Crossover from continuous to discontinuous propagation in the Portevin-Le Chatelier effect

The Portevin-Le Chatelier (PLC) effect in an Al-3.2%Mg alloy is investigated using velocity-driven tensile tests at room temperature, in a strain rate range where the crossover from continuous (type A bands) to discontinuous propagation (type B bands) occurs. Digital image correlation and infrared thermography techniques are used to render the strain, strain rate and temperature fields. Particular interest is paid to the critical tests where type A and type B bands are met sequentially. Attention is focused on the unsteadiness and anisotropy of the field variables and their consequences on the instability. It is shown that the decrease of strain rate at large strain and the elevation in temperature occurring at large driving velocity favor dynamic strain aging, reduce strain rate sensitivity and enhance strain localization. Interpretation for the crossover from type A to type B in a single test is provided accordingly. The effects of strain rate and temperature unsteadiness on the extent of the range of PLC effect, the type of instability and the critical plastic strain for the onset of serrations are discussed. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.