Serrated Flow and Enhanced Ductility in Coarse-Grained Al-Mg Alloys
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INTRODUCTION
ALUMINUM-MAGNESIUM alloys combine desirable strength, forming, and welding characteristics with a high resistance to corrosion.[1–4] These materials, however, exhibit limited formability[4,5] and are susceptible to dynamic strain aging (DSA) or serrated yielding when deformed at room (or a low) temperature.[6,7] Dynamic strain aging, i.e., the interaction between solutes and dislocations during deformation, results in a reduction in ductility as well as the appearance of undesirable stretcher strain surface markings on the alloy.[8] These effects, which are caused by DSA, are moreover attributed to the negative strain rate sensitivity regime found at room and low temperatures when deforming such alloys, where the stress decreases when the strain rate is increased. In the literature, the temperature-dependent flow stress behavior of Al-Mg alloys is found to undergo a transient region as a result of DSA, where the intensity of the strain aging increases with the Mg content.[6,9–12] Furthermore, the intermittent locking and unlocking of dislocations by solutes (substitutional Mg atoms in the case of Al-Mg alloys) gives rise to serrated stress–strain curves in the temperature/strain rate regime where strain EHAB SAMUEL, formerly Doctoral Student with McGill University, Montre´al, QC, Canada H3A 2B2, is now with the Aluminum Technology Centre, Saguenay, Chicoutimi, QC, Canada G7H 8C3. Contact e-mail: [email protected] JOHN J. JONAS, Birks Professor of Metallurgy Emeritus, is with the Department of Metals & Materials Engineering, McGill University. FAWZY H. SAMUEL, Professor and Senior Chairholder, is with the Department of Applied Sciences, Universite´ du Que´bec a´ Chicoutimi, Chicoutimi, QC, Canada G7H 2B1, and is also a Visiting Professor, CEREM, King Saud University, Riyadh 11451, Saudi Arabia. Manuscript submitted March 27, 2009. Article published online November 24, 2010 1028—VOLUME 42A, APRIL 2011
aging is prominent.[8,13,14] Consequently, the stresses observed are greatly increased via the strengthening effect of the pinned dislocations.[6] Improvements in ductility are possible at higher temperatures, such as under superplastic deformation conditions.[1] Although traditionally associated with a fine-grained microstructure and grain-boundary sliding,[15–22] a type of superplasticity or ‘‘enhanced ductility’’[19] can also take place in coarse-grained materials. This allows one to obtain suitable values of ductility without the need for added material processing and grain refinement. Ductility values observed in coarse-grained materials are more modest (~200 pct to 300 pct) than in fine-grained materials (>500 pct). Solute drag is usually accepted as the mechanism responsible for the enhanced ductility in the former case in view of the fact that (1) there is no grain size dependence and (2) a sufficiently high strain rate sensitivity (m ~ 0.3) is obtained.[4,19,20,23,24] In steel, where DSA is caused by interstitial C, the temperature/strain rate regime associated with DSA and negative rate sensiti
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