Experiment and FEM Analysis of Tensile Behavior of Bimodal Nanocrystalline Al-Mg Alloys
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Experiment and FEM Analysis of Tensile Behavior of Bimodal Nanocrystalline Al-Mg Alloys Zonghoon Lee, Johnathan Lee, Enrique J. Lavernia1 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 Davis, Davis, CA 95616, U.S.A.
ABSTRACT The tensile behavior of bimodal nanocrystalline Al-7.5Mg alloys was investigated using experiments and two-dimensional axisymmetric elastic-plastic finite element method (FEM). Cryomilled nanocrystalline powders blended with 15% and 30% unmilled coarse-grained powders were consolidated by hot isostatic pressing followed by extrusion to produce bulk bimodal nanocrystalline Al-7.5Mg alloys, which were comprised of nanocrystalline grains separated by coarse-grain regions. The calculated stress-strain curves have acceptable agreement with experimental curves of the bimodal structures. The bimodal Al-7.5Mg alloys show reasonable ductility while retaining enhanced strength compared to conventional alloys and nanocrystalline metals.
INTRODUCTION Nanocrystalline 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 annealing [1-3]. Previous reports suggest that the presence of coarser grains within the nanocrystalline matrix may enhance the ductility of nanocrystalline materials [4-6]. However, the selective grain growth and achievement of target size and phase of embedded coarse grains may be difficult or impossible to control by thermal treatment in practice. In contrast, a bimodal grain structure can be achieved by consolidation of blended powders. In this process, cryomilled nanocrystalline (NC) and coarsegrained (CG) powders can be combined in specific proportions, enabling the design and manufacture of materials that achieve the desired balance of enhanced strength with acceptable ductility and toughness. Moreover, mechanical alloying is possible to produce bulk samples in sufficient size while several other techniques have fabricated small samples to explore the intrinsic properties of the materials [7-9]. In the present work, ductile phase toughening in bimodal structured Al-7.5Mg was achieved by deliberate blending of CG powders with cryomilled powders in select proportions. The stress-strain behavior was investigated using uniaxial tensile tests and finite element method (FEM). In the FEM work, plain strain two-dimension solid axisymmetric nonlinear (elastic-
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plastic) anyalysis was used to reveal the stress-strain behaviors and deformation mechanisms of the bimodal nanostructures.
EXPERIMENTAL DETAILS Prealloyed Al-7.5%Mg (in weight percent) alloy po
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