A decrease in the mobility and multiplication of twinning dislocations in bismuth crystals exposed to constant magnetic field

Simultaneous application of a constant magnetic field and a concentrated load partly suppresses twinning in bismuth crystals by decreasing the mobility and multiplication of twinning dislocations. This significantly decreases both the total twinned volume and the total area of the twin-matrix boundaries.     

Influence of electric-current pulses on the operation of sources of twinning dislocations in bismuth single crystals

The influence of electric-current pulses on the operation of different sources of twinning dislocations is discussed. It is established that an electric current stimulates it and decreases the critical stresses needed for the onset of generation of dislocation loops. Mozyr State Pedagogical Institute, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 72, No. 3, pp. 592–594, May–June, 1999.     

The effect of nitrogen on the structure and mobility of dislocations in Fe-Ni-Cr austenite

The structure and the critical resolved shear stress for the motion of the straight a/2110 edge and screw dislocations in Fe-Ni-Cr and Fe-Ni-Cr-N austenite have been analysed using the conjugate gradient method to minimize the potential energy of the crystal and the embedded atom method to quantify atomic interactions at 0 K. In Fe-Ni-Cr austenite both the edge and the screw dislocations dissociate along one of the {111} planes forming a stacking-fault ribbon. The ribbon widths are comparable to their values calculated using continuum theory. Dissociated edge and screw dislocations require very similar levels of shear stress for their motion. In Fe-Ni-Cr-N austenite, the structure of the dislocation core of the a/2110 edge dislocation does not seem to be significantly affected by the presence of nitrogen. In sharp contrast, the core structure of the dissociated a/2110 dislocation undergoes a major change, resulting in spreading of the core on to two or more non-parallel planes. As a result, a significantly higher level of stress is required for the motion of a screw than an edge dislocation. Under certain conditions the interaction of nitrogen atoms with screw dislocations can result in pinning of the dislocations. The potential mechanism for the motion of the pinned screw dislocations by formation and motion of edge-type kinks is briefly discussed.     

Influence of dislocations and plasticity on the tensile behaviour of flax and hemp fibres

This paper presents finite element analyses of the tensile behaviour of elementary fibres of flax and hemp, focusing on the non-linear tensile behaviour of the fibres and the relationship between the stiffness and the diameter of the fibre. The non-linear tensile behaviour is modelled by introducing dislocations in the helical structure of the cellulose fibrils and assuming that the hemicellulose has an elastoplastic constitutive relation. The relationship between the elastic stiffness and the diameter of the fibres is analysed similarly, using an elastic constitutive behaviour of the hemicellulose. The results agree with experimental results found in the literature.     

Influence of dislocations on magnon-phonon couplings: A phenomenological approach

On the basis of a nonlinear theory of finitely deformable elastoviscoplastic ferromagnetic crystals developed in a companion paper, the present work presents an attempt at a phenomenological study of the influence of dislocations and viscoplastic flow on the behavior of spin waves (the collective modes of oscillations typical of ferromagnetism). This is achieved by linearizing the above mentioned nonlinear theory about a fundamental ferromagnetic phase with a practically vanishing viscoplastic threshold. The main results obtained after a study of wave modes and asymptotic evaluations in terms of a piezomagnetic coupling parameter are the evidence of a magnetoacoustic resonance between spin waves and left circularly polarized transverse elastoviscoplastic disturbances, a slight shift towards higher wave numbers of the corresponding critical wave number as compared to the perfectly elastic-crystal case and the fact that spin waves suffer a damping which is directly proportional to the piezomagnetic coupling parameter and to the reciprocal primary relaxation time (the relaxation time associated with the viscosity processes inherent in viscoplasticity, in the absence of restoring effects.     

Influence of heat-treatment and solute content on repeated precipitation at dislocations in Al-Cu alloys

The influence of heat-treating parameters and Cu content on repeated precipitation of the phase which occurs at climbing dislocations during quenching dilute Al-Cu alloys has been studied by transmission electron microscopy. In Al-3.85 wt % Cu the process occurs during quenching from all temperatures within the solid solution range to all temperatures in the range from room temperature to at least 300° C. It also occurs over a wide range of quench rates. Depending on the quench rate, it can occur at a variety of dislocation geometries, including climb sources, glide dislocations which climb off their slip planes, and collapsed vacancy loops. Quenching directly to the ageing temperature establishes the distribution of almost the entire volume fraction of by repeated precipitation during the quench. This process does not occur in alloys with Cu content below 1.5 ± 0.5 wt %.     

