MULTISCALE MODELLING AND SIMULATION OF DEFORMATION AND STRENGTH OF NANOSCALE METALLIC MULTILAYER SYSTEMS
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The objective of this research is to investigate the deformation behaviors of two types of NMMs at lower length scales: 1) One dimensional Cu-Ni, Au-Ni nanowires with coherent interfaces and 2) Two dimensional Cu-Nb multilayers with incoherent interfaces. Using molecular dynamics simulations, we investigate the different deformation mechanisms that govern the plastic behavior of the NMMs at different length scales. Based on the fundamental physics of deformation captured by these simulations, we propose models that explain the dependence of strength on layer thickness and identify the regions where the deformation is controlled by either dislocation propagation mechanism or dislocation nucleation mechanism. Chapter 2 investigates the deformation mechanisms of Cu-Ni composite nanowires subjected to uniaxial tensile loading by using MD simulations. The coupled effects of geometry and coherent interface on the twinning and pseudoelastic behavior of nanowires are investigated. It is shown that nanowires exhibit pseudoelastic behaviors when their layer thicknesses are below a critical thickness. We captured similar deformation mechanisms through MD simulations of Au-Ni nano ligaments that are assumed as building blocks of composite Au nanofoams with Ni shells, in chapter 3. Chapter 4 studies the deformation behaviour of Cu-Nb NMMs with incoherent interfaces. Using MD simulations, we investigate the strengthening effect of the weak interfaces interacting with glide dislocations by embedding artificial dislocations inside the layer. In addition, the effects of interfacial discontinuities such as ledges and steps on the strength of the NMMs are investigated. Chapter 5, studies the strengthening effects of the additional second phase particles inside the same Cu-Nb bi-layers. We developed an analytical model to explain the strengthening effect of the precipitates. The theoretical results show a qualitative agreement with the finding of the atomistic simulations. In chapter 6, the operative deformation mechanisms at different length scales for Cu-Nb multilayers under biaxial tensile deformation are determined. We established a unique viscoplastic continuum model able to address the macroscale plastic behaviour of bulk NMMs with layer thickness from few nanometers to hundreds of micrometers. An anisotropic yield function is proposed based on the plastic flow potential obtained from biaxial loading of the NMMs.