Nanostructures in Ti processed by severe plastic deformation
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J. Gubicza and T. Unga´r Department of General Physics, Eötvös University, Budapest, P.O. Box 32, H-1518, Hungary
Y.M. Wang and E. Ma Department of Materials Science and Engineering, The John Hopkins University, Baltimore, Maryland 21218
R.Z. Valiev Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, Ufa 450000, K. Marksa 12, Russia (Received 12 March 2003; accepted 23 May 2003)
Metals and alloys processed by severe plastic deformation (SPD) can demonstrate superior mechanical properties, which are rendered by their unique defect structures. In this investigation, transmission electron microscopy and x-ray analysis were used to systematically study the defect structures, including grain and subgrain structures, dislocation cells, dislocation distributions, grain boundaries, and the hierarchy of these structural features, in nanostructured Ti produced by a two-step SPD procedure—warm equal channel angular pressing followed by cold rolling. The effects of these defect structures on the mechanical behaviors of nanostructured Ti are discussed.
I. INTRODUCTION
Nanostructured materials have superior mechanical properties such as high strength1 and low-temperature or high-strain-rate superplasticity.2–4 While nanostructured materials synthesized by the consolidation of nanopowders often exhibit brittle behavior, especially under tensile loading,1,5 those produced by severe plastic deformation (SPD) techniques often have both high strength and good ductility.6–10 SPD techniques refine grains by introducing large plastic strains into a coarsegrained material, typically a metal or alloy.11 Metallic materials usually exhibit higher strength but lower ductility after being plastically deformed by conventional forming methods such as rolling, drawing, or extrusion.12,13 This trend is true at least for metals deformed by a low to medium plastic strain. It was also reported that once a metal is deformed beyond a certain plastic strain, the ductility of the metal could increase with additional plastic strain. 10,14 A combination of high strength and good ductility is attractive for structural applications of nanostructured materials. The superior mechanical properties of nanostructured materials are rendered by their unique nanostructures. The nanostructures determine the deformation mechanisms, which in turn determine the mechanical behaviors. It is not well understood why the SPD-processed 1908
J. Mater. Res., Vol. 18, No. 8, Aug 2003
nanostructured materials have high strengths while retaining good ductility. These materials are 100% dense and contamination free, which certainly contribute to their good ductility. More importantly, they have unique nanostructures that are significantly different from nanomaterials synthesized by consolidation of nanopowders. The first step toward understanding the mechanical behaviors of these materials is to systematically characterize their nanostructures. Huang et al. recently characterized the nanostructures of Cu produced by a SPD technique, repetiti
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