Finely striped structure at the edges of localized deformation bands

Under conditions of high-rate loading, plastic strain localization is a result of tension in the zone of interference of unloading waves rather than of thermal softening. At stresses close to the dynamic strength of the material, the microstructure of localized strain bands consists of strongly deformed material, with a large number of incipient microdiscontinuities. At stresses below the Hugoniot elastic limit, the microstructure looks as a set of barely visible stripes. The finely striped structure at the edges of the bands of a spall damage arises as a result of the stretching of initially rounded damage centers attached to the matrix material during dynamic deformation.     

Study of the relation between the thermal stability of color centers and the dislocation density in NaCl,KCl, and KBr crystals which are in the state of plastic deformation

The article relates to a study of the thermal stability of F-type color centers in NaCl, KCl, and KBr crystals which are in the state of plastic deformation. F-type color centers of two degrees of stability were established. A nonmonotonic change in the thermal stability (T_m) of the color centers of each group was observed at increasing plastic deformations.It is shown that the concentration n_v of destructive ions also changes nonmonotonically when the degree of plastic deformation increases. This change in n_v at increasing plastic deformations may be one of the main reasons for the nonmonotonic dependence of T_m upon the degree of plastic deformation in the case of the two types of color centers.     

The evolution of persistent slip bands in copper single crystals

The transformation of the so-called matrix structure into persistent slip bands (PSBs) during the fatigue of copper single crystals has been investigated by transmission electron microscopy (TEM). By cyclic pre-deformation a saturated, hard matrix structure was established which is not capable of further hardening. A sudden increase of the applied amplitude of the resolved plastic shear strain initiated the transformation of the matrix structure into PSBs. The number of deformation cycles with enlarged amplitude of resolved plastic shear strain was increased from experiment to experiment in order to obtain crystals with PSBs in consecutive stages of evolution. Surface observations indicated strain localization well before first fragments of the typical ladder-like dislocation pattern of PSBs could be identified in the bulk. From our experiments, we conclude that the transformation from the matrix structure into PSBs very likely starts from the centers of the veins which exhibit small dislocation-poor, soft areas. These areas are enclosed by a harder shell, where a high dislocation density is maintained and which may develop into first dislocation walls. During the evolution of PSBs the frequency distribution of the wall spacings narrows. This indicates that a shift of dislocation walls (1–2 nm/cycle) plays an important rôle in establishing the typical regular ladder-like dislocation pattern of well-developed PSBs.     

Influence of the degree of plastic deformation on the thermal photostability of F-centers in NaCl crystals

The effect of plastic deformations on the thermal stability and thermal photostability of F-centers in NaCl crystals was examined. The thermal photostability of F-centers and the thermal stability of some of these centers increases in proportion to the degree of plastic deformation. The concentration of the less stable F-centers increases due to plastic deformation. The effect was studied in crystals which were deformed before and after x-ray excitation. The results were interpreted from the viewpoint of an ionic activation mechanism in the thermal and photothermal decomposition of the color centers in alkalihalide crystals.The authors are indebted to Yu. L. Lukantsever for an evaluation of the results of this work.     

Strain-rate-induced bcc-to-hcp phase transformation of Fe nanowires

Using molecular dynamics simulation method, the plastic deformation mechanism of Fe nanowires is studied by applying uniaxial tension along the [110] direction. The simulation result shows that the bcc-to-hcp martensitic phase transformation mechanism controls the plastic deformation of the nanowires at high strain rate or low temperature; however,the plastic deformation mechanism will transform into a dislocation nucleation mechanism at low strain rate and higher temperature. Furthermore, the underlying cause of why the bcc-to-hcp martensitic phase transition mechanism is related to high strain rate and low temperature is also carefully studied. Based on the present study, a strain rate-temperature plastic deformation map for Fe nanowires has been proposed.     

Kinetics of Macrolocalization Patterns of Plastic Flow of Metals

The features of the macroscopic inhomogeneity of plastic deformation in the form of autowaves with a pulsating amplitude are analyzed, and data on the localization of sources of acoustic emission at different stages of plastic flow in the stretching of fcc mono- and polycrystals are presented. The relationship between the local components of the plastic distortion tensor in the strain localization zone is traced. The role of acoustic phenomena accompanying the localization of plastic strain in the development of the process of plastic deformation is considered.     

Plastic flow localization upon stretching Zr-1% Nb alloy

The stages in the plastic flow curve and the localization of plastic strains upon stretching Zr-1% Nb alloy are considered. The localization pattern is found to correlate with a strain hardening law upon plastic flow. Data for the dislocation structure in strain localization regions are reported.     

Plastic Strain Localization and Its Stages in Al-Mg Alloys

The paper describes an experimental technique based on the use of a Vic-3D contactless digital optical system and digital image correlation for research in the mechanical behavior of a solid and its plastic deformation with space-time inhomogeneities. Using this technique, we analyze the evolution of inhomogeneous strain and local strain rate fields in AMg2m alloy at constant uniaxial tension rates. The analysis reveals quasi-periodic strain field homogenization in jerky flow: alternating phases of active local plastic flow (shear banding) and macroscale strain levelling. Also analyzed are the parameters of localized microscale plastic flow such as the height and width of shear bands, their velocity, and coefficient of plastic strain inhomogeneity. From a series of mechanical tests, the influence of the specimen geometry and loading rate on these parameters is estimated.     

The effect of the strain amplitude changes on the stress amplitude and resistivity of torsionally fatigued copper

Polycrystalline copper wires were cyclically strained in torsion in liquid nitrogen and the effect of the strain amplitude changes on the stress amplitude and resistivity was followed. It was found that both stress amplitude and resistivity are able to increase or decrease in dependence on the applied plastic strain amplitude. When the strain amplitude was decreased, the reversibility was not complete. By means of intermediate annealing it was found that both the dislocation density and the point-defect concentration follow the changes of the plastic strain amplitude.     

