Interface Thermodynamics with Applications to Atomistic Simulations

Abstract

Interfaces are ubiquitous in natural phenomena. While the description of interfaces in fluid systems is well developed, solid-fluid and solid-solid interfaces are not well understood. This deficiency is especially true for solid-solid interfaces, which play critical roles in materials engineering, solid-state physics and solid-state chemistry. In this thesis, the Gibbs theory of interfaces is generalized to describe phase boundaries under non-hydrostatic stress in multicomponent systems. We obtain equations that describe coherent solid-solid interfaces with shear stresses parallel to the boundary plane, incoherent solid-solid interfaces for certain constraint variations, solid-fluid interfaces, grain boundaries and surfaces. In the second part of the thesis, the developed theory is applied to study particular types of interfaces using atomistic simulations. We modeled solid surface, solid-liquid interface and grain boundaries. The simulations allowed to calculate values of key thermodynamic properties, clarify behavior of these properties with temperature, composition and stress and test the predictions of the theory. Surface surface free energy and surface stress in a single component system were computed as functions of temperature. The values of these two excess properties do not converge near the melting point despite the extensive surface premelting. Solid-liquid interface free energy was computed using the developed thermodynamic integration technique as a function of composition in CuAg binary alloy and as a function of biaxial strain in a single component Cu system. In the later case the equilibrium states between the non-hydrostatically stressed solid and liquid were accurately predicted using the derived Clausius–Clapeyron type equation. We show that for non-hydrostatic equilibrium interfaces stress is not unique and compute different interface stresses using our simulation data. We also studied effects of elastic deformation, temperature and chemical composition on properties of a symmetrical tilt grain boundary in Cu and CuAg alloy. Excess grain boundary free energy was computed as a function of lateral strain, normal stress and shear stress parallel to the boundary plane. We also employed the derived thermodynamic integration method to compute grain boundary free energy as a function of temperature and composition. Maxwell type relations predicted by the adsorption equation were tested and verified. We proposed a thermodynamic model of liquid nucleation on superheated grain boundaries based on the sharp-interface approximation with a disjoining potential. The model predicts the shape and size of the critical nucleus by using a variational approach. Contrary to the classical nucleation theory, the model predicts the existence of a critical temperature of superheating and offers a simple formula for its calculation. The model is tested against molecular dynamic simulations in which liquid nuclei at a superheated boundary were obtained by an adiabatic trapping procedure.