Thermal effects and anomalies in the low-temperature plasticity of crystals

This paper discusses theoretically the mechanism that causes the temperature dependences of the yield point and yield stress, along with their rate coefficients, to deviate from behavior characteristic of thermally activated plastic strain at low temperatures (<30 K). At this time the existence of such deviations, e.g., anomalous decreases in the values of these characteristics in this temperature range, is explained by arguing that the process whereby dislocations overcome local barriers has inertia. It is shown that the observed anomalies can be caused by the development of thermal instability in the plastic strain at low temperatures. In contrast to inertia-related effects, thermal effects allow us to explain the plasticity of crystals at low temperatures without contradiction and within the framework of a single mechanism, including the unstable, discontinuous character of plastic strain that is characteristic of these temperatures. Fiz. Tverd. Tela (St. Petersburg) 40, 684–689 (April 1998)     

Influence of the superconducting state on the thermal instability of low-temperature plastic deformation in crystals

The influence of the superconducting transition on the unstable, discontinuous nature of plastic deformation in crystals at low temperatures (<10 K) is discussed theoretically. It is established, within the mechanism of the thermal instability of low-temperature plastic deformation, that the superconducting state promotes the stabilization of deformation in a superconductor because of the positive sign of the temperature sensitivity coefficient of the flow stresses below the superconducting-transition temperature and the reduced level of energy dissipated by moving dislocations in comparison to the normal metal. The temperature-rate regions for stable and unstable deformation of a superconductor and the dependence of the stress-jump amplitudes on temperature and deforming stress are determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1778–1784 (October 1998)     

Athermal luminescence of a tungsten surface irradiated with laser pulses

An investigation of athermal luminescence triggered at a tungsten surface by thermal deformations produced by laser pulses is reported. The spectral composition and time dependence of individual spectral components of the luminescence are investigated. Oscillations of the luminescence intensity are observed, and they are interpreted as a definite sequence of emergence of dislocations at the surface. Fiz. Tverd. Tela (St. Petersburg) 41, 1538–1542 (September 1999)     

Features of slip-band formation during the plastic straining of layered crystals

The effects of the narrowing and branching of screw slip bands during the plastic straining of nonuniformly doped or nonuniformly irradiated (layered) crystals are discussed theoretically on the basis of the equations of dislocation kinetics. Band formation is treated as a process involving the self-organization of dislocations in a dislocation ensemble at the mesoscopic level. The distributions of the densities of mobile and immobile dislocations, as well as of the local plastic strain rate, in a slip band propagating in a layered crystal are obtained. It is found that the narrowing of bands is due to the lower rate of broadening of the bands in stiff layers than in soft layers, which have not been hardened by doping or irradiation, and that branching is due the low local strain rate in stiff layers compared with the strain rate per slip band assigned by the straining machine. In the latter case the nucleation of new bands or the branching of existing bands is required to restore the balance between these rates. Fiz. Tverd. Tela (St. Petersburg) 41, 252–258 (February 1999)     

Evolution of dislocation structures having various orientations in strained single crystals of the alloy Ni3Ge

The dislocation structure of strained single crystals of Ni₃Ge with various orientations is investigated by electron microscopy. The evolution of the dislocation structure parameters is studied as a function of the degree of strain, temperature, and orientation of the single crystals. Analysis of the experimental dependences of the yield stress on the density of dislocations leads to certain conclusions about how various mechanisms for dislocation drag make temperature-dependent contributions to the deforming stress, and about the nature of the thermal hardening of Ni₃Ge. Fiz. Tverd. Tela (St. Petersburg) 40, 672–680 (April 1998)     

Self-consistent dynamics of an ensemble of screw dislocations during plastic deformation of crystals

A system of self-consistent field equations is formulated for an ensemble of continuously distributed screw dislocations. Rapid relaxation of dislocation charges owing to the elastic interaction of the dislocations leads to a diffusive ensemble dynamics. The conditions for development of an instability in the homogeneous state are determined. Self-organization of the dislocations and their spatial ordering are described in the two-dimensional case. Fiz. Tverd. Tela (St. Petersburg) 39, 1066–1071 (June 1997)     

Local heating and quasiathermal plastic deformation of crystals at low temperatures

A theoretical discussion is given of the relationship between local heating of a crystal by glide lines and bands and deviations observed in the temperature dependences of the flow stresses and their rate coefficients at low temperatures (<10 K) from the dependences characteristic of thermally activated plastic deformation. The appearance of plateaus in these dependences is currently explained by the onset of quantum-mechanical, athermal mechanisms for overcoming local barriers. In this paper it is shown that softening and the apparent athermicity of low-temperature deformation are caused by heating of sites where the strain is localized. Fiz. Tverd. Tela (St. Petersburg) 39, 2019–2022 (November 1997)     

