Atomic-Scale Study of Plastic-Yield Criterion in Nanocrystalline Cu at High Strain Rates
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TRODUCTION
NANOCRYSTALLINE metals, due to their enhanced strength and wear resistance, show significant promise for a number of future high-technology applications such as microelectromechanical systems, nanoelectromechanical systems, and nanoscale devices. To maximize the performance and reliability of these devices, a fine-tuning of the nanostructure is required, which necessitates a fundamental understanding of the deformation and failure mechanisms.[1] Nanocrystalline metals have been extensively studied,[2–9] to understand the micromechanisms governing their macroscopic mechanical behavior. At the atomic scale, plastic deformation mechanisms in nanocrystalline materials can be classified into dislocation-based and grain-boundary (GB)–based processes. A reduction in grain size results in an increase in the yield strength of materials, a relation known as the Hall–Petch effect,[10] which states that the yield strength varies inversely with the square root of the grain size. Recent studies[11–14] indicate that the increase in strength with decreasing grain size A.M. DONGARE, NRC Research Associate, Department of Mechanical and Aerospace Engineering and Department of Materials Science and Engineering, D.W. BRENNER, Kobe Steel Distinguished Professor, Department of Materials Science and Engineering, and M.A. ZIKRY, Professor, Department of Mechanical and Aerospace Engineering, are with North Carolina State University, Raleigh, NC 27695-7907. Contact e-mail: [email protected] A.M. RAJENDRAN, Chair and Professor, is with the Department of Mechanical Engineering, University of Mississippi, Oxford, MS 38677. B. LAMATTINA, Program Manager, is with the United States Army Research Office, Research Triangle Park, NC 27703. Manuscript submitted April 1, 2009. Article published online December 2, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
reaches a maximum after which a further decrease in the grain size (less than ~ 12 nm) can result in weakening of the metal. This weakening of the metal, due to the shift in the dominating mechanism of plastic deformation from dislocation-induced plasticity in the case of coarse-grained materials to GB sliding in the case of ultrasmall grain sizes, is referred to as the inverse Hall–Petch behavior.[15] Nanocrystalline metals with ultrasmall grain sizes (d £ 30 nm) have gained considerable attention due to their increased strengths during deformation at high strain rates.[16] In addition, shock loading of these ultrasmall nanocrystalline metals at speeds that are greater than the speed of sound (strain rates ‡ 108 s1) limits the GB sliding mechanism and thus results in ultrahigh strength values for the nanocrystalline metal.[17] This limiting of GB sliding at ultrahigh strain rates may lead to modifications in the yield criteria/ behavior at the ultrasmall grain sizes. To maximize the performance and reliability of these devices, a finetuning of the nanostructure is required, which necessitates a fundamental understanding of the deformation and failure of the constituent materials of the devi
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