Crossover in the scale-free statistics of acoustic emission associated with the Portevin-Le Chatelier instability

Abstract

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.

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

“…ε a is increased. The faster loading must be accommodated by a more intense plastic deformation and, therefore, lead to a globally stronger clustering and also the superposition of dislocation avalanches, as was confirmed by virtue of the AE technique [29,36]. At the same time, the smoothing of the recurrent heterogeneities becomes less efficient because of shorter reloading periods, so that there constantly exist some parts of the material close to the instability threshold.…”

“…The application of the AE method to the PLC effect provided an explicit explanation of the scale separation [29,36]. It occurred that despite the coexistence of two scale ranges for stress fluctuations at low .…”

“…Recent studies showed that the AE accompanying the plastic deformation of alloys obeys such critical-type statistics at all strain rates, even though the decrease in . ε a results in a transition to peaked distributions of stress drops [29,36]. Therewith, the AE does not only occur at the instants of stress drops but also during reloading periods corresponding to macroscopically stable deformation.…”

“…Instead, a distinction occurs between their durations because stress serrations are accompanied by clustering of the AE, giving rise to events in a millisecond range for deep stress drops, while individual AE events accompanying the reloading have durations in the microsecond range. Based on these observations, abrupt serrations with a characteristic size were attributed to a brief synchronization of many dislocation avalanches, proceeding through their successive triggering [29,36,50].…”

“…This apparent contradiction was solved due to the observation of a correlation between the occurrence of macroscopic stress serrations and the clustering (chaining) of AE events [29]. Accordingly, taking into account a relationship between the relaxation oscillations and the phenomenon of synchronization in lattice models [33,34], the transition from power-law distributions of AE to distinct kinds of statistics of stress serrations was ascribed to the synchronization of dislocation avalanches, depending on the strain rate [35,36].…”