Deformation behavior of bimodal nanostructured 5083 Al alloys
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I. INTRODUCTION
LARGE increases in strength are commonly observed in nanostructured metals, a phenomenon that is explained in part by grain refinement and the well-known Hall–Petch relation. However, low ductility, and the associated loss of toughness, is a serious deficiency in most of these metals.[1] High ductility in nanostructured materials has been observed in a few cases, but these cases appear to be exceptional,[2,3] often occurring only at high temperatures[4] or with microtensile samples using specialized testing equipment.[5] Inspection of the relevant literature reveals that several approaches have been proposed to enhance the ductility of nanostructured materials.[6,7] In one recent study, high tensile ductility was observed in nanostructured Cu with a bimodal grain structure that was achieved by partial annealing.[6] In earlier studies, Tellkamp and co-workers reported on the mechanical properties of a nanostructured cryomilled 5083 Al alloy, with a yield strength of 334 MPa, ultimate strength of 462 MPa, and an elongation of 8.4 pct.[8] They suggested that the presence of coarse grains in the nanocrystalline matrix was responsible for the observed ductility, an observation that sparked the genesis of multiscale materials. In this particular case, the coarse grains arose as a result of thermomechanical processing of the nanostructured powders, i.e., consolidation and extrusion. Multiscale structures were recently produced in a cryomilled Al-Ti-Cu alloy.[9] The microstructure consisted of nanocrystalline grains and elongated coarse-grained bands. Under tensile loading, microcracks nucleated in nanocrystalline regions and propagated to ductile coarsegrained regions, where they were effectively arrested by a combination of crack blunting and crack bridging. Among the various approaches that are currently being investigated to generate nanostructured materials, cryomilling B.Q. HAN, Assistant Researcher, and E.J. LAVERNIA, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Contact e-mail: [email protected] Z. LEE, Postdoctoral Researcher, and S. NUTT, Professor, are with the Department of Materials Science, University of Southern California, Los Angeles, CA 90089. D. WITKIN, Research Assistant, is with the Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697. Manuscript submitted September 27, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
affords unusual flexibility in microstructural design and the ability to produce bulk samples.[8] As with other methods for producing nanostructured alloys, decreasing grain size is accompanied by decreases in ductility.[10] However, recent reports demonstrate that cryomilled aluminum alloys with an ultrafine-grained microstructure[11] or with a bimodal grain size distribution[8] can attain elongations of 7 to 8 pct, rendering them attractive for many engineering applications. The present investigation was motivated by the hypothesis that the duct
