Effect of Grain Refinement on Jerky Flow in an Al-Mg-Sc Alloy
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IT is well known that the plastic flow of metallic alloys may become unstable within certain temperature and strain rate ranges, which is manifested as serrations on the stress–strain curve. This phenomenon, often denoted as jerky flow or repetitive yielding, is associated with heterogeneous localization of plastic deformation within the bulk of a material and is referred to as the Portevin-Le Chatelier (PLC) effect.[1–4] Al-Mg alloys present one of the widely used materials exhibiting this phenomenon.[4–7] The PLC effect is known to result in a reduction in ductility and the formation of deformation bands that leave undesirable traces on the surface of the sheet product.[5,6,8] The microscopic mechanism of this phenomenon is most often ascribed to dynamic strain aging (DSA), i.e., interaction of dislocations with solute atoms.[3–5,9–11] In particular, a recent atomic-scale investigation on an Al-4.3 pct Mg alloy proved that this interaction leads to segregation of Mg solute atoms on dislocations,[3] i.e., the formation of Cottrell clouds, under tension in the DSA regime.[4,5,12] Detachment of mobile dislocations from the solute atmospheres causes local softening and, hence, promotes slip localization
ANNA MOGUCHEVA, Senior Research Associate, DIANA YUZBEKOVA, Engineer, and RUSTAM KAIBYSHEV, Head, are with the Laboratory of Mechanical Properties of Nanostructural Materials and Superalloys, Belgorod State University, Pobeda 85, Belgorod, Russia 308015. Contact e-mails: [email protected], [email protected] TATIANA LEBEDKINA, Research Fellow, and MIKHAIL LEBYODKIN, Head, are with the Laboratoire d’Etude des Microstructures et de Me´canique des Mate´riaux (LEM3), CNRS UMR 7239, Universite´ de Lorraine, Ile du Saulcy, 57045 Metz, France. Manuscript submitted November 5, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
and planar slip. Besides the observations of strain localization in wide PLC deformation bands, which correspond to a millimeter scale, it is known that the initial microstructure influences the mesoscale topographical features and the surface roughness developed during plastic deformation.[8,13] However, there are little experimental data on the relationship between the initial microstructure, the PLC effect, and the surface markings. Therefore, examination of the effect of initial structure on the PLC instability is important for searching ways to improve the surface roughness and ductility impacted by plastic instability in industrial Al-Mg alloys. The behavior of the PLC effect shows generic signatures depending on the strain rate and temperature. They are usually classified into three types according to the serration patterns and kinematics of the PLC bands. A more detailed nomenclature including five types was also proposed to discriminate some specific behaviors (Figure 1).[3–5] Type A serrations are distinguished as rises in the flow stress, followed by a drop to or below the general stress level (the so-called ‘locking’ serrations). They are observed at high strain rates or low temperatures and are associate
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