Processing of nanostructured metals and alloys via plastic deformation
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Introduction Nanostructured materials are defined as solids with grain, subgrain, twin, or dislocation cells with sizes below 100 nm.1–4 Such materials usually have superior mechanical and physical properties, including high strength,1–4 improved corrosion resistance,5 and higher wear resistance.6 Two complementary approaches have been developed for synthesizing nanostructured solids. The first is the “bottom-up” approach, in which nanostructured materials are assembled from individual atoms or from nanoscale building blocks such as nanoparticles.7 The second is the “top-down” approach, in which existing coarsegrained materials are processed to produce substantial grain refinement and nanostructure. The most successful top-down approaches involve the application of large plastic deformation, in which materials are subjected to plastic strains typically larger than 4–6. Plastic deformation refines grains by a combination of several mechanisms, including dislocation glide, accumulation, interaction, annihilation, tangling, and spatial rearrangement.8–10 For materials with medium or low stacking fault energies, deformation twinning could also play a significant role, especially in the nanocrystalline grain size range.10 Detailed microstructural evolution may vary with the nature of materials as well as deformation mode, strain rate, and temperature. Hansen and co-workers have done extensive work on the grain refinement
mechanism during rolling with strains less than 100%.8 Their general observations also apply to other deformation modes.9 In coarse-grained fcc materials, each grain is divided into many subgrains during plastic deformation.8 Each subgrain deforms under fewer than five slip systems, but a group of adjacent subgrains acts collectively to fulfill the Taylor criterion for maintaining uniform deformation.11 Each subgrain is usually subdivided into dislocation cells. With increasing strain, large subgrains may further divide into smaller subgrains, and the misorientations between subgrains may increase to form lowangle and high-angle (>15°) grain boundaries. Under rapid dynamic strain rates, the grains may be further refined to form ultrafine grains and nanostructures.1–3 Lu et al. systematically studied the formation of nanostructures under surface mechanical attrition treatment (SMAT).10,12 They found that the subgrains become elongated, and their widths become smaller with increasing plastic strain. Finally, the subgrain width equals the dislocation-cell size, forming lamellar subgrains containing a string of dislocation cells. The misorientations across cell boundaries increase with further plastic strain, transforming dislocation cells into subgrains. The equiaxed subgrains further divide into smaller dislocation cells, which, in turn, convert into smaller subgrains as well as nanometer-sized grains with increasing strain. Grain rotation may play a significant role in the formation of the nanometer-sized grains with high-angle
Yuntian Zhu, North Carolina State University, Raleigh, NC 27695, USA; [email protected] R
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