Laser-induced dislocation structures in copper single crystals

Mechanical Stress fields of nearly radial symmetry are produced by focused laser irradiation on (111) surfaces of copper single crystals. The dislocation structures in the vicinity of the heat-affected zone can be revealed with the aid of an etch pit technique. The observed etch pit pattern is in very good agreement with calculations based on dislocation theory.     

Variation of cutting forces in machining of f.c.c. single crystals

In this study, micro-machining of f.c.c. single-crystal materials was investigated based on a hybrid modelling approach combining smoothed particle hydrodynamics and continuum finite element analysis. The numerical modelling was implemented in the commercial software ABAQUS/Explicit by employing a user-defined subroutine VUMAT for a crystal plasticity formulation to gain insight into the underlying mechanisms that drive a plastic response of materials in high deformation processes. The numerical studies demonstrate that cutting force variations in different cutting directions are similar for different f.c.c. crystals even though the magnitudes of the cutting forces are different.     

Internal stresses and dislocation structure in germanium single crystals

The thermoelastic stress developing during the growth of disk-shaped germanium single crystals for IR applications has been evaluated theoretically. The results have been shown to correlate with the dislocation structure of large germanium single crystals grown by the Stepanov method and directional solidification.     

Mechanical properties and dislocation mobility in barium fluoride single crystals

Conclusions The elastic limit temperature dependence between 50 and 1000°C consists of three regions differing in the mechanisms governing dislocation motion. The general form of the dependence, however, is similar to that for metals. Schmid's law is not obeyed between 50 and 250°C for compressional deformations along the [110] and [112] directions. The increased elastic limit for the [110] orientation is explained by an additional interaction between dislocations lying in the same slip plane but pertaining to different systems.The dislocation velocity dependence on applied shear stress is rapid at 150°C (m-12–18) but decreases quite strongly as the impurity content changes. According to calculations the activation area is (100–200)b² for all the studied BaF₂ crystals. This indicates that dislocation motion is not limited by Peierls' barriers in the stress region in question, but by barriers connected with point defects.Chernogolovka, Leningrad. Institute of Solid-State Physics, Academy of Sciences of the USSR, Moscow. S. I. Vavilov State Optical Institute. Translated from Problemy Prochnosti, No. 6, pp. 10–16, June, 1970.     

Interaction between a dislocation and impurities in KCI single crystals

The strain-rate cycling test during the Blaha effect at 77–220 K has been carried out for three kinds of single crystals, KCI, KCI doped with Sr²⁺, and KCI doped with various impurities. The temperature and impurity dependence of the relation between strain-rate sensitivity and stress decrement, as well as the effective stress, _p1, due to only one type of impurity lying on the dislocation with the largest separation, has been investigated. From the temperature dependence of _p1, the force-distance profile between a dislocation and impurities was obtained. The critical temperature,T _c, for KCI: Sr²⁺ was found to be about 227 K.     

Growth of low dislocation density single crystals of nickel

Low dislocation density single crystals of nickel have been grown at high ambient pressure by the Czochralski method. X-ray Laue picture shows that the crystals are strain-free. The dislocation density was determined to be <10³/cm² by the etching procedure. It was found that the necking and cone regions are very critical in the dislocation introduction in the crystals. An increase in the ambient pressure used during the growth seems to aid the crystal quality.     

Dislocation structures in deformed Cu-Ni-Zn alloy single crystals

It is known that Cu-Ni-Zn alloy has an ordered structure Cu₂NiZn (Ll₂) by annealing between 573 and 623 K. In the present experiments, the effects of annealing on the dislocation structure were studied on Cu-Ni-Zn single crystals with several compositions. Thin foils cut parallel to the {111} planes were observed in a transmission electron microscope. The results obtained are as follows. (i) In Cu-5Ni-5Zn, Cu-10Ni-10Zn and Cu-15Ni-15Zn (at%), the stress-strain behaviour, slip mode and dislocation structure did not change by annealing at 573 K. However, the slip mode became more concentrated and localized, and dislocations emitted from a source tended to stay on the same slip plane, as the nickel and zinc concentrations increased. (ii) However, those properties in Cu-20Ni-20Zn and Cu-25Ni-25Zn changed drastically by annealing. As the ordering proceeded, uniform distributions of superlattice dislocations were observed. A typical dislocation configuration, with long screw and wavy-edged superlattice dislocations, took the place of piled-up unit dislocations. (iii) The facts that edge-type superlattice dislocations formed dipoles and their clusters, and that the secondary dislocation density was much lower than the primary one, implied that the elastic interaction of the primary edge-type superlattice dislocations on the nearby parallel slip planes would control the work-hardening of ordered Cu₂NiZn alloy single crystals.     

Interaction between a dislocation and various divalent impurities in KCl single crystals

The strain-rate cycling test during the Blaha effect measurement at 77–254 K was carried out for four kinds of single crystals: KCl doped with Mg²⁺, Ca₂₊, Sr²⁺ or Ba²⁺ as a weak obstacle. It was found that the critical temperature, at which the effective stress is zero, increases when the divalent ionic size approaches increasingly that of the K⁺ ion. Furthermore, the activation energy for the interaction between a dislocation and the divalent ion-vacancy dipole was determined. The activation energy increases with the divalent ionic size. Fleischer's model attributes this to the difference of tetragonality around the dipole for each specimen.     

