Nanocrystalline grain structures developed in commercial purity Cu by low-temperature cold rolling
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Nanocrystalline pure copper was obtained by cold rolling a commercial bulk Cu to very large extensions at subambient temperatures. The eventual formation of nanocrystalline grain structures is attributed to dynamic grain refinement (recrystallization) mechanisms activated by the low-temperature continuous plastic deformation that leads to ultrahigh densities of dislocations. Nanocrystalline metals, conventionally defined as polycrystals with grain sizes less than 100 nm, are of considerable scientific and technological interest at present.1,2 Of the many techniques used for processing nanocrystalline materials, the severe plastic deformation (SPD) process is arguably the most promising for producing bulk nanostructured metals3,4 while avoiding the consolidation step that often leaves artifacts in the samples.5 A number of SPD techniques have been developed in recent years, typical examples being equal channel angular pressing and high-pressure torsion4 conducted at room temperature (RT). However, the grain sizes achieved so far through these routes for a pure metal such as Cu are actually outside the nanocrystalline regime,3,4 i.e., of the order of a few hundred nanometers (commonly referred to as the ultrafine grained range). This is also true for conventional bulk formation processes such as cold plate rolling, where a 100 times thickness reduction can only bring the scale of the microstructure down to the order of 200 nm for Cu.6 In these heavily worked metals, large fractions of the grain boundaries are low-angle ones evolved from dislocation cell walls.7 Interestingly, there are also known deformation processes that can lead to extremely small grain sizes of the order of 20 nm in Cu, with predominantly high-angle grain boundaries. Examples are ball milling of powders,8–10 friction deformation under large sliding loads,6 and ultrasonic shot peening.11 As these techniques concentrate deformation in powder particles or surface layers that are a few microns in size/depth, the plastic strain experienced by the small volume of material is extremely large and the strain rate is very high, making it efficient to store high densities of dislocations that decompose the grains. Unfortunately, these processes are not easily adaptable for processing bulk samples. We note that lowering the deformation temperature may have similar a) b)
Department of Mechanical Engineering. Address all correspondence to this author. e-mail: [email protected]
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J. Mater. Res., Vol. 17, No. 12, Dec 2002 Downloaded: 18 Mar 2015
effects in enhancing the efficiency of plastic deformation and providing the dislocation dynamics favorable for the formation of nanocrystalline grains.10,12,13 In this paper, we demonstrate a new scheme of developing truly nanocrystalline grain structures in a commercial purity metal through low-temperature (LT) large plastic deformations of a bulk sample, using Cu as an example. A commercial purity Cu (99.9+%) bar 12.7 mm in diameter was annealed at 900 °C for 2 h (the resulting grain si
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