The influence of thermal processing on the relative mobility of edge and screw dislocations in NaCl crystals

The effect of thermal processing on the mobility of dislocations is investigated in NaCl crystals doped with impurities of various types—high-solubility impurities (Ca²⁺) and low-solubility impurities (Pb²⁺). The results obtained after aging and thermal processing indicate that the type of impurity and its state (e.g., level of aggregation) in the crystal have a strong effect on the relative mobility of edge and screw dislocations, and also on the parameters of double transverse slip. Fiz. Tverd. Tela (St. Petersburg) 41, 1041–1043 (June 1999)     

Deformation characteristics of nanocrystalline copper and nickel at low temperatures

Measurements were made of the deformation and fracture characteristics of nanocrystalline copper and nickel at temperatures between 4.2 and 300 K. It was observed that the flow stresses are sensitive to the sign of the load while deformation instability was observed at temperatures close to liquid-helium temperature. The temperature dependence of the yield stress was obtained. It was found that there is a range of a thermal deformation at low temperatures which extends to 60 K for nickel and 200 K for copper. Possible reasons for these characteristics in the deformation behavior of nanocrystalline materials are discussed, especially the role of quantum effects in the low-temperature deformation. Fiz. Tverd. Tela (St. Petersburg) 40, 1264–1267 (July 1998)     

The atkermal stress component in neutron irradiated mild steel

Abstract

The temperature dependence of the tensile lower yield stress of an annealed aluminium grain size controlled mild steel has been investigated in the range 23–250 °C and at a strain rate of 1.67 × 10^?4 sec^?l before and after neutron irradiation to 2.3 × 10¹⁸ n/cm² (fission). The yield stress of the irradiated steel decreases with increasing temperature due to thermal activation of the radiation damage and is predicted to reach asymptotically that of the unirradiated steel at ～285 °C; the maximum test temperature was below that at which thermal annealing of the damage occurs. This implies that the athermal stress component due to irradiation is zero and hence there is negligible long range interaction between dislocations and radiation-induced defects.     

Fluctuation mechanism for formation of discontinuous tracks by fast ions in crystals

The effect of multiple loss and trapping of electrons on the formation of discontinuous tracks by high-energy ions in crystals is examined. A proposed model of charge fluctuations makes it possible to estimate quantitatively the longitudinal size of the defects in a discontinuous track, while an extended thermal spike model, which includes Coulomb repulsion, yields reasonable values for the transverse dimensions of the defects. Zh. Tekh. Fiz. 68, 42–45 (September 1998)     

Barriers participating in the excitation of electroluminescence of ZnS:Cu

It is shown that heterojunctions of the type Cu_xS-ZnS:Cu cannot determine the brightness and quantum yield of electroluminescence of particles of powdered luminophors. The main contribution comes from the excitation of electroluminescence in surface barriers at places where dislocations reach the surface of crystals. State University for Means of Communication, Moscow. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2. pp. 89–92, February, 1998.     

Electronic level diagram and structure of metastable centers in CdF2:Ga and CdF2:In semiconducting crystals

A study is reported of the optical properties of wide-gap, predominantly ionic cadmium fluoride crystals in photo-and thermally stimulated transformations of metastable indium and gallium centers. An analysis of these properties leads one to a conclusion of gallium having two metastable states (two types of deep centers). The deep-center binding energies and the barriers separating the shallow (hydrogenic) and deep centers have been determined for both impurities. Configuration-coordinate diagrams are developed, and microscopic models for the deep centers are proposed. It is concluded that these centers are identical with the metastable DX centers in typical semiconducting crystals. Thus cadmium fluoride is the most ionic among the crystals where DX centers have thus far been found. The potential of using such crystals for optical information recording is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 2130–2136 (December 1997)     

Size effects under plastic deformation of microcrystals and nanocrystals

A theoretical analysis of size effects in plastically deformed crystals with transverse sizes in micro and nanometer ranges has been performed in the framework of the dislocation-kinetic approach. The analysis is based on the evolution equation of the dislocation density in these crystals and takes into account the generation of dislocations from surface dislocation sources and the escape of dislocations from the crystal through the crystal surface. It has been established that the generation of dislocations from the sources leads to a strong strain hardening of the crystal and that the escape of dislocations through the crystal surface results in a fast equilibration of these two kinetic processes. As a result, there occurs a strong “exhaustion” of strain hardening of thin crystals at the early stage of their plastic deformation in accordance with experiments. According to the theory, the flow stresses σ and transverse sizes D of microcrystals and nanocrystals are related by the expressions σ ∼ D ⁻ⁿ (n = 0.625–1.0), which are in agreement with the experiment.     

