The dynamic behavior of dislocations under load was analyzed in terms of a balance between mutual interactions and lattice friction, defining a screening distance which was compared to dislocation separation. Large friction stresses or low dislocation densities obviously resulted in individual dislocation motion; a few examples taken from transmission electron microscopic in situ experiments illustrate how mechanisms recorded at the dislocation scale could help in understanding the macroscopic mechanical behavior of such materials. The pathologic case of strength anomalies was then analyzed: the effect of lattice friction was overwhelmed by a strong strain localization arising from a very low value of the strain rate sensitivity. The resulting collective and intermittent plastic flow made difficult any direct analysis of dislocation mechanisms within avalanches, whereas observations made in lower density regions might not be representative of the mechanisms responsible for the strength anomaly. Beyond such transient regimes, the screening distance tends to infinity as the lattice friction vanishes. An obstacle-free and fully collective dislocation motion appeared (domino effect), characterized by scale-free avalanche size distributions, in which avalanches of any sizes could occur. Such behavior was reminiscent of the well known self-organized criticality, making questionable any micro-macro homogenization procedure based upon a supposed representative elementary volume.

From Individual Dislocation Motion to Collective Behaviour. F.Louchet: Journal of Materials Science, 2006, 41[9], 2641-6