Self-organization Of Dislocations Research Articles

Dislocation self-organization under single slip straining and dipole properties

Spontaneous microstructural organization under single slip is investigated by transmission electron microscopy. The formation and the structure of dislocation entanglements are analyzed on three types of fcc-based systems, Al, Cu and TiAl, all deformed by 〈1 1 0〉 {1 1 1} slip. Differences are found that depend on stacking fault energy and lattice friction. The importance of dipolar configurations is outlined. Selected properties of dipoles are analyzed theoretically under isotropic and anisotropic elasticity in cubic systems. At variance from screw and near-screw dipoles, the stress-free equilibrium angle of an edge dipole is little dependent on the material's elastic anisotropy. In Cu, for instance, a screw dipole is at equilibrium at around 59° from the slip plane, and this angle is unchanged over a range of dislocation characters of approximately ±20°. On the other hand, given a dipole height, the passing stress is a maximum in the screw orientation. It is, however, not a minimum in the edge orientation. Static and dynamic dipole properties are but little affected by dissociation down to a dipole height of the order of a few times the dissociation distance.

Dislocation micromechanisms under single slip conditions

Abstract The microstructures formed in the early stage of deformation of Cu, Al, and TiAl in single slip and at low temperatures are analysed. While dislocations are highly mobile in Cu and Al, they are subject to a significant lattice friction in TiAl. Also, they are dissociated over a fewnm in Cu, not in Al and TiAl. These various properties are manifested in the self-organization of dislocations in bundles. The role of nanometric prismatic loops in producing specific debris is emphasized in the context of the so-called collinear interaction. Possible artefacts arising from microstructural relaxation after unloading are discussed and limitations of weak-beam analyses assessed.

Equilibrium and Dynamic Properties of Dislocation Dipoles

AbstractThis paper embraces several aspects related to dislocation self-organization in single slip. The formation of prismatic loops, seminal to the nucleation and growth of dislocation braids, is a dynamical process involving elastic dipolar interactions. Preliminary simulations of the concerted process of loop generation are presented. Elastic properties of dislocation dipoles are analysed as a function of dipole character. Equilibrium properties are investigated under the isotropic and anisotropic elastic approximations. The orientation dependence of the so-called dipole passing stress is examined, and its implication in the elementary mechanisms that take place under constrained deformation conditions, such as fatigue cycling, are discussed.

Effects of metal characteristics and experimental conditions on dislocation self-organization

Dislocations produced during plastic deformation tend to gather in well defined patterns. Both the shape and the structuring degree of these clusters depend on material parameters and on loading conditions. The present work analyzes these relations and classifies the observations in a schematic structural map. While a strong correlation exists between dislocation patterns and mechanical behaviours, it is shown that there is no direct effect of the substructure on either the stress level or the work-hardening rate of the material.

Analysis of the parameters of a submicron dislocation structure in metals subjected to severe plastic deformation

Equations of dislocation kinetics are used to quantitatively compare the mechanisms of formation and evolution (with deformation) of cellular dislocation structures at moderate strains and of submicron block dislocation structures at high plastic strains. In both cases, the formation of nonuniform dislocation structures is a result of dislocation self-organization, more specifically, the self-organization of statistically random dislocations during the formation of cellular structures and the self-organization of geometrically necessary dislocations (which appear due to the nonuniform character of plastic deformation on the micron scale) during the formation of block structures.

Spectral analysis of acoustic emission during cyclic deformation of copper single crystals

Abstract Acoustic emission (AE) is investigated during fatigue at constant-plasticstrain control of copper single crystals oriented for easy glide, aiming at AE monitoring of structural transformation and dislocation self-organization during cyclic deformation. The results of AE analysis are compared with the cyclic response and hysteresis loop shape on different stages of fatigue. It is shown that the modern AE technique employing spectral analysis is sensitive to all essential structural changes in dislocation ensembles during fatigue such as matrix hardening, formation of veins and channels, persistent slip band (PSB) nucleation and development until crack initiation. The evidence is given for early identification of strain localization by means of AE. It is shown that the maximum of median frequency of the AE power spectral density at the intermediate stage of rapid hardening can be considered as an indicator of local softening followed by strain localization in PSBs. The moment of crack nucleation is...

