In this dissertation, simulation techniques are used to understand the role of surfaces and grain boundaries in the deformation response of metallic materials. This research utilizes atomistic scale modeling to study nanoscale deformation phenomena with time and spatial resolution not available in experimental testing. Molecular dynamics techniques are used to understand plastic deformation of grain boundaries and surfaces in metals under different configurations and loading procedures.
Stress and strain localization phenomena are investigated at plastically deformed boundaries in axially strain thin film samples. Joint experimental and modelling work showed increased stress states at the intersections of slip planes and grain boundaries. This effect, as well as several other differences related to stress and strain localization are thoroughly examined in digital samples with two different grain boundary relaxation states. It is found that localized stress and strain is exacerbated by initial boundary disorder.
Dislocation content in the randomly generated boundaries of these samples was quantified via the dislocation extraction algorithm. Significant numbers of lattice dislocations were present in both deformed and undeformed samples. Trends in dislocation content during straining were identified for individual samples and boundaries but were not consistent across all examples. The various contributions to dislocation content and the implications on material behavior are discussed.
The effects of grain boundary hydrogen on the deformation response of a digital Ni polycrystalline thin film sample is reported. H content is found to change the structure of the boundaries and effect dislocation emission. The presence of dispersed hydrogen caused a slight increase in yield strength, followed by an increase in grain boundary dislocation emission and an increase in grain boundary crack formation and growth.
An atomistic nano indenter is employed to study the nanoscale contact behavior of the indenter-surface interface during nano-indentation. Several indentation simulations are executed with different interatomic potentials and different indenter orientations. A surface structure is identified that forms consistently regardless of these variables. This structure is found to affect several atomic layers of the sample. The implications of this effect on the onset of plasticity are discussed.
Finally, the implementation of an elastic/plastic continuum contact solution for use in mesoscale molecular dynamics simulations of solid spheres is discussed. The contact model improves on previous models for the forces response of colliding spheres by accounting for a plastic regime after the point of yield. The specifics of the model and its implementation are given in detail.
Overall, the dissertation presents insights into basic plastic deformation phenomena using a combination of experiment and theory. Despite the limitations of atomistic techniques, current computational power allows meaningful comparison with experiments. / Ph. D. / Certain engineering metals have a remarkable bend-then-break quality. This allows a metal component to withstand damage without totally failing. The process of permanent distortion is called plastic deformation. Metals, in nearly all practical forms, contain defects. During plastic deformation, defects are generated, moved, changed and annihilated. The rates of these actions govern the mechanical behavior of metals. There are several types of defects and several ways in which they can interact, forming a complex interplay during plastic deformation. The focus of this dissertation is on plastic deformation associated with two particular defect types: surfaces and grain boundaries. Because these defects occur on a very small length scale, the details of their behavior can best be observed via simulation. For this reason, Molecular Dynamics was employed as the primary research tool, and other methods were used for validation. This allows fully 3D rendering of our simulated samples with atom-scale resolution, and complete stress/energy information. In each of the 6 manuscripts presented in this dissertation, new insights into the plastic deformation around surfaces or grain boundaries is presented.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/78704 |
| Date | 15 August 2017 |
| Creators | Kuhr, Bryan Richard |
| Contributors | Materials Science and Engineering, Farkas, Diana, Corcoran, Sean G., Hin, Celine, Reynolds, William T. Jr. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/x-zip-compressed, application/pdf, application/pdf, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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