Millisecond dynamics of deformation bands during discontinuous creep in an AlMg polycrystal.

Formation of bands of macroscopic strain localization during staircase creep in an AlMg polycrystal is studied by the acoustic emission (AE) technique and high-speed video recording with an image acquisition rate up to 50 000 frames per second. The simultaneous measurements by two methods allow us to distinguish different types of embryo deformation bands and concomitant AE signals, and to establish correlations between the band evolution and the acoustic response. It is found that the fastest stages of band formation, associated with its emergence to the surface and subsequent accelerated expansion, generate complex AE bursts in the frequency band ∼0.05-1 MHz. The correlations hidden in the complex structure of an individual acoustic burst are investigated by methods of statistical and fractal analysis. On the other hand, relationships between average parameters of various physical responses to discontinuous creep are assessed. Particularly, a close correspondence is found between the envelope of the acoustic burst and the rate of stress change during formation of a single deformation band. Evolution of dynamical behavior of embryo bands with increasing creep stress is discussed. Notably, a qualitative change in the AE waveform observed on approaching the ultimate stress is considered from the viewpoint of anticipation of the oncoming fracture.

Correlation versus randomization of jerky flow in an AlMgScZr alloy using acoustic emission.

Jerky flow in solids results from collective dynamics of dislocations which gives rise to serrated deformation curves and a complex evolution of the strain heterogeneity. A rich example of this phenomenon is the Portevin-Le Chatelier effect in alloys. The corresponding spatiotemporal patterns showed some universal features which provided a basis for a well-known phenomenological classification. Recent studies revealed peculiar features in both the stress serration sequences and the kinematics of deformation bands in Al-based alloys containing fine microstructure elements, such as nanosize precipitates and/or submicron grains. In the present work, jerky flow of an AlMgScZr alloy is studied using statistical analysis of stress serrations and the accompanying acoustic emission. As in the case of coarse-grained binary AlMg alloys, the amplitude distributions of acoustic events obey a power-law scaling which is usually considered as evidence of avalanchelike dynamics. However, the scaling exponents display specific dependences on the strain and strain rate for the investigated materials. The observed effects bear evidence to a competition between the phenomena of synchronization and randomization of dislocation avalanches, which may shed light on the mechanisms leading to a high variety of jerky flow patterns observed in applied alloys.

Experimental investigation of the effect of thresholding on temporal statistics of avalanches.

Avalanchelike behavior reflected in power-law statistics is a ubiquitous property of extended systems addressed in a number of generic models. The paper presents an experimental investigation of the effect of thresholding on the statistics of durations and waiting times between avalanches using acoustic emission accompanying unstable plastic deformation. It is found that durations of acoustic events obey power-law statistical distributions robust against thresholding. The quiescent time distributions follow the Poisson law for low threshold values. Both these results corroborate the hypothesis that plastic deformation is akin to the phenomena associated with self-organized criticality (SOC), often advanced on the basis of power-law amplitude statistics. Increasing the threshold height enforces deviation from the Poisson distributions toward apparent power-law behavior. Such a thresholding effect may hinder the experimental determination of SOC-like dynamics because of the inevitable noise.

Role of superposition of dislocation avalanches in the statistics of acoustic emission during plastic deformation.

Various dynamical systems with many degrees of freedom display avalanche dynamics, which is characterized by scale invariance reflected in power-law statistics. The superposition of avalanche processes in real systems driven at a finite velocity may influence the experimental determination of the underlying power law. The present paper reports results of an investigation of this effect using the example of acoustic emission (AE) accompanying plastic deformation of crystals. Indeed, recent studies of AE did not only prove that the dynamics of crystal defects obeys power-law statistics, but also led to a hypothesis of universality of the scaling law. We examine the sensitivity of the apparent statistics of AE to the parameters applied to individualize AE events. Two different alloys, MgZr and AlMg, both displaying strong AE but characterized by different plasticity mechanisms, are investigated. It is shown that the power-law indices display a good robustness in wide ranges of parameters even in the conditions leading to very strong superposition of AE events, although some deviations from the persistent values are also detected. The totality of the results confirms the scale-invariant character of deformation processes on the scale relevant to AE, but uncovers essential differences between the power-law exponents found for two kinds of alloys.

Multifractality and randomness in the unstable plastic flow near the lower strain-rate boundary of instability.

The unstable plastic flow of an AlMg alloy, associated with the Portevin-Le Chatelier effect, was studied near the lower strain-rate boundary of instability using multifractal analysis. Self-similarity of deformation curves, indicating long-range time correlations of stress serrations, was detected within the strain-rate range where serrations are commonly ascribed to the occurrence of uncorrelated deformation bands. The deformation curves display a wide range of shapes that are characterized by different groupings of serrations. Multifractal analysis provides a method to quantify the observed complexity and compare it to known Portevin-Le Chatelier effect regimes. The measurement noise effect on the multifractal spectra determined from experimental data was mimicked by superposing multifractal Cantor sets with random noise. Such tests using standard multifractal data sets justify the separation of self-similar and random components of the serrated deformation curves. Furthermore, these results shed light on the general problem of the effect of experimental noise on the apparent multifractal properties of physical fractals.

Multifractal analysis of evolving noise associated with unstable plastic flow.

Plastic flow instability attracts increasing interest as a self-organization phenomenon showing various dynamical regimes, including deterministic chaos and self-organized criticality. The analysis of the associated nonrandom noise--drastic jumps of the mechanical stress--however, confronts the variation of the noise average parameters due to the evolution of the dislocation microstructure. The present paper examines some limitations of the multifractal approach to the study of the evolving noise. The applicability of the multifractal analysis to practical situations is proven using the example of discontinuous deformation curves observed under conditions of the Portevin-Le Châtelier effect in an A1Mg alloy, as well as model signals generated by stretching multifractal Cantor sets. It is found that the smooth trends in the stress serration parameters may narrow the range of the scale invariant behavior associated with the multifractal structure, but do not essentially mask it.