Plastic Flow Properties and Microstructural Evolution in an Ultrafine-Grained Al-Mg-Si Alloy at Elevated Temperatures
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INTRODUCTION
HIGH strength and light weight requirements for the aerospace and automotive sectors have led to the development of new Al alloys and also to modifications in composition and processing of established alloys. While there has been significant progress in thermomechanical processing to optimize the physical and mechanical properties of ultrafine-grained (UFG) 2xxx and 7xxx grades of Al alloys in the 1980s,[1–3] it is only recently that the focus has shifted to the 6xxx series Al alloys.[4–7] This is in spite of the fact that the 6xxx Al alloys are in wide use due to their superior corrosion resistance and low cost, along with good formability and weldability.[8] It is now realized that a better understanding of strengthening and flow properties of the Al alloys of the 6xxx series, by manipulating the precipitation and grain refinement processes, can enable more
B.P. KASHYAP, Professor, is with the Department of Metallurgical Engineering and Materials Science, IIT Bombay, Powai, Mumbai, 400076, India. P.D. HODGSON, Director of Research, M.R. BARNETT, Associate Professor, and I. TIMOKHINA, Academic Researcher, are with the Centre for Material and Fibre Innovation, Institute of Technology Research and Innovation, Deakin University, Waurn Ponds, Vic. 3217, Australia. Y. ESTRIN, Professor, is with the CSIRO Division of Materials Science and Engineering, Clayton South, Vic. 3168, Australia. I. SABIROV, Researcher, is with the Instituto Madrilen˜o de Estudios Avanzados de Materiales (IMDEA-Materiales), E.T.S. de Ingenieros de Caminos, 28040, Madrid, Spain. Contact e-mail: [email protected] P.D. HODGSON, Y. ESTRIN, and M.R. BARNETT are also with ARC Centre of Excellence for Design in Light Metals, Department of Materials Engineering, Monash University, Clayton, Vic. 3800, Australia. Y. ESTRIN, is also with Deakin University, Waurn Ponds, Vic., Australia. Manuscript submitted May 5, 2009. Article published online October 27, 2009 3294—VOLUME 40A, DECEMBER 2009
effective exploitation of these important alloys. Troeger and Starke[5] developed a thermomechanical treatment for an alloy, within the compositional ranges of 6013 and 6111 Al alloys, which can yield a sufficiently fine grain size for superplasticity, with a maximum tensile elongation of 375 pct at 540 C. It should be mentioned that, prior to that work, there existed a significant body of research on the high strain rate superplasticity of composites having a 6xxx series matrix.[9–12] Grain boundaries in UFG and nanostructured materials act as a source of strengthening at low temperatures, whereas the same boundaries can facilitate plastic deformation at elevated temperatures,[13–20] or even at room temperature.[21–25] Morris et al. used equal channel angular pressing (ECAP)[7] for grain refinement, and their analysis of the contributions of grain size, dislocation density, and precipitation to the strength of an AA6082 alloy demonstrated that the Hall–Petch type grain boundary strengthening was the predominant mechanism. The extent to which grain boundary p
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