Exploring for 3D photonic bandgap structures in the 11 f.c.c. space groups

The promise of photonic crystals and their potential applications has attracted considerable attention towards the establishment of periodic dielectric structures that in addition to possessing robust complete bandgaps, can be easily fabricated with current techniques. A number of theoretical structures have been proposed. To date, the best complete photonic bandgap structure is that of diamond networks having Fd3m symmetry (2-3 gap). The only other known complete bandgap in a face-centred-cubic (f.c.c.) lattice structure is that of air spheres in a dielectric matrix (8-9 gap; the so called 'inverse-opal' structure). Importantly, there is no systematic approach to discovering champion photonic crystal structures. Here we propose a level-set approach based on crystallography to systematically examine for photonic bandgap structures and illustrate this approach by applying it to the 11 f.c.c. groups. This approach gives us an insight into the effects of symmetry and connectivity. We classify the F-space groups into four fundamental geometries on the basis of the connectivity of high-symmetry Wyckoff sites. Three of the fundamental geometries studied display complete bandgaps--including two: the F-RD structure with Fm3m symmetry and a group 216 structure with F43m symmetry that have not been reported previously. By using this systematic approach we were able to open gaps between the 2-3, 5-6 and 8-9 bands in the f.c.c. systems.     

High-temperature creep and dislocation structure of MgO single crystals at low stresses

MgO single crystals oriented toward (100) have been compressively deformed to strains in the range of 0.04 to 0.09 at temperatures between 1948 and 2023 K using stresses under 6 MPa. Microstructural developments in the crept samples were monitored by etch pitting, SEM and HVEM. Optical microscopy revealed an equiaxed network of subgrains with 〈110〉 forming the framework for the formation of subgrains. Measurement of the dislocation density not associated with cells reveals that the stress dependence of the steady-state values of dislocation density can be described by the relationϱ ∫σ ^2.15. HVEM observations show that cell boundaries are formed by the process of knitting. Drastic unloading results in a fraction of the sub-boundaries straightening, but the majority of the boundaries are destroyed. It is concluded that creep of MgO at low stresses and high temperature is similar to those of fcc metals.     

Photoelectrical properties of layered GaS single crystals and related structures

The photoconductivity of pure and Cu-doped layered gallium monosulfide (GaS) single crystals, and of Ni-GaS(Cu)-In and GaS(Cu)-ZnO structures, is investigated. The activation energies of surface states were found as 0.10 eV, 0.40 eV and 17 meV, ∼400 meV for GaS and GaS(Cu), respectively. Cu acceptor levels are localized at 0.44 eV and 0.52 eV above the valence band of GaS. ZnO-GaS(Cu) heterojunctions show remarkable photosensitivity in the wavelength range of 250-700 nm.     

Deformation of NiO single crystals below room temperature: dislocation configurations

NiO single crystals prepared by two crystal growth techniques (zone melting in an arc-image furnace and Verneuil crystallization have been deformed by compression along 001 at temperatures as low as 4.2 K, and the dislocation substructure observed by transmission electron microscopy. The Peierls mechanism has been suggested as the mechanism controlling the mechanical behaviour at the lower temperatures. The dislocations generated at cavities found in the zone-melted crystals may be responsible for the increase of the flow stress of these crystals compared with the Verneuil ones.     

Dielectric properties of LiF single crystals X-ray irradiated under d.c. fields

The dielectric constant (K), loss (tan ) and a.c. conductivity () of LiF single crystals subjected to X-ray irradiation in the presence of cl.c. fields up to 16 kVcm^–1 are measured in the frequency range 10² to 10⁵ Hz and in the temperature range 30 to 400° C. The space charge and Bipolar effects observed from this study are discussed in the light of optical absorphon data.     

The identification of faulted prismatic dislocation loops in single crystals of undoped InP

High-voltage transmission electron microscopy has shown that undoped single crystals of indium phosphide, grown by the liquid-encapsulated Czochralski technique, can contain linear arrays of faulted dislocation loops. The plane of the loops is (1 1 0), the fault vector is 1/n [1 1 0] and the Burgers vector of the dislocation loop is 1/n [1 1 0]. A direct correlation has been obtained between these loops and arrays of both ridge and prism features revealed by chemical etching. Sequential use of two etchants has also established a direct link between the faulted dislocation loops and the slip dislocations induced by thermal stresses during crystal growth. A study of a number of crystals grown from differently prepared starting materials suggests that the formation of the loops is associated with departures from stoichiometry and inhibited by the presence of dopant impurities.     

A dislocation model of the plastic deformation of single crystals of ice

A new dislocation model is constructed on the basis of various characteristics of dislocations revealed by X-ray diffraction topography, for interpreting plastic deformation behaviour of ice single crystals in basal glide. The model of a pair of screw dislocation arrays of opposite sign exhibits resistance for the movement, which depends upon both the configuration and the stress. Orowan's relation between the macroscopic strain rate and characteristic of dislocations in the crystal (density and velocity) is rewritten in a dynamical style taking into account the resisting stress and the empirically established linear relationship between the dislocation velocity and the stress. In this formulation, a new concept of fractional dislocation density is introduced.Examples of fractional density as a function of maximum stress are obtained from our stress relaxation experiments. Assuming that the initial fractional density profile for a fresh ice single crystal is similar to those obtained above, stress-strain curves are calculated numerically for various crosshead speeds of the test machine. Computed results coincide well with the characteristic stress-strain curves previously obtained experimentally.