Crossover from continuous to discontinuous propagation in the Portevin–Le Chatelier effect H. Ait-Amokhtar a, * , C. Fressengeas b a Laboratoire de Technologie des Mate ´riaux et de Ge ´nie des Proce ´de ´s, Universite ´ de Be ´jaia, 06000 Be ´ jaia, Algeria b Laboratoire de Physique et Me ´canique des Mate ´ riaux, Universite ´ Paul Verlaine – Metz/CNRS, Ile du Saulcy, 57045 Metz Cedex, France Received 25 June 2009; received in revised form 17 September 2009; accepted 23 October 2009 Available online 20 November 2009 Abstract 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 inter- est is paid to the critical tests where type A and type B bands are met sequentially. Attention is focused on the unsteadiness and anisot- ropy 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. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Portevin–Le Chatelier effect; Dynamic strain aging; Unsteadiness; Digital image correlation; Infrared thermography 1. Introduction The Portevin–Le Chatelier (PLC) effect is an outstand- ing manifestation of dynamic strain aging (DSA), i.e., the dynamic interaction between mobile dislocations and sol- ute atoms whereby solute atoms diffuse to and age mobile dislocations during their temporary arrest at local obstacles (forest dislocations, precipitates, etc.) [1–3]. DSA reduces the strain rate sensitivity (SRS) of the flow stress, which may become negative in a limited range of temperature, strain and strain rate. Within this range, instability of homogeneous flow occurs and strain localizes into narrow bands associated with stress drops on the stress–strain curve, a regime also referred to as “discontinuous yielding” or “serrated flow” [3,4]. According to the internal state of the deformed material and the externally imposed conditions of temperature and strain rate, three types of instability can be observed in polycrystals [3–8]. Instability shifts from type C to type B then to type A when strain rate is increased [3,5,7,8] or tem- perature is decreased [3,9,10]. Type C bands appear ran- domly along a tensile sample and produce large stress drops on the stress–strain curve. In the type B regime, dis- continuous propagation (hopping bands) is observed as well as regular stress drops of smaller size. Type A bands are associated with weak undulations on the stress–strain curve and are characterized by continuous propagation. Recent analyses [11,12] suggested that the shift in these dynamical regimes could be due to the varying degree of spatial correlation between plasticity events in the material. Spatial correlation was thought to be associated in partic- ular with the elastic internal stresses arising from lattice incompatibility in the neighborhood of the bands. Plastic relaxation of these internal stresses occurs in time. Its intensity is controlled by the ratio of the characteristic time for plastic relaxation to the delay time for reloading after a stress drop. At low strain rates, this ratio is small and full 1359-6454/$36.00 Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2009.10.038 * Corresponding author. Tel.: +213 662 17 88 15; fax: +213 34 21 59 86. E-mail address: aitamokhtar_h@yahoo.fr (H. Ait-Amokhtar). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 58 (2010) 1342–1349