Interaction Of Edge Dislocation Research Articles

Anchoring-Mediated Interaction of Edge Dislocations with Bounding Surfaces in Confined Cholesteric Liquid Crystals

We employ the fluorescent confocal polarizing microscopy to image edge dislocations in cholesteric liquid crystals. Surface anchoring at the bounding plates determines the structure and behavior of defects. Two types of plates set in-plane director orientation but differ in the type of associated anchoring potentials. Plates with strong polar anchoring and nonzero azimuthal anchoring repel the dislocations, while plates with weak polar anchoring and no azimuthal anchoring allow the dislocations to escape through the boundary. To explain the results, we propose a coarse-grained model of cholesteric anchoring.

Interaction of Transonic Edge Dislocations with Self-interstitial Loop

The interaction of transonic edge dislocations with a sessile self-interstitial dislocation loop (SIL) in BCC tungsten is investigated using atomistic Molecular Dynamics method. The moving velocity of the transonic edge dislocations is higher than the transverse sound wave velocity. It was found that SIL has a strong pinning effect to the moving edge dislocations. The SIL was eventually annihilated through forming jogs on the moving edge dislocations. A transonic edge dislocation can be stopped by SIL at the pinning point, and then the resulting jogs could move above the sound barrier afterwards under high shear.

Dislocation drag in the low-temperature phase of C60 fullerite due to orientational relaxation of the molecules

The interaction of edge dislocations with pentagonal (p) and hexagonal (h) orientation states of the molecules in the low-temperature simple cubic phase of C60 fullerite is discussed. The temperature interval considered is Tg<T<Tc, where Tc=260 K is the phase transition temperature and Tg=90 K is the orientational glass point. The nonuniform distribution of the h configurations around a sessile dislocation line is described, and the starting force Fs(T) needed to break the dislocation away from the cloud of h configurations formed by it is determined. The dynamic drag force FD(T,V) arising as a result of thermally activated transitions between the p and h configurations under the influence of the elastic field of a dislocation moving at a constant velocity V is calculated and analyzed.

Atomistic simulation of thermally activated glide of dislocations in FeNiCrN austenite

Thermally activated glide of edge and screw dislocations with the Burgers vector b = a/2〈110〉 in FeNiCr austenite without and with nitrogen has been analyzed using the conjugate gradient method to minimize the potential energy of the crystal (subject to appropriate constraints) and the embedded-atom method to quantify the atomic interactions. The results of the analyses were compared with their experimental counterparts. In austenite without nitrogen, the variation of the crystal energy with the dislocation position gives rise to a Peierls frictional stress. However, the magnitude of this stress is relatively low and both edge and screw dislocations can readily overcome it by the assistance of thermal activation. The presence of nitrogen, on the other hand, causes the core of a screw dislocation to dissociate onto two {111} planes containing the dislocation line. This renders the screw dislocation sessile. Such changes in the core structure of an edge dislocation were not observed. Instead, the interaction of nitrogen with edge dislocations was found to give rise to both an athermal (long-range) and a thermal (short-range) component of the frictional stress. A detailed analysis of the interactions between the nitrogen atoms and the edge dislocations suggests that the athermal component of the frictional stress observed in the present work is most likely responsible for the experimentally observed athermal flow stress in FeNiCr polycrystalline austenite. On the other hand, with a possible exception of very low temperature, the contribution of the thermal frictional stress associated with the interaction of edge dislocations with nitrogen to the strength of FeNiCr polycrystalline austenite is small.

Elastic interaction of edge and screw dislocations with point defects in cubic crystals

Elastic interaction of edge and screw dislocations with point defects in cubic crystals

Plastic anisotropy of doped alkali halide single crystals

The critical resolved shear stress (CRSS) of various alkali halides doped with divalent ions was measured for slip on {110} and {100} planes in compression and shear. The results are explained in terms of the interaction of edge dislocations with dopant-vacancy dipoles. While {110} glide around room temperature has to overcome long-range elastic (Snoek-type) interaction, in general a Fleischer-type interaction with both elastic and electrostatic contributions is responsible for the glide resistance. The so-calculated plastic anisotropy at T = 0 is in qualitative agreement with that extrapolated from experiment. The observed correlation of the plastic anisotropy and solution hardening parameters with the polarizability (Σα) of the ions of the host lattice is explained by the dependence of the electrostatic interaction energy between dislocation and dipole on Σα.

Solution hardening by aliovalent cations in ionic crystals

Theories of solution hardening in ionic crystals due to aliovalent cations are reviewed. The electrostatic interaction between a dislocation and an impurity cation-cation vacancy complex is calculated and found to be too small to explain the observed solution hardening rates in the alkali halides, MgO, and Al 2O 3. The elastic interaction between a dislocation and the “tetragonal” distortion associated with the various complexes is calculated for screw and edge dislocations using anisotropic elasticity; it is found that in most materials, the edge dislocation interaction dominates, and that the observed solution hardening rates can be explained by reasonable values for the tetragonal distortion. The origin of the tetragonal distortion around the divalent cation-impurity-cation vacancy complex in the alkali halides is discussed and it is considered to arise from the relaxation around the complex and possibly involves off-center displacements of the divalent cation.

Critical shear stress temperature dependence in Al?Mg single crystal alloys of various concentrations in the range 1.6?300�K

1. The critical shear stress temperature dependence for Al−Mg single crystal alloys and for Al crystals had three regions in the studied temperature range: (a) 300−(120–130)°K, in which the CSS is temperature independent; (b) 120−4.2°K, in which the CSS rises with falling temperature (the temperature T0, above which τ = τG, slightly shifting to lower temperatures side with increasing Mg content in the alloy); and (c) below 4.2°K, for which the CSS decreases with decreasing temperature. 2. The magnitude of the critical shear stress in the studied temperature and concentration ranges is proportional to √c and is described satisfactorily using Fleischer's concepts of edge dislocation interaction with the maximum elastic stress fields and localized shear modulus concentration variations. 3. The absolute fall in the CSS at temperatures below 4.2°K rises proportionally to √c, its relative value not depending on the alloy Mg concentration.

Mechanical Behavior of Samhire

The present state of the knowledge of the mechanical behavior of sapphire is reviewed. Sapphire deforms plastically at temperatures above 900°C, the most common mode of deformation being slip on the basal plane in the 〈110〉 direction. Under certain conditions, however, slip may occur on prism planes or twinning may occur. A yield point is often observed, which appears to be related to the multiplication of dislocations by a mechanism controlled by the motion of dislocations through the lattice, rather than by the tearing of dislocations from an impurity (or defect) atmosphere. The dynamics of yielding and flow is similar and can be expressed by either of two thermally activated equations. In one, the effect of stress is principally on the pre-exponential factor; in the other, its effect is principally on the activation energy. There are insufficient data to permit a positive decision between the two, or to positively identify the rate-controlling dislocation mechanism. For 60°-oriented crystals fracture generally occurs on a plane approximately normal to the tensile stress and the fracture surface is conchoidal. In the range 1100° to 1500°C, the tensile fracture stress decreases with increase in plastic strain, independent of temperature and strain rate. The mechanism of failure in this case seems to be the interaction of edge dislocations with pre-existing cracks.