Atomistic studies of nano-structural mechanical behavior and point defects

Abstract: The content of this thesis consists of two parts. For the first part, size and crystallographic orientational influence on the elastic properties of bcc nanowires and films have been studied through static molecular simulations. In order to explicitly account for surface effects, the concept of surface free energy has been employed. Comparisons between continuum mechanical calculations and results from atomistic simulations reveal that the relaxation strains for thin films are in very good agreement with continuum mechanical predictions. For $(110)$ and $(111)$ surfaces, inclusion of non-linear elastic terms improves the correspondence between the continuum mechanical results and the results from simulations. On the other hand, for (100) surfaces non-linear elasticity increases the discrepancies and the linear elastic predictions are in better agreement with the simulations. For the size dependence of [110] wires, both the linear and non-linear calculations predict similar relaxation strains and Young's moduli to those of the simulations. When regarding [100] wires, the linear as well as the non-linear predictions fail to describe the relaxation strains accurately. Comparing Young's modulus for [100] wires, when incorporating non-linear elastic constants into the continuum mechanical solution, the prediction is that the nanowires stiffen, whereas for linear elasticity the wires are found to be more compliant with decreasing size. From the simulations the iron wires stiffen while tungsten wires weaken with decreasing size. This may be an artifact originating from the fact that it may not be sufficient to describe the elastic properties of these rectangular cross-sections solely based on properties of surfaces, but it may also be required to incorporate explicit edge effects. Comparing the bending stiffness calculated from tensile simulations with those obtained from bending simulations reveals that there is a consistency between the two. Studying the rotations of the cross sections in the bending simulations reveals that the shearing is small. For the second part, a set of EAM potentials for bcc transition metals have been constructed, and have been adjusted to give stable self-interstitials corresponding to those found in experiments and from emph{ab initio} simulations. These potentials are then used to evaluate the stabilities of divacancies and the migration paths for mono and divacancies. For all the potentials, the migration and activation energies for monovacancies are in good agreement with experimental data. All the potentials are found to give the second nearest neighbor as the most stable divacancy configuration and the nearest neighbor paths are found to be the plausible migration paths. Comparing the divacancy activation energies with the available experimental data, it can be seen that for V the activation energy is in good agreement. For Mo, Ta, and W the activation energies are slightly overestimated by the potentials in comparison with the available experimental data.

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