Spatiotemporal aspects of jerky flow in Al–Mg alloys, in relation with the Mg content H. Ait-Amokhtar a, * , S. Boudrahem a , C. Fressengeas b a Laboratoire de Technologie des Mate ´riaux et de Ge ´nie des Proce ´de ´s, De ´partement de physique, Universite ´ de Be ´jaia, 06000 Be ´jaia, Algeria b Laboratoire de Physique et Me ´canique des Mate ´riaux, Universite ´ de Metz-CNRS, Ile du Saulcy, 57045 Metz Cedex, France Received 15 January 2006; received in revised form 26 February 2006; accepted 1 March 2006 Available online 3 April 2006 Abstract The spatiotemporal aspects of the Portevin–Le Chatelier effect in Al–Mg alloys are investigated using direct observations of the sample surface with a digital camera. Optical measurements of the surface profile using a white light interferometer allow the estimation of the local strain and strain rate associated with the bands. The effect of Mg content on instability characteristics is analyzed. The reloading time is shown to depend on the Mg content only at the strain and strain rates where type C bands are observed. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Portevin–Le Chatelier effect; Local measurements; Mg content; Critical strain 1. Introduction Jerky flow or the Portevin–Le Chatelier (PLC) effect is one of the most striking manifestations of plastic instability [1–4]. It is due to dynamic strain aging (DSA), i.e., a dynamic interaction between solute atoms and mobile dislocations. The solute atoms diffuse to and age mobile dislocations during their temporary arrest at local obstacles (forest dislocations, precipitates, etc.) [5,6]. DSA reduces the strain rate sensitivity of the flow stress, which may become negative in some range of temperature, strain and strain rate. In such a case, strain localizes into narrow bands associated with stress drops on the stress–strain curve [2,3]. According to the standard dynamics of the nucleation and motion of PLC bands, three types of insta- bilities can be observed in polycrystals loaded at constant velocity (hard machine) [1,2]. At low strain rates, type C bands occur randomly on the sample surface. They are static and associated with large stress drops on the stress–strain curve. In type B bands, observed at medium strain rates, spatial correlation leads to an apparent prop- agation (hopping bands) and to regular stress drops with a smaller size. At large strain rates, type A bands produce weak undulations on the stress–strain curve and are char- acterized by a continuous propagation due to strong spatial correlations. Recently, new methods [3,7,8] of strain field measure- ment have been developed for the analysis of heteroge- neous deformation. Ziegenbein et al. [7] used the ‘‘laser scanning extensometer’’ method and Shabadi et al. [3] employed a ‘‘laser speckle technique’’. These methods are indirect in the sense that they do not use plain video record- ings of the sample surface. Direct observations of PLC bands like those carried out by Chihab et al.[2] on Al– 5at.%Mg alloy use video recordings of the sample surface illuminated by ordinary light. In the present study, we studied jerky flow in Al– 4.5%Mg alloys during tensile tests at imposed strain rate and at room temperature. Direct observations of the sample surface using a digital camera allowed the study of the dynamics of the bands as a function of strain. A white light interferometer (WLI) was used to measure the surface profile of the deformed sample and to estimate the local strain and strain rate due to PLC band formation. 1359-6462/$ - see front matter Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.03.006 * Corresponding author. Tel.: +213 62 17 88 15; fax: +213 34 21 51 05. E-mail address: aitamokhtar_h@yahoo.fr (H. Ait-Amokhtar). www.actamat-journals.com Scripta Materialia 54 (2006) 2113–2118