Chernov Luders Research Articles

Structure of Chernov–Luders Band in Normalized Steel 09G2S. Part 1. Bands of Localized Strain

Structure of Chernov–Luders Band in Normalized Steel 09G2S. Part 1. Bands of Localized Strain

Effect of Strain Aging on Stages of Plastic Strain and Tensile Fracture of Specimens of Steel 08G2B. Part I. Yield Plateau and the Stage of Strain Hardening

Mechanisms of Luders deformation in tensile tests of specimens of steel 08G2B with ultrafine ferrite-bainite structure are studied by the method of digital image correlation. The fields of displacements of eyy and exy and their distribution over the length of a specimen are determined. Two kinds of Luders deformation differing in the behavior of the Chernov–Luders bands are detected. The results of the measurement of microhardness in Chernov–Luders bands and of the EBSD analysis are used to determine the interrelation between the strain aging effect and the variation of the characteristics of strength (σy , σr ) and ductility (δpl , δu , δ, n ) at tension.

Development Kinetics of the Plastic Wave Front at the Metal Interface

The paper considers an inhomogeneous development of plastic strain at the Chernov–Luders band front in the bimetal consisting of carbon and austenitic steel layers. It is found that in the low carbon steel basal layer, the distribution of localized deformation at the yield point represents two zones similar to the Chernov–Luders band. In the austenitic steel coating layer, the distribution of localized deformation at the yield point represents two plastic wave fronts similar to the Portevin–Le Chatelier effect. The paper proposes a mechanism of nucleation of localized deformation bands in the frame of the model of the solid fracturing.

A Study of Plastic Strain Before and on the Yield Plateau of Steel 08G2B by Correlation of Digital Images. Part II. Functioning of Flow Channels and Zones in them

Regular features of Luders strain due to appearance and propagation of one Chernov–Luders band (CLB) in superfine-grained steel 08G2B are investigated. Active, relaxation and peripheral zones are distinguished in the source of strain uniting the CLB nucleus; go-ahead-activated flow channels are defined. The functions of the zones are considered. Growth in the dislocation density and in the energy spent in the zones upon motion of the CLB front is evaluated. Mechanisms of appearance of new slip bands in the CLB and of unlocking of dislocations are analyzed.

Structure and Plastic Flow Heterogeneities of the 12Kh18N9T Steel–St3 Steel Bimetal during Tension

The plastic deformation of a three-layer bimetallic composite, namely, structural low-carbon St3sp steel clad by an austenitic corrosion-resistant 12Kh18N9T steel on both sides is studied. The laws of Chernov–Luders band propagation during uniaxial tension and the load that corresponds to the yield strength are considered.

Luders deformation of low-carbon steel

The development of Chernov–Luders bands on elastoplastic transition in low-carbon steel is investigated. The main factors responsible for the creation and development of the bands are identified. The kinetics of the mobile band boundaries (fronts) is of particular interest. The characteristic speeds are determined. The nucleation rate of Chernov–Luders bands exceeds their expansion rate by more than an order of magnitude. The simultaneous development of more than one band, with the appearance of several moving fronts, is considered. In all cases, the fronts of the Chernov–Luders bands move at matched speeds, so that, at any time, the generalized expansion rate of the deformation zone is constant. The influence of the strain rate on the kinetics of the band fronts is analyzed. Both the generalized expansion rate of the deformed zone and the speeds of individual fronts increase with increase in the loading rate. This is a nonlinear dependence (a power law). The fronts of the Chernov–Luders bands are complex in structure. Different sections of the front may move at nonuniform speeds, so that the front is locally distorted and split. Ahead of the front, in the undeformed sample, precursors whose configuration resembles that of the incipient Chernov–Luders bands may be observed. When they meet, the fronts of adjacent bands cancel out. Annihilation of the band fronts is a complex process, characterized by the formation of precursors and secondary diffuse Chernov–Luders bands. These findings indicate that the simplified concept of the Chernov–Luders bands as a deformed region in a loaded sample, as the front of a band, or as the boundary between deformed and undeformed zones must be revised. A microscopic theory of Luders deformation is based on the cascade growth in density of mobile dislocations on account of their breakaway from the points of attachment and their subsequent multiplication, which occurs instantaneously at the upper yield point within the crystallite (grain). At the same time, the formation of a mobile macroscopic strain front calls for the transfer of plastic deformation by adjacent grains, without strengthening. In other words, grain-boundary accommodation is required. The results obtained suggest that the Chernov–Luders band is such an accommodation zone, and so it has a complex structure.

