From individual dislocation motion to collective behaviour

The dynamical behaviour of dislocations under load is analysed in terms of a balance between mutual interactions and lattice friction, defining a screening distance which is compared to dislocation separation. Large friction stresses or low dislocation densities obviously result in individual dislocation motion; a few examples taken from TEM in situ experiments illustrate how mechanisms recorded at the dislocation scale may help in understanding the macroscopic mechanical behaviour of such materials. The pathologic case of strength anomalies is then analysed: the effect of lattice friction is overwhelmed by a strong strain localisation arising from a very low value of the strain rate sensitivity. The resulting collective and intermittent plastic flow makes difficult any direct analysis of dislocation mechanisms within avalanches, whereas observations made in lower density regions may 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 appears (domino effect), characterised by scale-free avalanche size distributions, in which avalanches of any sizes can occur. Such behaviour is reminiscent of the well known self-organised criticality (SOC), making questionable any micro-macro homogeneization procedure based on a supposed representative elementary volume. (c) 2006 Springer Science + Business Media, Inc.