A continuum model for heterogeneous nucleation - atomistic simulations on diffusive time scales
Nucleation plays an important role in material processing. The initial state of microstructural forming of materials is often started by a nucleation event. And the initial microstructure can influence the property and/or quality of the produced material. Thus, understanding and controlling of the nucleation is a key to control material properties. In a macroscopic limit, when the energy of a system is just described by bulk energy of the phases and the interfacial energy, the nucleation is well understood. But in a microscopic regime, the bulk and interface energies may be dependent on the microscopic structure of the material.
We used the phase field crystal (PFC) model in order to describe the material spatial on a microscopic scale and diffusive time scale. The free energy of the PFC model can be motivated as a local approximation to classical density functional theory. The free energy is minimized by a field which resamples a density wave, describing the liquid or crystalline state.
Nucleation is an activated process, thus in order to find the nucleation barrier a saddle point in the energy landscape of PFC has to be found. In order to find the minimum stable grain, we use a chain of states method. A simplified string method is applied to the PFC model in order to find nucleation barriers and specify the nucleation path for homogeneous and heterogeneous nucleation. We found for the standard PFC the critical grains, are not bulk like, and thus the nucleation cannot be described by a macroscopic nucleation theory. Introducing contact to a substrate or a wall shows, that the nucleation is greatly influenced by details of the interaction on the microscopic scale.
Project duration: 08/2007 - 04/2016
Funded by: DFG (SPP 1296)