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"The mechanical behavior of Metallic Nanolayered Composites (MNCs) is governed by their underlying microstructure. In this dissertation, the roles of the interlayer spacing (grain size, d) and the intralayer biphase spacing (layer thickness, h) on mechanical response of Cu/Nb MNCs are examined by Molecular Dynamics (MD) simulations. The study of the strength of MNCs show that small changes in both d and h play a profound role in the relative plastic contributions from grain boundary sliding and dislocation glide. The interplay of d and h leads to a very broad transition region from grain boundary sliding dominated flow, where the strength of the material is weak and insensitive to changes in h, to grain boundary dislocation emission and glide dominated flow, where the strength of the material is strong and sensitive to changes in h. The study of the fracture behavior of MNCs shows that cracks in Cu and Nb layers may exhibit different propagation paths and distances under the same external loading. Interfaces can improve the fracture resistance of the Nb layer in Cu/Nb MNCs by providing mobile dislocation sources to generate the plastic strain at the crack tip necessary for crack blunting. Increasing the layer thickness can further enhance the fracture resistance of both Cu and Nb layers, since the critical stress for activating dislocation motion decreases with increasing the layer thickness. A novel atomistic-informed interface-dislocation dynamics (I-DD) model has been developed to study Metal-Ceramic Nanolayered Composites (MCNCs) based on the key deformation process and microstructure features revealed by MD simulations. The I-DD predicted results match well with the prior experimental results where both yield stress and strain hardening rate increase as the layer thickness decreases. This I-DD model shows great potential in predicting and optimizing the mechanical properties of MNCs"--Abstract, page iv.
The goal of this thesis is to understand the interaction between dislocations and various metallic interfaces in nanoscale metallic multilayers (NMM). At lower strain rates, this mean understanding the effect of interfaces to the strain hardening of the NMMs; at higher strain rates, this means the effect of the interfaces on the spallation strengths of the NMMs. NMMs possess ultra-high strength level which is owing to the interactions between single dislocations (i.e. no pile-up) and interfaces. In this thesis, aiming at the goal, using atomistic simulations several nanoscale metallic multilayers subjected to different loading conditions and strain rates are being considered.
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.
A novel interatomic potential of ternary Al-TiN has been developed to study the deformation behavior of Al-TiN nanolaminates. The ternary nanolayered Al-TiN composite has attracted a lot of interest due to its combination of strength and ductility. The current analysis on the system has been primarily concentrated on continuum models which are inadequate to explain the key deformation events such as nucleation and interaction of dislocations. Progress in the preferred atomistic approach has been hampered however by the lack of available interatomic potential optimized for the ternary system. I developed a many-body potential based on embedded atomic model (EAM) by employing the force-fitting code Potfit to sample the energy and force data generated from the ab-initio molecular dynamics simulations of the ternary system using VASP code. The potential’s analytical EAM function was subsequently optimized and utilized to simulate structures of bulk Al & TiN and Al-TiN nanolaminates. I then focused on modeling the deformation behavior of Al-TiN multilayers under compression through classical molecular dynamics simulations. I found that the total bilayer thickness as well as volume ratio between TiN and Al nanolayers play a major role in controlling the dislocation nucleation and mobility and the stress accumulation at the layer interface and thus determine the deformation behavior and failure mechanisms of the nanolayered composites.
We developed here fundamental models of plasticity, based on dislocation dynamics and atomistic computer simulation methods for the prediction of the strength and plastic deformation at the nano-to-micro-length scales. The models are applied to the simulation of plastic flow in ultra-strong nano-laminates. The developed methods are: (1) An ab-initio based hybrid approach based on an extension of the parametric dislocation dynamics (DD) to bi-materials where the dislocation spreading over the interface is explicitly accounted; (2) a hybrid ab initio-discrete dislocation dynamics model to study the core structure in straight and curved dislocations, with application to single layers and across material interfaces; (3) Molecular dynamics (MD) modeling of dislocation motion and deformation in nano-layered composite materials and twins; and (4) Dislocation Dynamics (DD) modeling of dislocation motion and deformation modes of anisotropic, nano-layered composite materials.
