On the correspondence between a screw dislocation in gradient elasticity and a regularized vortex

We show the correspondence between a screw dislocation in gradient elasticity and a regularized vortex. The effective Burgers vector, nonsingular distortion and stress fields of a screw dislocation and the effective circulation, smoothed velocity and momentum of a vortex are given and discussed.     

The effect of pressure on transformation temperatures and some physical parameters of Fe–32Mn–6Si–3Cr shape memory alloy

An experimental investigation of the effect of pressure on shape memory behavior of Fe–32%Mn–6%Si–3%Cr alloy was undertaken. There are significant differences in the Gibbs free energy, dislocation density and transformation temperatures of the alloy due to applied pressure.     

The mechanisms governing the activation of dislocation sources in aluminum at different strain rates

This article examines the time to activate Frank–Read sources in response to macroscopic strain rates ranging from 10¹ s⁻¹ to 10¹⁰ s⁻¹ in aluminium under athermal conditions. We develop analytical models of the bowing of a pinned dislocation segment as well as numerical simulations of three dimensional dislocation dynamics. We find that the strain rate has a direct influence on both the activation time and the source strength of Frank–Read sources at strain rates up to 10⁶ s⁻¹, and the source strength increases in almost direct proportion to the strain rate. This contributes to the increase in the yield stress of materials at these strain rates. Above 10⁶ s⁻¹, the speed of the bowing segments reaches values that exceed the domain of validity of the linear viscous drag law, and the drag law is modified to account for inertial effects on the motion of the dislocation. As a result the activation times of Frank–Read sources reach a finite limit at strain rates greater than 10⁸ s⁻¹, suggesting that Frank–Read sources are unable to operate before homogeneous nucleation relaxes elastic stresses at the higher strain rates of shock loading. Elastodynamic calculations are carried out to compare the contributions of Frank–Read sources and homogeneous nucleation of dislocations to plastic relaxation. We find that at strain rates of 5×10⁷ s⁻¹ homogeneous nucleation becomes the dominant generation mechanism.     

Electronic properties of dislocations introduced mechanically at room temperature on a single crystal silicon surface

This paper focuses on the effects of temperature and environment on the electronic properties of dislocations in n-type single crystal silicon near the surface. Deep level transient spectroscopy (DLTS) analyses were carried out with Schottky electrodes and p⁺–n junctions. The trap level, originally found at E_C−0.50 eV (as commonly reported), shifted to a shallower level at E_C−0.23 eV after a heat treatment at 350 K in an inert environment. The same heat treatment in lab air, however, did not cause any shift. The trap level shifted by the heat treatment in an inert environment was found to revert back to the original level when the specimens were exposed to lab air again. Therefore, the intrinsic trap level is expected to occur at E_C−0.23 eV and shift sensitively with gas adsorption in air.     

Analysis of a cracked concrete containing an inclusion with inhomogeneously imperfect interface

The distributed dislocation technique is applied to determine the behavior of a cracked concrete matrix containing an inclusion. The analysis of cracked concrete in the presence of inclusions such as steel expansions is a practical problem that needs special attention. The solution to the problem of interaction of an edge dislocation with a circular inclusion having circumferentially inhomogeneously imperfect interface is available in the literature. This analytical solution is used in the distributed dislocation technique to obtain the stress intensity factor for the cracked concrete in the presence of inclusion. The interface of the matrix and the inclusion is assumed inhomogeneously imperfect and the stress intensity factor is determined for the cracked concrete for a case of two identical cracks on diametrically opposite sides of the inclusion. Consideration of this general inhomogeneously imperfect interface is the contribution of this paper. The variation of the inhomogeneity parameters is studied and presented. Additionally, the general assumption for the interface is simplified to the special case of perfectly bonded interface. The observations for the perfect interface are coincident with the previously reported results.     

Dislocation theory as a physical field theory

Dislocations are the elementary carriers in many situations of plastic flow. Since they can be seen, counted and typified, e.g. in the electron microscope, and since their presence changes the state of the (elastoplastic) medium, the dislocations have the status of a physical state quantity. In spite of this a continuum theory of elastoplasticity can be built up which does not use the concept of dislocation. However, disregarding the dislocations implies approximation, and this approximation is not always good, in spite of the smallness of the dislocation diameter (order atomic distance). It is discussed how a field theory of dislocations can be developed which takes full account of the dislocational degrees of freedom.

