Jerky flow in dilute alloys, or the Portevin-Le Chatelier effect, is investigated using statistical analysis of time series characterizing the evolution of the plastic activity at distinct scales of observation, namely, the macroscopic scale of stress serrations and a mesoscopic scale pertaining to the accompanying acoustic emission. Whereas the stress serrations display various types of statistical distributions depending on the driving strain rate, including power-law, peaked and bimodal histograms, it is found that acoustic emission is characterized by power-law statistics of event size in all experimental conditions. The latter reflect intermittency and self-organization of plastic activity at a mesoscopic scale. This shift in the observed dynamics when the observation length scale is decreased is discussed in terms of the synchronization of small-scale events.

International audiencePlasticity, a key property in the mechanical behavior and processing of crystalline solids, has been traditionally viewed as a smooth and homogeneous flow. However, using two experimental methods, acoustic emission and high-resolution extensometry, to probe the collective dislocation dynamics in various single crystals, we show that its intermittent critical-like character appears as a rule rather than an exception. Such intermittent, apparently scale-free plastic activity is observed in single-slip as well as multislip conditions and is not significantly influenced by forest hardening. Strain bursts resulting from dislocation avalanches are limited in size by a nontrivial finite size effect resulting from the lamellar character of avalanches. This cutoff explains why strain curves of macroscopic samples are smooth, whereas fluctuations of plastic activity are outstanding in submillimetric structures

When envisioned at the relevant length scale, plasticity of crystalline solids consists in the transport of dislocations through the lattice. In this paper, transport of dislocations is evidenced by experimental data gathered from high-resolution extensometry carried out on copper single crystals in tension. Spatiotemporal kinematic fields display spatial correlation through characteristic lines intermittently covered by plastic activity. Intermittency shows temporal correlation and power-law distribution of avalanche size. Interpretation of this phenomenon is proposed within the framework of a field dislocation theory attacking the combined problem of dislocation transport and long-range internal stress field development. Intermittency and transport properties show remarkable independence from sample size, aspect ratio, loading rate, and strain-rate sensitivity of the flow stress.

The jerky flow of dilute alloys, or the Portevin-Le Chatelier effect, has a burst-like intermittent character at different fluctuation size levels. Multifractal analysis is applied to both the macroscopic stress serrations and the acoustic emission accompanying the plastic deformation. Multifractal scaling is found for both kinds of time series. The scaling range of the stress serrations is limited from below by their characteristic frequency. Unexpectedly, the scaling range for acoustic bursts not only covers this range but spreads to much shorter time scales with the same scaling exponent. This result testifies that the deformation processes revealed by the acoustic emission at a mesoscopic scale have a similar nature during both stress serrations and smooth plastic flow. The implications on the crossovers in the dynamics of jerky flow are discussed.

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Statistics of acoustic emission accompanying plastic deformation and of stress serrations caused by the Portevin-Le Chatelier effect are studied during tension of an Al3%Mg alloy at room temperature. Power-law distributions of acoustic emission reflecting selforganization of dislocations and intermittency of plastic flow are found, irrespective of the strain rate, both before and after the critical strain for the onset of the serrated flow. In contrast, several regimes including both power-law and peaked distributions are observed at the macroscopic scale of stress serrations, depending on the applied strain rate.

The acoustic emission accompanying plastic deformation obeys scale-free statistics reflecting avalanche-like dislocation motion. This feature holds out for the macroscopically unstable deformation of alloys. However, stress serrations display peaked distributions at low enough strain rates. This occurrence of a characteristic macroscopic scale was supposed to result from the synchronization of dislocation avalanches. In the present work, the synchronization mechanism is studied using statistical analysis of different subsets of acoustic events. A crossover in the power-law exponents is detected for the events occurring during deep stress drops. It is described to a transition from chaining to overlapping dislocation avalanches.

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.