Plastic deformation of metals usually occurs as a series of rapid and localized deformation events of various sizes. During these events, the line-like defects of the crystal lattice, called dislocations, move collectively in a local part of the crystal. The experimental investigation of this avalanche-like phenomenon was initially carried out on bulk samples using acoustic emission (AE) measurements. It was found that the distribution of both the energy and amplitude of individual events are scale-free, which indicates the critical nature of plastic deformation. This phenomenon was later also observed during the compression of micropillars, typically a few um in size, where it manifests in random jumps in the stress-strain curve.

 

In the first half of the lecture, the outcome of the combination of these two experimental techniques will be presented, the aim of which was to obtain a more detailed picture of the dynamics of dislocation avalanches. To this end, micropillars were milled from Zn single crystals using the focused ion beam technique. The samples were attached to an AE sensor and compressed in situ in a scanning electron microscope using a special device designed for this purpose. We found that the acoustic events measured during compression are perfectly correlated with the stress drops experienced during compression. The statistical analysis of the data obtained by the two methods revealed the complex spatio-temporal dynamics of a single stress drop: a dislocation avalanche consists of many smaller events that show similarities to earthquakes through different phenomenological laws. These results, which are also confirmed by discrete dislocation dynamics simulations, provide the missing link between the quantities measured by AE and the mechanical characteristics of individual events [1]. As a continuation of the research, we present the stability analysis of the discrete dislocation systems, which reveals the development of dynamic modes that can span the entire volume. The emergence of these modes provides an explanation for the previously experienced anomalous system size dependence of avalanche sizes and the unusual behavior of local flow stresses [2].

 

[1] PD Ispánovity, D Ugi, G Péterffy, M Knapek, Sz Kalácska, D Tüzes, Z Dankházi, K Máthis, F Chmelík, I Groma, Nature Communications 13, 10 (2022).

[2] D Berta, G Péterffy, PD Ispánovity, arXiv:2202.08224 (2022).