  Previous acoustic emission (AE) experiments on ice single crystals, as well
as numerical simulations, called for the possible occurrence of self-organized
criticality (SOC) in collective dislocation dynamics during plastic
deformation. Here, we report AE experiments on hcp metallic single crystals.
Dislocation avalanches in relation with slip and twinning are identified with
the only sources of AE. Both types of processes exhibit a strong intermittent
character. The AE waveforms of slip and twinning events seem to be different,
but from the point of view of the AE event energy distributions, no distinction
is possible. The distributions always follow a power law, even when multi-slip
and forest hardening occur. The power law exponent is in perfect agreement with
those previously found in ice single crystals. Along with observed time
clustering and interactions between avalanches, these results are new and
strong arguments in favour of a general, SOC-type, framework for crystalline
plasticity.