Spectral analysis of acoustic emission during cyclic deformation of copper single crystals

Abstract Acoustic emission (AE) is investigated during fatigue at constant-plasticstrain control of copper single crystals oriented for easy glide, aiming at AE monitoring of structural transformation and dislocation self-organization during cyclic deformation. The results of AE analysis are compared with the cyclic response and hysteresis loop shape on different stages of fatigue. It is shown that the modern AE technique employing spectral analysis is sensitive to all essential structural changes in dislocation ensembles during fatigue such as matrix hardening, formation of veins and channels, persistent slip band (PSB) nucleation and development until crack initiation. The evidence is given for early identification of strain localization by means of AE. It is shown that the maximum of median frequency of the AE power spectral density at the intermediate stage of rapid hardening can be considered as an indicator of local softening followed by strain localization in PSBs. The moment of crack nucleation is...

Phase field microelasticity theory and modeling of multiple dislocation dynamics

The phase field theory and model of a multidislocation dynamics in an elastically anisotropic crystal under applied stress is developed. The proposed three-dimensional (3D) model is a particular case of our phase field microelasticity model of the stress-induced martensitic transformations. A spontaneous self-organization of dislocations in the evolving ensemble, which involves multiplication/annihilation and movement of dislocations controlled by their elastic interaction, is described by the Ginzburg–Landau kinetic equation. Examples of 3D computer simulation of dislocation dynamics are considered.

Mechanical effects of the self-organization of dislocations and related critical characteristics

The effects of the dislocation pattern formed due to the self-organization of the dislocations in crystals on the macroscopic hardening and dynamic internal friction (DIF) during deformation are studied. The classic dislocation models for the hardening and DIF corresponding to the homogeneous dislocation configuration are extended to the case for the non-homogeneous one. In addition, using the result of dislocation patterning deduced from the non-linear dislocation dynamics model for single slip, the correlation between the dislocation pattern and hardening as well as DIF is obtained. It is shown that in the case of the tension with a constant strain rate, the bifurcation point of dislocation patterning corresponds to the turning point in the stress versus strain and DIF versus strain curves. This result along with the critical characteristics of the macroscopic behavior near the bifurcation point is microscopically and macroscopically in agreement with the experimental findings on mono-crystalline pure aluminum at temperatures around 0.5Tm. The present study suggests that measuring the DIF would be a sensitive and useful mechanical means in order to study the critical phenomenon of materials during deformation.

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.

Protsessy samoorganizatsii dislokatsii i plastichnost' kristallov

Protsessy samoorganizatsii dislokatsii i plastichnost' kristallov

Self-organization approach to cyclic microplasticity: A model of a persistent slip band

Abstract The evolution of a persistent slip band is treated as a consequence of instability transitions accompanied by a process of self-organization of dislocations stored in the deformed metal crystal. The proposed qualitative model is analysed by the perturbation method. The first order perturbation analysis of the model at the state of zero hardening which simulates the disintegrating dislocation vein structure predicts an appearance of shear bands of localized slip. The second order perturbation analysis indicates that within the lamella of localized slip a new dislocation structure of the ladder type is formed. The thickness of the PSB lamella is controlled by the internal stresses caused by the ladder structure in the band.

The model of PSB formation

The evolution of persistent slip bands (PSBs) is treated as a consequence of an instability transition accompanied by a process of self-organization of dislocations stored in the deformed metal crystal. The proposed qualitative model is analysed by the perturbation method. The first order perturbation analysis of the model at the state of zero hardening, which simulates a disintegrating dislocation vein structure, predicts an appearance of shear bands of localized slip. The second order perturbation indicates that within the bands of localized slip a new dislocation structure of the ladder type may appear. The thickness of the PSBs is estimated using minimization of the elastic strain energy of the internal stresses caused by the walls of the ladder structure.