Time distribution of stress drops, critical strain and crossover in the dynamics of jerky flow

The distribution in time of stress drops in the jerky flow of several Al–Mg alloys is investigated in velocity controlled tensile tests, in relation with the crossover from hopping to propagating bands. The distribution evolves from randomness for static bands to increasing regularity for hopping bands. This evolution culminates in a power law distribution, which breaks down into a highly disordered behaviour at the crossover with propagating bands. The critical incubation strain for the Portevin–Le Chatelier (PLC) effect is shown to follow a parallel evolution when the applied strain rate is increased. An ‘inverse’ evolution is observed until the crossover is met, followed by a ‘normal’ increase of the critical strain in the propagating regime. Both aspects are discussed in relation with characteristic time scales and microstructure evolution with strain.