Skip to main content
eScholarship
Open Access Publications from the University of California

UC Berkeley

UC Berkeley Electronic Theses and Dissertations bannerUC Berkeley

Atomistic Simulations of Extended Defects in Metals

Abstract

In this dissertation the thermodynamic and kinetic properties of extended defects in metals are investigated using atomistic simulation methods, i.e., methods in which the behavior of individual atoms is explicitly considered. The dissertation is divided into two main applications. The first presents a study of surface steps where a theory for the thermodynamic properties of steps on faceted crystalline surfaces is introduced, leading to the development of an equation for the temperature dependence of the step free energy. The application of this new formalism is demonstrated in thermodynamic-integration (TI) calculations of the step free energy, based on Molecular Dynamics (MD) simulations, considering <110> steps on the {111} surface of a classical potential model for elemental copper for a temperature range of zero up to the melting point. Calculated results for the step free energy show relatively weak temperature dependencies up to a homologous temperature of 0.6, above which the temperature dependence becomes strong and the step free energy becomes more isotropic. It is found that the step free energy remains finite up to the melting point, indicating the absence of a roughening temperature for this {111} surface facet, but the step free energy decreases by roughly fifty percent from the zero-temperature value. At high temperatures the step becomes configurationally disordered due to the presence of appreciable capillary fluctuations; these fluctuations are investigated by computing the fluctuation spectrum near the melting temperature. Step stiffnesses are derived from the fluctuation analysis and the values obtained are compared to the step free energy as obtained from the TI study. Results from the capillary-fluctuation analysis and TI calculations yield statistically significant differences that are discussed within the framework of statistical-mechanical theories for configurational contributions to step free energies. The second application of the dissertation focuses on the study of quantum effects on dislocation motion. The thermally activated motion of dislocations in alpha-iron is investigated using a simulation method that rigorously accounts for quantum effects on the dynamics of condensed-phase systems, namely the Ring-Polymer Molecular Dynamics method. Calculated results for the flow stress of 1/2 <111> screw dislocations indicate that arguments based solely on the zero-point energy of the system overestimate the observed reduction in the Peierls stress by a factor of four as they do not account for the atomic quantum dispersion, i.e., the finite size of the atomic wave function as opposed to the point-particle representation in classical mechanics.

Main Content
For improved accessibility of PDF content, download the file to your device.
Current View