Studying Deformation Behaviors in Austenitic Stainless Steels within a Temperature Range of 143 K < T < 420 K

This work deals with studying staging and macroscopic strain localization in austenitic stainless steel 12Kh18N9T within a temperature range of 143 K < T < 420 K. The visualization and evolution of macroscopic localized plastic deformation bands at different stages of work hardening were carried out by the method of the double-exposure speckle photography (DESP), which allows registering displacement fields with a high accuracy by tracing changes on the surface of the material under study and then comparing the specklograms recorded during uniaxial tension. The shape of the tensile curves σ(ε) undergoes a significant change with a decreasing temperature due to the γ-α'-phase transformation induced by plastic deformation. The processing of the deformation curves of the steel samples made it possible to distinguish the following stages of strain hardening, i.e. the stage of linear hardening and jerky flow stage. A comparative analysis of the design diagrams (with the introduction of additional parameters of the Ludwigson equation) and experimental diagrams of tension of steel 12Kh18N9T for different temperatures is carried out. The analysis of local strains distributions showed that at the stage of linear work hardening, a mobile system of plastic strain localization centers is observed. The temperature dependence of the parameters of plastic deformation localization at the stages of linear work hardening has been established. Unlike the linear hardening, the jerky flow possesses the propagation of single plastic strain fronts that occur one after another through the sample due to the γ-α' phase transition and the Portevin-Le Chatelier effect. It was found that at the jerky flow stage, which is the final stage before the destruction of the sample, the centers of deformation localization do not merge, leading to the neck formation.

Experimental study of the Portevin-Le Chatelier effect under complex loading of Al-Mg alloy: procedure issues

The aim of this work is to solve the methodological issues of the experimental study of the nucleation and propagation of deformation bands due to the Portevin-Le Chatelier effect under conditions of complex loading. It is of interest to determine the boundaries of unstable plastic deformation of the AMg6 alloy under complex loading conditions. A technique for controlling the loading process with a given rate of deformation intensity of materials has been worked out. The results showed that short-term stops and unloading during loading influence on the value of critical deformation, at which the manifestation of the jerky flow begins. The evolution of inhomogeneous strain fields and local strain rates under conditions of manifestation of jerky flow during tension with torsion tests of Al-Mg alloy samples.

Kinetics of Plastic Deformation Localization Bands in Polycrystalline Nickel

Jerky flow has recently aroused interest as an example of complex spatiotemporal dynamics resulting from the collective behavior of defects in Al- and Mg-based alloys under loading. This paper presents the results of the study of the macroscopic strain localization kinetics in Nickel 200 (99.5 wt % purity). Uniaxial tension of flat samples is monitored at room temperature in the load–unload mode at a constant strain rate and total deformation increment up to 5%. The stress–strain curves reveal jerky flow from the yield point to the formation of the neck. The digital speckle correlation method evidences the movement of localized plastic deformation bands under the conditions of the Portevin–Le Chatelier effect (PLC). It is shown that stress drops during jerky flow in Ni are accompanied by the formation of morphologically simple single PLC bands. It is established that, with an increase in total deformation, the number of PLC bands and their velocity of motion along the sample decrease, while their time period increases. Moreover, an increase in total deformation leads to an increase in the parameters of the force response (i.e., time period and stress drop magnitude). It is found that the criterion of damage for PLC bands as a function of the total strain has a sigmoidal shape.

Strain Hardening of Low-Carbon Steel in the Area of Jerky Flow

Purpose. The aim of this work is to assess the effect of ferrite grain size of low-carbon steel on the development of strain hardening processes in the area of nucleation and propagation of deformation bands. Methodology. Low-carbon steels with a carbon content of 0.06–0.1% C in various structural states were used as the material for study. The sample for the study was a wire with a diameter of 1mm. The structural studies of the metal were carried out using an Epiquant light microscope. Ferrite grain size was determined using quantitative metallographic techniques. Different ferrite grain size was obtained as a result of combination of thermal and termo mechanical treatment. Vary by heating temperature and the cooling rate, using cold plastic deformation and subsequent annealing, made it possible to change the ferrite grain size at the level of two orders of magnitude. Deformation curves were obtained during stretching the samples on the Instron testing machine. Findings. Based on the analysis of stretching curves of low-carbon steels with different ferrite grain sizes, it has been established that the initiation and propagation of plastic deformation in the jerky flow area is accompanied by the development of strain hardening processes. The study of the nature of increase at dislocation density depending on ferrite grain size of low-carbon steel, starting from the moment of initiation of plastic deformation, confirmed the existence of relationship between the development of strain hardening at the area of jerky flow and the area of parabolic hardening curve. Originality. One of the reasons for decrease in Luders deformation with an increase of ferrite grain size of low-carbon steel is an increase in strain hardening indicator, which accelerates decomposition of uniform dislocations distribution in the front of deformation band. The flow stress during initiation of plastic deformation is determined by the additive contribution from the frictional stress of the crystal lattices, the state of ferrite grain boundaries, and the density of mobile dislocations. It was found that the size of dislocation cell increases in proportion to the diameter of ferrite grain, which facilitates the development of dislocation annihilation during plastic deformation. Practical value. Explanation of qualitative dependence of the influence of ferrite grain size of a low-carbon steel on the strain hardening degree and the magnitude of Luders deformation will make it possible to determine the optimal structural state of steels subjected to cold plastic deformation.

Scaling in the Local Strain-Rate Field during Jerky Flow in an Al-3%Mg Alloy

Jerky flow in alloys, or the Portevin-Le Chatelier effect, presents an outstanding example of self-organization phenomena in plasticity. Recent acoustic emission investigations revealed that its microscopic dynamics is governed by scale invariance manifested as power-law statistics of intermittent events. As the macroscopic stress serrations show both scale invariance and characteristic scales, the micro-macro transition is an intricate question requiring an assessment of intermediate behaviors. The first attempt of such an investigation is undertaken in the present paper by virtue of a one-dimensional (1D) local extensometry technique and statistical analysis of time series. The data obtained complete the missing link and bear evidence to a coexistence of characteristic large events and power laws for smaller events. The scale separation is interpreted in terms of the phenomena of self-organized criticality and synchronization in complex systems. Furthermore, it is found that both the stress serrations and local strain-rate bursts agree with the so-called fluctuation scaling related to general mathematical laws and unifying various specific mechanisms proposed to explain scale invariance in diverse systems. Prospects of further investigations including the duality manifested by a wavy spatial organization of the local bursts of plastic deformation are discussed.