Interrelation between the Portevin Le-Chatelier effect and necking in AlMg alloys

Plastic flow instability caused by the Portevin Le-Chatelier (PLC) effect and its influence on the necking instability were studied in a binary and a precipitation-strengthened AlMg alloy using the digital image correlation (DIC) technique. Coarse-grained structure and two different finegrained states distinctly distinguished by dislocation density were produced in both alloys using similar routes of thermomechanical processin

Scaling and complexity of stress fluctuations associated with smooth and jerky flow in a FeCoNiTiAl high-entropy alloy

Recent observations of jerky flow in high-entropy alloys (HEA) revealed a high role of self-organization of dislocations in their plasticity. The present work reports first results of investigation of stress fluctuations during plastic deformation of a FeCoNiTiAl alloy, examined in a wide temperature range covering both smooth and jerky flow. These fluctuations, which accompany the overall deformation behavior representing an essentially slower stress evolution controlled by the work hardening, were processed using complementary approaches comprising the Fourier spectral analysis, the refined composite multiscale entropy, and multifractal formalisms. The joint analysis at distinct scales testified that even a macroscopically smooth plastic flow is accompanied with nonrandom fluctuations, disclosing self-organized dynamics of dislocations. Qualitative changes in such a fine-scale "noise" were found with varying temperature. The observed diversity is significant for understanding the relationships between different scales of plasticity of HEAs and crystal materials in general.Comment: 13 pages main body, 6 figures, 2 appendices, 65 citations (22 pages overall

On the Role of Hazard and Particle Failure Statistics on the Variation of Fracture Parameters of Ductile-Brittle Composites

The behavior of a simple computer model considering a random distribution of brittle spherical particles in a ductile matrix is examined in order to highlight the intrinsic variations of the fracture conditions due to the probabilistic nature of the particle cleavage. The model is qualitatively supported by experimental data on stress-strain behavior and damage accumulation in an Al-Si alloy with unconnected equiaxed Si particles. It is used to evaluate the effect of the particle failure statistics on the fracture characteristics (strength, ductility, and fraction of cracked particles) and their scatter, which occur to be strongly dependent on the specific shape of the probabilistic law. In particular, it is found that the variations of the fracture conditions may depend non-monotonously on its sharpness, in the case of the well-known Weibull statistics controlled by the value of the respective modulus of the material of hard particles. The existence of a maximum scatter leads to a suggestion that the choice of the reinforcements may influence not only on the average value of the fracture resistance but also on the quality of its prediction

Onset of the Portevin-Le Chatelier effect: role of synchronization of dislocations

International audienceThe problem of the onset of the Portevin-Le Chatelier (PLC) effect is revised by combining a study of the kinetics of the flow stress evolution upon abrupt changes in the applied strain rate and acoustic emission (AE) accompanying plastic deformation of an AlMg alloy. The kinetic measurements allow evaluating the strain-rate sensitivity of the flow stress and the time characteristics of transient processes as functions of plastic strain. Using known criteria of plastic instability, domains of instability are constructed in the (strain, strain rate) plane. A particular accent is put on the strain-rate range corresponding to the so-called “inverse” behavior. The comparison of such maps with experimental data on the critical strain testifies to the insufficiency of these criteria for explaining the onset of the PLC effect. Moreover, the slow transient kinetics contradicts observations of the fast development of stress drops. The AE measurements bear witness that the stress serrations are associated with bursts in duration of acoustic events generated by the collective motion of dislocations. The possible role of synchronization of dislocation dynamics on the onset of plastic instability is discussed

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

International audienceJerky 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

Complexity and Anisotropy of Plastic Flow of α-Ti Probed by Acoustic Emission and Local Extensometry

