Microstructural evolution and deformation of cryomilled nanocrystalline Al-Ti-Cu Alloy
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6/11/03
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Microstructural Evolution and Deformation of Cryomilled Nanocrystalline Al-Ti-Cu Alloy Z. LEE, R. RODRIGUEZ, R.W. HAYES, E.J. LAVERNIA, and S.R. NUTT The microstructural evolution during processing and tensile deformation of a nanocrystalline Al-Ti-Cu alloy was investigated using transmission and scanning electron microscopy. Grain refinement was achieved by cryomilling of elemental powders, and powders were consolidated by hot isostatic pressing (“HIPing”) followed by extrusion to produce bulk nanocrystalline Al-Ti-Cu alloys. In an effort to enhance ductility and toughness of nanocrystalline metals, multiscale structures were produced that consisted of nanocrystalline grains and elongated coarse-grain (CG) bands of pure aluminum. Examination of bulk tensile fracture samples revealed unusual failure mechanisms and interactions between the CG bands and nanocrystalline regions. The ductile CG bands underwent extensive plastic deformation prior to fracture, while nanocrystalline regions exhibited nucleation and growth of voids and microcracks. Cracks tended to propagate from nanocrystalline regions to the CG bands, where they were effectively arrested by a combination of crack blunting and crack bridging. These processes were instrumental in enhancing the toughness and ductility of the nanocrystalline alloy.
I. INTRODUCTION
THE behavior and processing of nanocrystalline materials have been reviewed in several recent treatises.[1,2,3] Although several techniques have been developed to prepare nanocrystalline materials, mechanical alloying is one of the principal methods used to produce nanocrystalline powders in sufficient quantities for bulk material processing.[4,5] The mechanical behavior of such bulk nanocrystalline alloys is only beginning to be understood. For example, in one recent study, elastic–nearly perfectly plastic stress-strain behavior was observed and reported in an Al-10Ti-2Cu alloy produced by mechanical alloying using cryomilling.[6,7,8] Such behavior is atypical for coarse-grain alloys of similar composition, and is like the behavior of metallic glasses, which, however, typically show only limited ductility. In an effort to enhance ductility and toughness of bulk nanocrystalline Al-Ti-Cu alloys, multiscale structures were recently prepared in the current program. The intent was to create structures comprised of nanocrystalline grains separated by coarse-grain (CG) regions, thereby achieving ductile phase toughening. Before embarking on a full-scale processing effort to optimize a toughened alloy, a more thorough understanding of microstructure and of structure-property relations in Al-10Ti-2Cu was required. Thus, in this work, the microstructural evolution during processing of nanocrystalline powders and bulk nanocrystalline Al-10Ti-2Cu alloy was investigated using electron miZ. LEE, Graduate Student, and S.R. NUTT, Professor, are with the Department of Materials Science, University of Southern California, Los Angeles, CA 90089-0241. Contact e-mail: [email protected] R. RO
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