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Title:Defect mediated plasticity in Cu/Ag nanoscale multilayered metal composites: deformation twinning and wavy interface formation
Author(s):Li, Ruizhi
Advisor(s):Chew, Huck Beng
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:M.S.
Genre:Thesis
Subject(s):Nanolayered metals
flaws
deformation twinning
interface migration
wavy interfaces
molecular dynamics
Abstract:The exceptional mechanical properties of metallic nanolayers originate from the high density of nanoscale interfaces. However, conflicting observations in the relationship between mechanical properties and interlayer thicknesses, as well as discrepancies in measured strength between experiments and simulations, suggest that microstructural flaws play an essential role in the deformation behavior of actual metallic nanolayers. In this thesis, molecular dynamics simulations are used to uncover two distinct nanoscale plasticity mechanisms activated by the presence of micro-cracks and columnar grain boundaries in Cu/Ag nanolayers under tension. The first mechanism is deformation twinning, caused by emission of twinning partials from the micro-cracks and columnar grain boundaries. These deformation microtwins are transmitted across multiple Cu/Ag interlayers and facilitate the communication between spatially separated flaws. In addition, the intersections of microtwins on non-parallel slip planes produce formidable locks, which serve as stress concentration sites for incipient crack growth. The second mechanism is interlayer interface migration, which results in the morphological transition of initially planar Cu/Ag nanolayer to become wavy. This planar-to-wavy transition is driven by energetics, and is facilitated by dislocation climb along columnar grain boundaries. The above tensile-activated plasticity mechanisms are distinctly different from the strengthening mechanism associated with interface crossings of single dislocations under compression. Implications of these mechanisms to the ductility of metallic nanolayers, as well as the activation of novel nanoscale plastic recovery processes, are discussed.
Issue Date:2013-08-22
URI:http://hdl.handle.net/2142/45307
Rights Information:Copyright 2013 Ruizhi Li
Date Available in IDEALS:2013-08-22
Date Deposited:2013-08


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