Speaker
Description
The Portevin-Le Chatelier (PLC) effect describing unstable plastic flow in alloys is an exceptional example of self-organization phenomena in plasticity. Its dynamics stems from the mechanism associated with nonmonotonous behavior of the strain-rate sensitivity of stress, negative in a strain-rate interval allowing for dynamic strain ageing of dislocations by solutes. If the motion of all dislocations were identical, this nonlinearity would give rise to periodic relaxation oscillations. The intrinsic heterogeneity of plastic deformation leads to complex behaviors of real materials. Investigations of the PLC effect using acoustic emission (AE) revealed that the microscopic dynamics of dislocations is characterized by scale invariance reflected in power-law statistics of acoustic events. Moreover, such AE statistics were found to be a ubiquitous feature of plastic deformation in the absence of macroscopic instability. These observations led to a conclusion on the avalanche nature of the dislocation dynamics at fine scales. In contrast, the analysis of stress serrations caused by the PLC effect revealed a wealth of dynamics on the macroscopic scale, including avalanche behavior, deterministic chaos, or relaxation oscillations. To shed light on the distinction between different scales, the present study involves analyses with intermediate resolutions. The work will present statistics of local strain-rate maps built for an AlMg alloy exhibiting the PLC effect. The data testify to a coexistence of large characteristic events and power laws for smaller events. The scale separation is interpreted in terms of phenomena of self-organized criticality and synchronization in complex systems. Furthermore, it is observed that bursts in the local strain rate are organized in wavy patterns. This duality, little studied so far, is of great interest for understanding the correlations between temporal instabilities and spatial heterogeneities in the dynamics of crystal defects.
| Speaker Country | France |
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