Collective Dislocation Dynamics and Avalanches during Fatigue of Aluminum

Abstract

We present a study of collective dislocation dynamics and plasticity during fatigue of pure Aluminum from the analysis of continuous and discrete acoustic emission (AE). The three stages of macroscopic fatigue behavior (strain‐hardening, shakedown, and strain softening) are clearly differentiated in terms of AE. During the first loading cycles, collective dislocation dynamics consists in dislocation avalanches of various sizes and clustered in time. Once a microstructure of dislocation cells and walls is formed, the spreading of such avalanches is restrained, and the discrete AE strongly decreases. Instead, a symmetrical (tension‐compression) continuous AE, maximal at plastic yield, is observed, likely associated to a superposition of numerous, small and uncorrelated motions such as dislocation loops initiation from cell walls. However, some discrete AE activity remains during shakedown, a possible signature of sudden rearrangements of the microstructure occurring at scales larger than its wavelength. Finally, the onset of strain softening is associated to a strong increase of discrete AE, in relation with microcracking. Our results suggest that collective dislocation instabilities and the emergence of a dislocation microstructure are interrelated, and challenge future numerical modeling developments of dislocation assemblies.