Using isothermal-isochore and isoenthalpic-isobaric molecular dynamics simulations, with either periodic boundary conditions or free boundary conditions, we explore in detail the vacancy annealing mechanisms in monolayers with or without substrate. A new mechanism, named dislocation mediated annealing (DMA), is observed. In this mechanism, vacancies condense rapidly into dislocation dipoles, with an associated shear modulus collapse, and anneal out of the system. External pressure, mobility of the vacancies and increased range of the interactions all favour DMA. We find that the other mechanism observed, annealing of the vacancies by the formation of voids, is a slower process, by at least an order of magnitude. We clarify the concept of reference configuration and volume in the calculation of the elastic constants in "equilibrium" fluctuation methods and give the general expressions. We derive the correct fluctuation formulae for elastic constants in uniform dilation systems. We show that the traditional thermodynamic potentials cannot be used in stability problems for an anisotropically stressed system. We derive general expressions for the mechanical stability criteria of a stressed material. We find that for a system under isotropic initial stress, the elastic stiffness coefficients which govern stress-strain relations can be used as stability criteria. However, for a system under anisotropic initial stress, stability criteria are different from either elastic constants or elastic stiffness coefficients. We show that the stability conditions in the constant pressure ensemble are stronger than in the constant volume ensemble. Exact solutions for perfect systems at zero temperature with three types of interactions of different range are found to be consistent with those obtained from long-wavelength expansion and computer simulations. We study the rupture of models of solid membranes for several interparticle interactions. We show that rupture at zero temperature occurs at the mechanical instability point. We verify that in general rupture at finite temperature takes place before the mechanical instability point. Two regimes are observed in the variation of the critical rupture pressure ($P\sb{c}$) with temperature (T) for both one and two dimensional (2D) systems; $P\sb{c}$ drops very fast at low T, but relatively slowly at high T. In ideal 2D membranes under isotropic tension, a linear decrease of $P\sb{c}$ was observed at high T. The kinetics in non defected systems, involve, just before rupture, the creation of gliding dislocation dipoles. In-grown vacancies reduce the influence of dislocations and can lead to direct cavitation.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/9941 |
| Date | January 1996 |
| Creators | Zhou, Zicong. |
| Contributors | Joos, Bela, |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 168 p. |
Page generated in 0.0023 seconds