Atomistic simulations have been used to study the effect of various types of point defects on the mechanical response of FCC single crystals in nanoindentation and uniaxial tests. To study the effect of spatial distribution of defects in nanoindentation testing, various point defects were located in different relative positions to the indenter. When the defect position was close to the regions of high shear stresses the nucleation of dislocations was related to the location of the defect; however homogeneous nucleation of dislocations was also observed for defect-containing crystals. The effect of the point defects was independent of the indenter size, and the applied pressure needed to initiate plasticity, when compared to defect-free crystals, was a reduction of approximately 10%, 20%, 20% and 50% for a single vacancy, di-vacancy, self-interstitial atom and stacking fault tetrahedron (SFT), respectively. The stochastic nature of the pop-in loads was further explored for different orientations using molecular dynamics and complementary nanoindentation experiments on (100), (101) and (111) single crystals of copper and Ni200. The sensitivity of the crystal to the presence of internal structural defects depends strongly on its crystallographic orientation. The simulations suggest that the first event observed experimentally may not correspond to the first plastic deformation event. Anisotropy effects were also studied for various orientations in uniaxial tests in the presence of a centered SFT. Both the normal stresses to the slip plane and the relative values of Schmid factor in compression and tension affect the observed compression/tension yield asymmetry. The reduction in yield stress was found to be larger in compression than in tension for almost all orientations. The simulations suggest that compression is a more reliable experimental tool for studying the effect of structural defects on the mechanical behavior of the FCC crystals, while tension may be more useful to determine size effects in deformation. Finally, simulations at high temperatures showed that internal defects are capable of reducing the temperature sensitivity of yielding in various crystal orientations, especially when the stress field is mainly compressive like those in nanoindentation and compression tests.
A simulation framework was developed for studying the deformation behavior of metallic materials. Atomistic simulations were employed to study dislocation nucleation during nanoindentation and to correlate dislocation behavior and overall material response in thin-film crystals. An instrumented indenter was acquired to study the indentation behavior of metallic composites. Experimental and continuum- based modeling works on indentation of discontinuously reinforced metal matrix composites were also conducted. Detailed microscopic features were analyzed, which aided in our fundamental understanding of plastic deformation in these materials.
Metallic Glass-Based Nanocomposites: Molecular Dynamics Study of Properties provides readers with an overview of the most commonly used tools for MD simulation of metallic glass composites and provides all the basic steps necessary for simulating any material on Materials Studio. After reading this book, readers will be able to model their own problems on this tool for predicting the properties of metallic glass composites. This book provides an introduction to metallic glasses with definitions and classifications, provides detailed explanations of various types of composites, reinforcements and matrices, and explores the basic mechanisms of reinforcement-MG interaction during mechanical loading. It explains various models for calculating the thermal conductivity of metallic glass composites and provides examples of molecular dynamics simulations. Aimed at students and researchers, this book caters to the needs of those working in the field of molecular dynamics (MD) simulation of metallic glass composites.
This book addresses issues pertinent to mechanics and stress generation, especially in recent advanced cases of technology developments, spanning from micrometer interconnects in solar photovoltaics (PV), next-gen energy storage devices to multilayers of nano-scale composites enabling novel stretchable/flexible conductor technologies. In these cases, the mechanics of materials have been pushed to the extreme edges of human knowledge to enable cutting-edge, unprecedented functionalities and technological innovations. Synchrotron X-ray diffraction, in situ small-scale mechanical testing combined with physics-based computational modeling/simulation, has been widely used approaches to probe these mechanics of the materials at their extreme limits due to their recently discovered distinct advantages. The techniques discussed in this manuscript are highlights specially curated from the broad body of work recently reported in the literature, especially ones that the author had led the pursuits at the frontier himself. Extreme stress generation in these advanced material leads to often new failure modes, and hence, the reliability of the final product is directly affected. From the recent topics and various advanced case studies covered in this book, the reader gets an updated knowledge of how new mechanics can and has been applied in Design-for-Reliability (DfR) for some of the latest technological innovations known in our modern world. Further, this also helps in building better designs, which may avoid the pitfalls of the current practiced trends.