Current progress in the prediction of mechanical behavior of solids requires understanding of spatiotemporal complexity of plastic flow caused by self-organization of crystal defects. It may be particularly important in hexagonal materials because of their strong anisotropy and combination of different mechanisms of plasticity, such as dislocation glide and twinning. These materials often display complex behavior even on the macroscopic scale of deformation curves, e.g., a peculiar three-stage elastoplastic transition, the origin of which is a matter of debates. The present work is devoted to a multiscale study of plastic flow in α-Ti, based on simultaneous recording of deformation curves, 1D local strain field, and acoustic emission (AE). It is found that the average AE activity also reveals three-stage behavior, but in a qualitatively different way depending on the crystallographic orientation of the sample axis. On the finer scale, the statistical analysis of AE events and local strain rates testifies to an avalanche-like character of dislocation processes, reflected in power-law probability distribution functions. The results are discussed from the viewpoint of collective dislocation dynamics and are confronted to predictions of a recent micromechanical model of Ti strain hardening

Auto-organisation des ensembles de dislocations dans la plasticité cristalline

L’effet Portevin – Le Chatelier est un phénomène d’instabilité qui résulte du comportement collectif et de l’interaction de défauts cristallins tels que les dislocations et les atomes de soluté. L’analyse statistique des variations de la contrainte appliquée (comportement temporel) et les mesures de déplacement 1D ultra-rapides (structuration spatiale) lors d’essais de traction d’alliages Al-Mg, révèlent l’organisation de ces défauts à des échelles intermédiaires. On montre la nécessité de prendre en compte ces effets collectifs dans les lois de comportement

Multiscale analysis of acoustic emission during plastic flow of Al and Mg alloys: from microseconds to minute

International audienceRecent studies of plastic deformation using high-resolution experimental techniques bear witness that deformation processes are often characterized by collective effects emerging on an intermediate scale between the scales describing the dynamics of individual crystal defects or the macroscopic plastic flow. In particular, the acoustic emission (AE) reveals intermittency of plastic deformation in various experimental conditions, which is manifested by the property of scale invariance, a characteristic feature of self-organized phenomena. Some materials, e.g., Al or Mg alloys, display a macroscopic discontinuity of plastic flow due to the Portevin-Le Chatelier effect or twinning. These materials are therefore of special interest for the study of collective effects in plasticity. The present work reviews the results of a multiscale investigation of AE accompanying plastic deformation of such model alloys. The AE is analyzed by methods borrowed from the theory of nonlinear dynamical systems, including statistical and multifractal analyses

Avalanche dynamics in crumpled aluminum thin foils

International audienceUniaxial compression of crumpled aluminum thin foils with different relative densities was studied using an acoustic emission (AE) technique. The AE signal analysis reveals a power law linking the probable density and amplitudes of acoustic events, proving an avalanche dynamics of plastic deformation. The exponent found for the distributions of squared amplitudes that reflect the dissipated mechanical energy places the observed behavior among those found previously for bulk samples of various pure materials

Micromechanical modeling of hardening mechanisms in commercially pure alpha-titanium in tensile condition

International audienceTensile tests on commercially pure alpha-titanium show a three-stage behavior giving rise to a well on the strain dependence of the work hardening. An opposite strain rate effect on the well depth is found whether specimens are elongated along the rolling or the transverse direction. Slip lines analysis reveals an initial predominance of prismatic slip, particularly pronounced in specimens strained along the rolling direction. The relative activity of prismatic slip is then observed to decrease with the samples deformation. These results provide grounds for elaboration of an elasto-viscoplastic self-consistent model based on the translated field method and an affine linearization of the viscoplastic flow rule, and capable of explaining such peculiar work hardening behavior. The model considers crystal plasticity and deals separately with mobile dislocation density and dislocation velocity. It assumes lower strain rate sensitivity as well as higher dislocation multiplication rate for prismatic systems. Based on these assumptions, the model reproduces correctly the stress strain curves and gives sound estimates of Lankford coefficients, prismatic slip activity and textures evolution. Most importantly, the opposite effect of strain rate on the well depth with regard to the orientation of the tensile axis is qualitatively retrieved, which allows putting forward an explanation of the observed phenomena