Influence of nonlinear coupling of pulses on spatio-temporal compression

Abstract

Properties of a novel configuration of an optical (spatio-temporal) pulse compressor, that is based on a Kerr-type planar waveguide into which two pulses are simultaneously launched, are studied. It is assumed that the pulse which is the subject of the compression propagates in the anomalous dispersion regime, while the auxiliary pulse is in normal dispersion. The best parameters of the proposed compressor are obtained when duration of the auxiliary pulse is so large that this dispersion can be neglected, while energy of the second pulse is above the threshold of first-order soliton generation. It is observed that in such a configuration the compression occurs simultaneously with the generation of a soliton-like solution. It is argued that the proposed configuration with two simultaneously propagating pulses has advantages over the configuration with a single pulse, namely the maximal compression factor and the optimal length of the compressor is, respectively, more than 3 times larger and, at least, 10 times greater than the corresponding values of the compressor with a single pulse. It is also demonstrated that such a compressor can be considered as a universal device, since its operation depends only slightly on the initial parameters of the pulse subject to the compression.     

Generation of femtosecond current pulses using the inverse magneto-optical Faraday effect

A new method of generating ultrashort current pulses is proposed that is based on the optical pumping of a mesoscopic structure comprising a metal ring with a core made of a material possessing giant magneto-optical susceptibility. The main dynamic characteristics of the proposed device are calculated.     

Topological current structure of dislocations in the 4-dimensional gauge field theory of disclocation and disclination continuum

The topological current structure of dislocations is studied by means of decomposition of gauge potential in the 4-dimensional gauge field theory of dislocation and disclination continuum. The dislocations in defect continuum are globally classified in terms of winding numbers and locally characterized by Brouwer degrees and Hopf indices. It is shown that the topological dislocation current corresponds to the motion current of a set of strings, in which the strings are just the dislocation lines. And the topological dislocation current can be expressed as the cross product of the topological disclocation density and the velocity of the strings.     

Plastic deformation in electrical conductors subjected to short-duration current pulses

In this paper, we consider the viscoplastic response of Al 6061-T6 and Cu-102 when subjected to a combination of mechanical load and high-intensity electric current. The specimens are subjected to mechanical loading under fixed-grip and dead-load conditions; in addition, the specimen is subjected to a nearly sinusoidal current pulse (frequency 4 kHz, duration ∼1 ms, and intensity ∼10⁹ A/m²). The resulting temperature increase causes the yield stress to drop and enables accumulation of plastic strain. A viscoplastic model is used to simulate the process; comparisons of the simulation results to time resolved measurements of strain and temperature are used to calibrate the viscoplastic model.     

Influence of deposition parameters on hole mobility in polymorphous silicon

Time-of-flight transient photoconductivity measurements reveal a monotonic increase with the deposition pressure in the hole mobility in polymorphous silicon for samples deposited under hydrogen dilution. With helium dilution, a maximum mobility that matches the highest value from H-dilution samples is measured at the intermediate pressure of 1.4 Torr. The deposition rate of those samples is twice the rate for the H-dilution ones. For the samples with the best hole mobilities, the valence-band tail is comparable to the one of standard hydrogenated amorphous silicon.     

Ionic current pulses upon field desorption of cesium from graphene on iridium

Pulses of Cs⁺ ion current upon field desorption in constant electric field from an iridium point coated with a graphene film have been detected. Pulses with durations of less than 0.1 s were observed upon sputter deposition of cesium atoms onto graphene followed by their removal from the surface. During the pulse, one could observe a flash on the field desorption microscope display occupying all area of the images of {001} and {111} facets. Possible mechanisms of the flash formation using models of cesium intercalation in the graphene layer and the yield of cesium atoms from under the film under the action of the applied electric field of the electric field have been considered.     

Melting and crack growth in electrical conductors subjected to short-duration current pulses

In this paper, we examine the response of a crack tip in an electrically conducting material subjected to a combination of mechanical load as well as a high density electrical current. We present a detailed examination of the process of evolution of melting and ejection, as revealed by high speed photography. The critical mechanical and electrical parameters that govern crack extension are then determined for two different alloys. Finally, we present an evaluation of the phenomenon through a coupled field simulation to examine the nature of the interaction between the electric field and the thermo-mechanical response.