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Sommario In molte situazioni le dislocazioni sono i portatori puntiformi del flusso plastico. Poiché esse possono essere viste, contate e classificate, per esempio mediante il microscopio elettronico, e poiché la loro presenza cambia lo stato del mezzo (elastoplastico), le dislocazioni svolgono il ruolo di una variabile di stato. Nonostante ciò, si può formulare una teoria dell'elastoplasticità che non usa il concetto di dislocazione. Tuttavia, il trascurare le dislocazioni comporta un'approssimazione, e tale approssimazione non è sempre buona nonostante la piccolezza delle dislocazioni (dell'ordine della distanza interatomica). Si discute come si possasviluppare una teoria di campo delle dislocazione che tenga conto completo del loro grado di mobilità.

     

On physical foundations of plasticity

Complexity of plastic response is caused by the fact that plastically deformed solids are composites with a changing structure. At the mesoscale (m) a pattern of high and low dislocation density regions is spontaneously formed. The evolution of the composite structures is treated as a consequence of the structuralization process governed by four types of mechanisms: (i) the overproduction of dislocation, (ii) the screening of their elastic fields, (iii) the formation of dipolar dislocation patterns, (iv) the instability transition leading to the formation of subgrain boundaries, dislocation grids, microshear bands, or persistent slip bands. The proposed theoretical model consists of the balance law for the dislocation density of stored dislocations, the relations which determine the average shape of glide dislocations, and the continuum mechanics equations for stress and strain. To demonstrate the capacity of the model some available solutions simulating the mesoscale structure evolution are briefly reviewed.

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Sommario La complessità della risposta plastica è dovuta al fatto che i solidi soggetti a deformazioni plastiche sono corpi compositi di struttura variabile. A mesoscala (m) si forma spontaneamente un insieme di regioni di alta e bassa densità di dislocazioni. Si tratta il comportamento delle strutture composite come conseguenza di un processo di strutturalizzazione governato da quattro tipi di meccanismi: (1) sovrapproduzione di dislocazioni; (2) la schermatura del loro campo elastico;(3) la formazione di tipi di dislocazioni dipolari;(4) transizione di instabilità. Il modello teorico proposto è composto dalle equazioni di bilancio per la densità di dislocazione delle dislocazioni immagazzinate, dalle relazioni che determinano l'andamento medio delle dislocazioni di scorrimento e dalle equazioni della meccanica dei continui per sforzi e deformazioni. Per controllare la validità del modello, si riesaminano brevemente alcune soluzioni conosciute che simulano l'evoluzione della struttura a mesoscala.

     

The gauge theory of dislocations: A nonuniformly moving screw dislocation

We investigate the nonuniform motion of a straight screw dislocation in infinite media in the framework of the translational gauge theory of dislocations. The equations of motion are derived for an arbitrarily moving screw dislocation. The fields of the elastic velocity, elastic distortion, dislocation density and dislocation current surrounding the arbitrarily moving screw dislocation are derived explicitly in the form of integral representations. We calculate the radiation fields and the fields depending on the dislocation velocities.     

The role of dislocation walls for nanoindentation to shallow depths

Dislocation events are seen as excursions or pop-in events in the load–displacement curve of nanoindentation experiments. Two nanoindenters have been used to examine the difference between quasi-static and dynamic loading during indentation. Yield excursions were present in the load–displacement curves of both the statically and dynamically loaded single crystal nickel samples. Only one major excursion occurred in each quasi-static indent, nominally loaded at 100 μN/s while staircase yielding was observed under dynamic loading indentation with a 45 Hz oscillation of 2 nm superimposed on a 60 μN/s loading rate. Thermal activation analysis is used to explain the arrest and reinitiation of the yielding with activation volumes being modeled. For nanoindentation experiments differences between quasi-static and dynamic loading are described by the models presented. It is proposed that insight into the plastic deformation mechanisms associated with such plastic instabilities will provide one of the keys to length scale effects necessary to understanding nanostructures.