On the influence of N on residual microstrain in cryomilled Ni
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NANOCRYSTALLINE materials may be broadly classified as those having spatial attributes (e.g., particle size, grain size, film thickness, etc.) that fall in the 1 to 100 nm regime, although strict adherence to the upper bound (100 nm) is not a requirement. The unique properties of this class of materials are often attributed to the high concentration of interfaces and the unusual behavior that emerges when the physical size of a structure falls in the “nanometric” range. For example, in a nanocrystalline material, nearly 50 pct of all atoms in a grain volume will be located at or near grain boundaries when the grain size is 5 nm.[1,2] Interest in this class of materials increased rapidly, when it was realized that early skepticism on the availability of commercial quantities of nanocrystalline powders were unfounded. Various techniques have been successfully used to synthesize nanostructured materials, e.g., inert gas condensation,[3] rapid solidification,[4] electrodeposition,[5] sputtering,[6] crystallization of amorphous phase by annealing,[7] and chemical processing.[8] Mechanical alloying (MA)—ball milling— has also been widely used to synthesize nanostructured materials.[9] It is believed that mechanical alloying produces its nanostructures by the structural decomposition of coarsegrained structures as the result of severe plastic deformation. The first report of formation of nanostructured material synthesized by MA was by Thompson and Politis in 1987,[10] even though the specific mention of formation of “nanometer
KYUNG H. CHUNG, Postdoctoral Researcher, is with the Department of Chemical Engineering and Materials Science, University of CaliforniaIrvine, Irvine, CA 92697-2575. ENTRIQUE J. LAVERNIA, Professor, is with the Department of Chemical Engineering and Materials Science, University of California-Davis, Davis, CA 95616, and University of California-Irvine. Contact e-mail: [email protected] Manuscript submitted June 19, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
order crystalline structures produced by mechanical alloying” was by Shingu et al. in 1988.[11] Koch[12] has summarized the results on the synthesis and structure of nanocrystalline structures produced by mechanical attrition. Our current understanding of the mechanisms that govern the evolution of a nanocrystalline microstructure in singlephase materials during ball milling is summarized in the literature.[9,13] According to Fecht,[13] the evolution of microstructure or grain size refinement by mechanical attrition includes three stages. (1) The deformation is localized into shear bands consisting of an array of dislocations with high density. (2) At a certain strain level, these dislocations annihilate and recombine to form small subgrain boundaries separating the individual large grains. The subgrains formed via this route are already in the nanometer size range with diameters often between 20 and 30 nm. During further attrition, the sample volume exhibiting small grains extends throughout the entire specimen. (3) The orient
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