Plasticity, a key property in the mechanical behavior and processing of crystalline solids, has been traditionally viewed as a smooth and homogeneous flow. However, using two experimental methods, acoustic emission and high-resolution extensometry, to probe the collective dislocation dynamics in various single crystals, we show that its intermittent critical-like character appears as a rule rather than an exception. Such intermittent, apparently scale-free plastic activity is observed in single-slip as well as multislip conditions and is not significantly influenced by forest hardening. Strain bursts resulting from dislocation avalanches are limited in size by a nontrivial finite size effect resulting from the lamellar character of avalanches. This cutoff explains why strain curves of macroscopic samples are smooth, whereas fluctuations of plastic activity are outstanding in submillimetric structures.

We report a crossover from chaotic to self-organized critical dynamics in the Portevin–Le Chatelier effect in single crystals of Cu–10% Al in tension as a function of the applied strain rate. For low and intermediate strain rates, we provide an unambiguous support for the existence of chaotic stress drops by showing the existence of a finite correlation dimension and a stable positive Lyapunov exponent. A surrogate data analysis rules out the possibility that the time series is due to a power law stochastic process. As the strain rate is increased, the distributions of stress drops and the time intervals between the stress drops change from peaked to power law type with an exponent close to unity reminiscent of self-organized critical state. A scaling relation compatible with self-organized criticality relates the various exponents. The absence of a finite correlation dimension and a stable positive Lyapunov exponent at the highest strain rate also supports the evidence of crossover.