Investigating the Portevin-Le Chatelier effect in strain rate and stress rate controlled tests by the acoustic emission and laser extensometry techniques

The Portevin-Le Chatelier (PLC) effect is a spectacular effect of dynamic strain aging in many alloys deformed in certain intervals of strain rates and temperatures. The main feature of the PLC effect is a negative strain rate sensitivity of stress, which is associated with stress fluctuations, a macroscopic spatio-temporal localization of plastic deformation (nucleation and propagation of deformation bands) and an intense acoustic emission. Recent theoretical and experimental studies have pointed out that cooperative dislocation motion is a necessary condition for plastic instabilities to occur under conditions of negative strain rate sensitivity. In this work the potential of acoustic emission and laser extensometry to monitor in situ cooperative dislocation motion due to the PLC effect is revisited and examined in an Al-3.5 wt.% Mg alloy. The alloy was tested for two tensile modes, i.e. at a constant cross-head speed and at a constant stress rate. For each testing mode, the alloy exhibits specific PLC deformation behavior and unique characteristics of acoustic emission. The results demonstrate features of dislocation dynamics for both of these tensile modes. (C) 2006 Elsevier B.V. All rights reserved.