were studied. Specimens prepared using a focused-ion-beam technique were examined by transmission electron microscopy. Natures of dislocations were determined by the conventional g · b criterion, where the weak-beam method was also applied. The slip system in fluorapatite apatite was determined to be      

Dislocations in carbon nanotube walls

We present a novel interpretation of defected tubes based on a dislocation model. A scroll (a sheet rolled without closure) is considered as an axial edge dislocation in a multiwall nanotube (MWNT). Screw dislocation type defects separate scroll from nested-tubes within the same nanotube. The glide of the screw dislocation causes the transformation between these two forms. In some cases, the mechanism of formation of an MWNT could, therefore, start with the formation of a scroll which, by gliding of a screw dislocation, is transformed into the more stable MWNT We compare the structure and energetics of prismatic screw and edge dislocations in graphite and carbon nanotubes. We present calculations for the Peierls barrier of the first kind for graphite and we discuss this result for glide motion of screw dislocations in nanotubes. There is no evidence for stable sp3 atoms in any of the studied nanostructures.     

Screw Dislocations in Light Wavefronts

Abstract

We discuss screw dislocations of a phase surface as the one type of wavefront of a monochromatic wave. The simple method for construction of the optical wavefronts with an isolated screw dislocation is reported. Laser beams with the dislocations of different orders were experimentally achieved by using diffraction on computer-synthesized gratings.     

Erosive granular avalanches: a cross confrontation between theory and experiment

Results on two laboratory scale avalanches experiments taking place both in the air and under-water, are presented. In both cases a family of solitary erosion/deposition waves are observed. At higher inclination angles, we show the existence of a long wavelength transverse instability followed by a coarsening and the onset of a fingering pattern. While the experiments strongly differ by the spatial and time scales, the agreement between the stability diagram, the wavelengths selection and the avalanche morphology suggest a common erosion/deposition scenario. These experiments are studied theoretically in the framework of the “partial fluidization” model of dense granular flows. This model identifies a family of propagating solitary waves displaying a behavior similar to the experimental observation. A primary cause for the transverse instability is related to the dependence of avalanche velocity on the granular mass trapped by the flow.