Coherent effects in regular three-dimensional lattices of insulator nanocrystals in an opal matrix

Samples of the nanocomposite opal + NaCl with 100% filling of first-order voids in the opal by NaCl have been obtained. The thermal conductivity of the nanocomposite has been measured in the temperature interval 4.2–300 K. It is shown that NaCl, introduced into opal, forms a regular “matrix quasilattice” of microcrystals, which leads to suppression of coherent effects and, as a consequence, of properties characteristic of massive crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 348–353 (February 1999)     

On the generation of vacancies by moving dislocations

New experiments of Molenaar and Aarts, Blewitt and others seem to confirm the view of the author, previously based only on the experiments of Gyulai and Hartly and Stepanow on sodium chloride, that vacant lattice sites, and possibly interstitial atoms, are generated during plastic flow in ductile crystals, particularly in metals. It is pointed out that the average temperatures near a moving dislocation are probably not sufficiently high to evaporate vacant lattice sites or interstitial atoms as a result of thermal effects alone. Instead, one apparently must conclude that the imperfections are generated either by purely geometrical means during the looping of dislocations about appropriate obstacles, as the result of dynamical instability in the motion of a dislocation, possibly near a jog, or in the very high thermal pulses or ‘spikes’ which are generated either in the zone where two dislocations of opposite sign annihilate one another or near impediments where dislocations are strongly curved. It is pointed out that a pair of vacancies is probably stable near room temperature and may diffuse more rapidly than a single vacancy. It is also proposed that vacancies retained during quenching of Al-Cu alloys and those generated by cold-work play an important role in the precipitation process. The origin of work hardening in single crystals is discussed and several alternative interpretations, which involve the impediment of Frank-Read generators either directly or indirectly as a consequence of the generation of vacancies, are presented. The importance of prismatic dislocations formed by condensation of vacancies is restated. The role that vacancies formed by cold-work may play in determining the stored energy and decrease in density and in affecting processes such as creep and the hardening of latent slip planes is also discussed. Finally a few experiments are proposed, typical of those which could prove decisive in isolating the influence of vacancies.     

Recombination activity of dislocations on (0 0 0 1) introduced in wurtzite ZnO at elevated temperatures

The activity for non-radiative recombination at dislocations on (0 0 0 1) basal planes was examined in wurtzite ZnO bulk single crystals. In panchromatic cathodoluminescence intensity maps, the dislocations did not exhibit apparent contrast when they were introduced at elevated temperatures of 923–1073 K, while the dislocations introduced at low temperatures (below 623 K) were observed as dark bands. It was suggested that the dislocations formed complexes involving point defects, via the thermal migration of point defects at elevated temperatures, resulting in the suppression of the recombination activity. The complexes did not influence the existing emission lines in pre-dislocated crystals.     

The effect of multiple pulse treatment on the recrystallization behavior of Mg-3Al-1Zn alloy strip

The present study first reports the application of multiple pulse treatment (MPT) for recrystallizaion of Mg-3Al-1Zn (AZ31) alloy strip. It is surprising and exciting that the completed recrystallization state of sample subjected to the electropulsing can be obtained rapidly in ∼10 s at relatively lower temperatures compared with conventional heat treatment. MPT also features excellent mechanical property and typical ductile failure thanks to ultrafine grained microstructure. The favoring mechanism of recrystallization of MPT can be attributed to the coupled actions of the thermal and athermal effects, thereinto, the latter one activates dislocations climb effectively. PACS 81.40.Ef; 84.30.Sk; 62.20 Mk; 61.72.Cc; 66.30.Qa     

Diffusive fluxes associated with interfacial defeet motion and interaction

The diffusional flux associated with the motion of interfacial defects is described by an equation expressed in terms of the topological parameters which characterise defects, namely their Burgers vectors and step heights, the defect velocity and the concentration of each atomic species in the two adjacent crystals. This expression demonstrates that glide/climb behaviour of grain boundary defects is analogous to motion of dislocations in single crystals; climb motion results if a component of b is perpendicular to the interface plane. However, the situation is more complex in the case of interphase interface defects, but the present approach, which considers the step and dislocation portions of defects separately, enables a straightforward analysis. Several examples are illustrated to show the various possibilities, such as climb motion even when b is parallel to the interface, and glide motion when b is not. The latter case arises in martensitic transformation where the existence of an invariant-plane-strain relation at the interface leads to equal and opposite fluxes to the step and dislocation portions of transformation defects so that overall the motion is diffusionless.Interfacial processes involve the motion and interaction of defects. The present analysis facilitates the consideration of diffusive fluxes associated with defect interaction since the step and dislocation portions can be treated independently. A general expression is derived for the total flux arising, and a particular case, the interaction of transformation dislocations with crystal dislocations which have reached the interface during lattice-invariant deformation in martensite formation, is considered.