Bimodal Structured Bulk Nanocrystalline Al-7.5Mg Alloy
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Bimodal Structured Bulk Nanocrystalline Al-7.5Mg Alloy Zonghoon Lee, David B. Witkin1, Enrique J. Lavernia2 and Steven R. Nutt Department of Materials Science, University of Southern California, Los Angeles, CA 90089-0241, U.S.A. 1 Department of Chemical Engineering and Materials Science, University of California at Irvine, Irvine, CA 92697-2575, U.S.A. 2 Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616, U.S.A. ABSTRACT The microstructure, mechanical properties and deformation of bimodal structured nanocrystalline Al-7.5Mg alloy were investigated. Grain refinement was achieved by cryomilling of atomized Al-7.5Mg powders, and then cryomilled nanocrystalline powders blended with 15% and 30% unmilled coarse-grained powders were consolidated by hot isostatic pressing followed by extrusion to produce bulk nanocrystalline alloys. Bimodal bulk nanocrystalline Al-7.5Mg alloys, which were comprised of nanocrystalline grains separated by coarse-grain regions, show balanced mechanical properties of enhanced yield and ultimate strength and reasonable ductility and toughness compared to comparable conventional alloys and nanocrystalline metals. INTRODUCTION Nanocrystalline (or nanostructured) metals exhibit remarkable improvements in strength and the possibility of weight savings in structural applications. However, these materials generally suffer from insufficient ductility and reduced toughness compared to conventional alloys with coarser grains. Ductility enhancement of nanocrystalline metals has been tried in nanostructured metals through the incorporation of coarser grains in a fine-grained matrix by heat treatment of bulk samples at relatively high temperature. The annealing results in penalty of reducing strength due to normal and abnormal grain growth of nanocrystalline grains. In a recent report, a non-uniform bimodal grain size distribution, with micrometer-sized grains embedded inside a matrix of nanocrystalline (NC) and ultrafine (
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