Chernov–Lüders bands and the Portevin–Le Chatelier effect as plastic flow instabilities

The development of macroscopic plastic flow heterogeneities in metals in the form of Chernov–Luders bands and the Portevin–Le Chatelier effect is studied. The main laws of deformation development in these two cases are found, and the kinetics of the motion of Chernov–Luders band fronts and the serrated deformation during the Portevin–Le Chatelier effect is investigated. It is shown that both versions of heterogeneities can be considered as macroscopic autowave processes of switching and excitation in deformable media.

Chernov–Luders and Portevin–Le Chatelier deformations in active deformable media of different nature

The development of macroscopic inhomogeneities in the form of Chernov–Luders bands and serrated deformation bands (Portevin–Le Chatelier effect) in plastic metal flow is studied. For these two cases, regularities in the development of deformation inhomogeneity were established and the kinetics of motion of the fronts of Chernov–Luders bands and serrated deformation bands was studied. It is shown that the Chernov–Luders fronts and the serrated Portevin–Le Chatelier deformation can be considered as macroscopic autowave switching and excitation processes, respectively, in deformable media of different nature.

Plastic flow instability: Chernov–Lüders bands and the Portevin—Le Chatelier effect

We have studied the regularities in the evolution of macroscopic nonuniformities in the plastic flow of metals in the form of the Chernov–Luders bands and the Portevin—Le Chatelier effect. The regularities in the evolution of strain inhomogeneity in these two cases have been established, and the kinetics of motion of the Chernov–Luders fronts and abrupt deformation in the Portevin–Le Chatelier effect has been analyzed. It has been shown that the Chernov–Luders fronts and Portevin–Le Chatelier jumpwise straining can be treated as macroscopic autowave processes of switching and excitation, respectively, in deformed media of various origins.

INVESTIGATION OF LÜDERS DEFORMATION IN THE MILD STEEL

The features of macroscopic strain inhomogeneity in the form of Chernov–Luders bands development on elastic-plastic transition were investigated in the mild steel. The main regularities of nucleation and propagation of the bands are established. Particular attention was paid to the kinetics of moving boundaries (fronts) of bands, characteristic velocities were determined. It is shown that the rate of formation of nucleus of the Chernov–Luders band is more than an order of magnitude higher than the rate of its expansion. Situations are considered when more than one band develops simultaneously in the object and therefore several moving fronts are observed. It is established that in all cases the velocities of fronts of the Chernov–Luders bands are mutually consistent so that at any instant the generalized rate of expansion of the deformed zone is a constant value. The effect of the deformation rate on the kinetics of the Chernov–Luders fronts was analyzed. Both the generalized rate of expansion of the deformed zone and the speed of individual fronts increase with the increase of the loading rate. A nonlinear (power-law) character of this dependence is established. The fronts of the bands have a complex structure. Different parts of the front can move with unequal velocities, so that the front line is locally curved and split. Ahead of the front, in the undeformed part of the sample, the forerunners may appear, the configuration of which resembles the Chernov–Luders bands nuclei. When encountering the fronts of adjacent bands are annihilated. Annihilation of the fronts is a complex process, which is also characterized by the formation of a precursor and secondary diffusion Chernov–Luders bands. These facts demonstrate that a simplified view of the Chernov-Luders band as a deformed region in a loaded sample, and the front of the band as a boundary between deformed and undeformed zones, should be revised. The microscopic theory of Luders deformation is based on the avalanche growth of the density of mobile dislocations due to breaking from an obstacles and subsequent multiplication, which is realized simultaneously at the upper yield point within the crystallite (grain). At the same time, to form a mobile macroscopic deformation front it is necessary that plastic deformation should be transferred to neighboring grains without hardening, that is, grain-boundary accommodation is needed. The results obtained in the paper suggest that such a zone of accommodation is apparently the Chernov–Luders band front, and, therefore, it has a complex structure.

Localized plastic flow and spatiotemporal distribution of acoustic emission in steel

A relationship between the macroscopic localization of plastic flow and the spatiotemporal distribution of acoustic emission has been experimentally established in low-carbon steel deformed via the development of the Chernov-Luders (CL) band followed by the parabolic strain hardening. It is shown that, as the CL band front propagates at a constant velocity in the sample, the pattern of acoustic emission is different at various stages of motion of this strain localization focus. Immobile foci of the localized flow, which appear at the stage of parabolic strain hardening, are spatially related to the inhomogeneity of strain developed during the CL band propagation.

THE DEPENDENCE OF THE FORMATION PROCESSES OF DEFORMATION BANDS FROM THE STRUCTURAL PARAMETERS OF LOW-CARBON STEELS AFTER THE ANNEALING

The causes of directionaty of the yield point, stress of microyielding, and Chernov–Luders deformation in lowcarbon steel have been investigated. The observed of Chernov–Luders deformation be haviour is considered to be the result of a directional variation in the grain size of berrite to cause formation dislocation cell structure.