Bimodal Microstructure and Mechanical Properties of Cryomilled Nanocrystalline Al-7.5Mg
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Bimodal Microstructure and Mechanical Properties of Cryomilled Nanocrystalline Al-7.5Mg 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 and mechanical properties consisting of tensile behavior and hardness of bulk nanocrystalline Al-7.5Mg alloy were investigated. Grain refinement was achieved by cryomilling of atomized Al-7.5Mg powders, and then nanocrystalline powders blended with 15% and 30% coarse-grained Al-7.5Mg powders were consolidated by hot isostatic pressing (HIP) followed by extrusion to produce bulk nanocrystalline Al-7.5Mg alloys. Bimodal structures, which enhance ductility and toughness of nanocrystalline metals, were produced that consisted of nanocrystalline grains and elongated coarse-grain bands. Examination of indentation revealed unusual deformation mechanisms and interactions between the coarse-grain bands and nanocrystalline regions. The ductile coarse-grain bands underwent extensive plastic deformation near indentation, while nanocrystalline regions exhibited limited deformation. INTRODUCTION The relations between microstructures and mechanical properties of bulk nanocrystalline metals have been investigated in recent years. Although several materials and microstructures have been developed high strength nanocrystalline metals, nanocrystalline metals still suffer from ductility and toughness reduction in contrast with increasing its strength. In one recent study, elastic-nearly perfectly plastic stress-strain behavior was observed and reported in Al10Ti-2Cu alloy produced by mechanical alloying using cryomilling [1,2,3]. Such behavior is atypical for coarse-grain alloys of similar composition. Multi-scale structures, which were consisted of nanocrystalline grains and coarse-grain regions, enhanced ductility and toughness of bulk nanocrystalline alloys. In an effort to optimize toughened alloys based on multi-scale microstructures, more thorough understanding of microstructure and of structure-property relations in this structure was required. In the extended study, bimodal structures, which were comprised of nanocrystalline grains separated by coarse-grain regions, were produced to achieve ductile phase toughening in Al-7.5Mg. Thus in this work, the bimodal microstructure of bulk nanocrystalline Al-7.5Mg alloy was investigated using electron microscopy and optical microscopy. The stress-strain behaviors of bimodal Al-7.5Mg were estimated using uniaxial tensile test. In addition, Vickers hardness tests also were performed along the parallel and perpendicular to the extrusion direction to investigate the orientation dependent response of the bimodal microstructures and the interactions between c
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