On the behavior of microstructures with multiple length scales
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NANOSTRUCTURED (,200 nm) and ultrafine-grained (UFG, ,500 nm) materials have engendered considerable interest, not only as a result of their novel physical and mechanical attributes,[1] but also due to recent experimental[2–5] and theoretical studies[6–11] that suggest that the deformation mechanisms in these materials may be sizedependent. A review of the available literature shows that despite extensive efforts to prepare nanostructured or UFG materials via various methods (e.g., inert gas condensation, electron deposition, plasma synthesis, crystallization of amorphous solids, equal channel angular pressing, highpressure torsion, mechanical milling), a high strength was usually achieved at the expense of ductility when compared with the behavior of coarse-grained counterparts.[12] High ductility values have been reported for nanostructured materials in only a few cases,[13–16] and some of these appear to be exceptional,[13,14] or occurred at elevated temperatures.[15] Inspection of the relevant literature reveals that a number of approaches have been proposed in an effort to enhance the ductility of nanostructured materials.[16,17] For example, the measured high tensile ductility in a nanostructured Cu was attributed to the presence of a bimodal grain structure, which was achieved by partial annealing.[16] In earlier studies, Tellkamp et al. were the first to suggest that the presence of coarse-grained regions was responsible for an attractive ductility of 8.4 pct measured with a standard tensile specimen.[18] A multiscale microstructure that consisted of nanocrystalline grains and elongated coarse-grain bands of aluminum was also ZHIHUI ZHANG, Graduate Student Researcher, BING Q. HAN, Materials Scientist, and ENRIQUE J. LAVERNIA, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Contact e-mail: [email protected] KYUNG H. CHUNG, formerly Postdoctoral Researcher with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, is now Development Engineer with Heraeus MTD, Chandler, AZ 85226. Manuscript submitted July 25, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
reported in a nanostructured cryomilled Al-Ti-Cu alloy.[19] The improvement in ductility reported for materials containing coarse-grained regions has been rationalized in terms of two mechanisms. The first mechanism involves strain hardening (strain accommodation) in the coarsegrained regions;[16] the second mechanism involves crack blunting and crack bridging.[17] Review of the available literature reveals that despite encouraging mechanical behavior results, fundamental information regarding microstructure evolution, especially the occurrence of coarse grains (;1 mm), is very limited.[20,21] It is generally accepted that nanostructured crystallites produced by mechanical milling are primarily separated by high angle boundaries[22] and that the microstructure undergoes grain growth at elevated temperatures.[23,24] Furthermore, the
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