Heterogeneity in Plastic Deformation

Dynamic strain ageing (DSA) in aluminum alloys is associated with the diffusion of solute to, and along, mobile dislocations. Dislocations become pinned at obstacles (forest dislocations), and during this waiting time solute atoms diffuse to the dislocation core thereby increasing the glide resistance [1, 2, 3]. Periods of such ageing, interspersed with free flight between obstacles, lead to 'jerky glide.' The glide resistance can be decomposed into two components [1, 3], activation and ageing. The interaction between the two time scales, associated with waiting time and diffusion, imparts a negative contribution to the overall rate sensitivity. Should the contribution be sufficient to render a negative rate sensitivity, the Portevin-LeChatelier (PLC) effect ensues. The PLC effect is a topic of continued study and fascination. It is from a macroscale, say over the dimension of the usual test specimen, that one observes negative rate sensitivity and concomitant localization. Alternatively, one may take a local view at the level of some 'representative volume element' within the specimen. Here, rate sensitivity must be (locally) positive and jerky flow is in fact a de-localization following an initial instability caused by some inherent material property'.