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Note: Thesis (Ph.D. -- Brown University (2016)
Name: Personal
Name: Personal
Name: Personal
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Abstract: Both interfaces and spatial gradients in engineering structures exhibit distinct mechanical properties in contrast to their bulk counterparts. The demands of both the aerospace and computer industries continue to migrate towards smaller, more highly precisioned components. A precise mechanistic understanding and description of size-dependent plasticity is therefore of increasing engineering significance. In the first part of this thesis we will focus on two examples of size-dependent plasticity where interactions between dislocations and characteristic lengths on the order of tenths of microns are responsible for the emergence of a size-dependence. Experiments have demonstrated a mechanical size-dependence, however only in the presence of a stress gradient. These effects will be rationalized through both 3D Discrete Dislocation Dynamics simulations and a simple analytical model, both of which capture the observed experimental phenomena. The following chapter will focus on a size effect intrinsic to Molecular Dynamics (MD) simulations of dislocations. While these simulation methodologies have successfully brought insight to strengthening and hardening mechanisms, often little to no attention is paid to the interaction between a bowing dislocation and a finite simulation cell, where both the cell size and cell shape behave analogous to a thin film, where thinner is stronger. This strengthening effect manifests itself in the form of (spurious) image forces which markedly degrade the quality and reliability of MD computations. Through a simple mechanistic model, these forces are quantified and used to rationalize inconsistent MD results throughout the literature. A unique simulation cell shape is found to minimize unduly effects. Both examples provide guidance and a simple mechanistic framework for understanding size-dependence in metals. We will then examine a size effect ubiquitous to commercial alloys containing precipitates and dislocations, namely the dislocation line tension (Γ). Despite its importance, the precise value of Γ in a given material has proven difficult to assess, with many already existent measures encompassing a wide range of values for the same material. We will present results from a multiscale (image free) simulation and robust analysis of the dislocation line tension for two widely-used interatomic potentials for Al. A central part of the analysis involves an effective Peierls Stress applicable to curved dislocation structures that markedly differs from that of perfectly straight dislocations but is required to describe the bow-out both in loading and unloading. Our results indicate that continuum line dislocation models, based on elasticity theory and various core-cut-off assumptions, are fundamentally unable to reproduce full atomistic results, thus hampering the detailed predictive ability of such continuum models.
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Identifier: DOI: 10.7301/Z0Z036K0
Access Condition: rights statement (href="http://rightsstatements.org/vocab/InC/1.0/"): In Copyright
Access Condition: restriction on access: Collection is open for research.
Type of Resource (primo): dissertations