Synthesis and microstructure of bulk nanocrystalline copper
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I. INTRODUCTION
NANOCRYSTALLINE copper (NC-Cu) has been shown to have exceptional mechanical properties, from high strength and ductility[1] to near-perfect elastoplasticity,[2] high toughness and ductility,[3] and “super extensibility.”[4] Superplasticity may even be achievable for small-enough grain sizes.[5] Both the study of mechanical properties and the development of potential applications require the preparation of sufficiently large amounts of bulk nanostructured material. So far, results have mainly been reported by two specimen-preparation techniques that produce very different microstructures. Equal-channel angular pressing[6] is able to produce extremely large specimens. The grain size of such specimens is quoted to be in the range of 150 nm, limited by the underlying dislocation-based mechanism. Other severe plastic deformation (SPD) techniques produce variants in the microstructure; high-pressure torsion (HPT)[7] allows further grain refinement (up to 20 nm), and severe cold rolling at liquid-nitrogen temperature[3] produces an interesting bimodal nanomicrostructure (with grain sizes of 200 nm and 1 to 3 m). Electrodeposition is another technique that produces reasonably large specimens. For example, the lowest normalized superplastic temperature was reported for Ni,[8] and super extensibility was obtained for copper. The grain size was quoted to be as low as 20 nm, although an extremely large proportion will be subgrains separated by low angle GBs. C. LANGLOIS, formerly Ph.D. Student, Centre d’Etudes de Chimie Métallurgique CECM-CNRS, is Associate Professor, Laboratoire Matériaux Et Phénom`enes Quantiques, LPS, ESPCI 10 Rue Vauquelin 75252 Paris Cedex ¨ 05 France. M.J. HYTCH, S. LARTIGUE-KORINEK, and Y. CHAMPION, Senior Researchers, are with the Centre d’Etudes de Chimie Métallurgique CECM-CNRS, 94407 Vitry-sur-Seine, France. Contact e-mail: champion@ glvt-cnrs.fr P. LANGLOIS, Senior Researcher, is with the Laboratoire d’Ingénierie des Matériaux et des Hautes Pressions LIMHP-CNRS, Université Paris XIII, 93430 Villetaneuse, France. Manuscript submitted June 28, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
The SPD and electrodeposition are seen as the most relevant preparation techniques, because they produce fully dense bulk materials. The SPD technique, as an in-situ refinement process, has the additional advantage of producing noncontaminated samples. In contrast, powder metallurgy (P/M) processing, either of nanopowders or ball-milled powders, is often criticized regarding bulk materials fabrication. The main reasons are that sample purity and ultimate densification are difficult to obtain. Contamination can be avoided by compacting nanopowders in situ at high pressure, but the resulting specimens are small.[9] Larger specimens can be made by keeping particles oxidized[10] and with in-situ processing, but specimens then suffer from residual porosity.[11] If chemistry and densification can be controlled, P/M technology offers new perspectives for nanostructures. The grain size and the nature o
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