On the localization of plastic flow under compression of NaCl and KCl crystals

The plastic flow localization patterns for alkali halide crystals under compression are investigated. The main spatiotemporal regularities of the strain localization at the stages of deformation hardening in these single crystals are established. The relation is traced between the orientation of localized strain zones and the crystallography of slip systems of the test specimens at the initial stages of plastic deformation. The velocity of motion of localized strain zones under compression is determined.     

Variations of work function and corrosion behaviors of deformed copper surfaces

Surface work function (WF) and the corrosion behavior of copper under influence of plastic strain were investigated using experimental and computational approaches. It was observed that both the corrosion potential (Ecorr) and the WF decreased while the corrosion rate increased with an increase in plastic strain, indicating that the strained surface layer became more electrochemically active. Ecorr and WF eventually became stable when the plastic strain reached a certain level. However, the corrosion rate continuously increased. It was demonstrated that this continuous increase in corrosion rate could be dominated by the dislocation density rather than the corrosion potential. The study has shown that the WF is closely related to the corrosion potential and could thus be a sensitive parameter for investigating mechanisms responsible for corrosive wear. The investigation of the effects of plastic deformation on the corrosion behavior would help to fundamentally understand the synergism of wear and corrosion.     

Plastic strain localization and necking in Zr-Nb alloy

Plastic strain macrolocalization at the parabolic stage of strain hardening in Zr-1 wt% Nb alloy is studied. The plastic flow is found to be unstable, which shows up as the oscillatory periodic space-time variation of the local strain distribution. The results are discussed in terms of a synergetic model of plastic flow evolution.     

受驱无序胶体动力学

利用Langevin分子动力学，本数值研究点钉扎中心随机分布的二维胶体动力学．随着钉扎中心强度的提高，我们发现了从弹性脱钉到塑性脱钉的渡越，并伴随临界钉扎力在渡越区的明显提高，类似于超导体中的峰值效应．另外，我们首次发现：当塑性流动发生时，高速运动胶体粒子感受到的平均钉扎力在从玻璃态到液态的转变过程中会出现峰值效应，并伴随有速度-驱动力曲线的交叠．     

Effect of the fields of randomly spaced charged centers on the luminescence polarization

This paper considers the effect produced by randomly spaced charged centers in a crystal on the luminescence polarization associated with Jahn-Teller centers under conditions of uniaxial strain.     

Macrolocalization of plastic strain in creep of fine-grain aluminum

The evolution of the plastic strain macrolocalization pattern in low-temperature creep of commercial purity aluminum is studied. The localization pattern depends on a stage in the creep curve. At the stage of steady-state creep, localization zones propagate in the form of a wave traveling with a velocity proportional to the rate of buildup of the total strain. It is found that the volumes where the creep and strain localization wave propagation are activated equal each other. Based on estimates of the activation volumes, it is shown that the velocity of plastic strain localization waves is governed by thermally activated dislocation movement.     

On the plastic equation of state with one structure variable

Summary The general expression for a plastic equation of state with one structure variable and with a scaling relationship is presented. It is demonstrated that this equation can be written, in general, in terms of stress, plastic strain rate and a function of plastic strain. Finally, the results are compared with previous formulations of the problem presented in the literature to show that they are particular cases of the general formalism presented. The authors of this paper have agreed to not receive the proofs for correction.     

Characteristics of plastic flow localization autowaves

The dispersion relations for localized plastic strain autowaves are considered for some metals at the stages of easy slip and linear strain hardening. The quadratic form of this relation, the type of the dependences of the phase and group velocities of such autowaves on the wavenumber, and the quantitative relation between the characteristics of plastic flow localization wave processes and elastic wave parameters in solids being deformed are established and explained. An invariant for strain effects at micro- and macroscopic levels is introduced.     

NUMERICAL STUDIES ON THE DYNAMICS OF DISORDERED VORTEX LATTICE

Flow behavior of the driven two-dimensional vortex lattice is numerically studied with different densities of randomly distributed pointlike pinning centers. Different features in the curves of velocity-force dependence are found between the elastic and plastic regimes. Scaling fit between force and velocity above the critical driving force can be obtained in the elastic regime but fails in the plastic regime. Transition from the lastic to plastic regimes is accompanied by maximum peaks in the differential curves of velocity-force dependence in the disordered vortex lattice.     

The influence of a static magnetic field up to 15 T on the manifestation of the Portevin-Le Chatelier effect in NaCl: Eu crystals

The influence of a static magnetic field on the instability of plastic flow (the Portevin-Le Chatelier effect) is revealed in NaCl: Eu quenched crystals. It is found that, in an external magnetic field, the yield stress of the crystals is reduced, the probability of plastic strain jumps and their amplitude decrease, and the amplitude distribution of the plastic strain jumps becomes random. The number of shear bands formed on the surface of crystals strained in the magnetic field is halved as compared to that observed without a magnetic field.     

Effects of elastic and plastic deformations on the electron work function of metals during bending tests

The high sensitivity of the electron work function (EWF) to surface conditions has attracted increasing interest in the application of the electron work function (EWF) to investigate tribological phenomena using the Kelvin probing technique. In this study, the correlation between the EWF and both the elastic and plastic deformation of copper and aluminium during bending tests was investigated. It was demonstrated that, in the elastic range, tensile strain decreased the EWF, while compressive strain increased the EWF. However, in the plastic range, the EWF always decreased with plastic deformation irrespectively of whether it was tensile or compressive. The mechanism responsible for the variation in EWF with elastic and plastic deformation is analysed and discussed.