Multiscale Modeling of Interfaces

Licentiatavhandling, 2007

Understanding strength and stability of interfaces between dissimilar materials or phases is a highly active field of contemporary materials research. Knowledge of interface structure and energetics is important for modeling of complex materials both on a mechanical and thermodynamical level as well as for many functional properties. Interfaces is a collective term including surfaces, interphase boundaries and grain boundaries. A general interface can be sharp or diffuse, planar or corrugated. The interface between two crystals is usually quite sharp, and can be classified as coherent, semicoherent or incoherent depending on the mismatch between the adjoining crystals. The goal of this Thesis is to obtain a better understanding of interfacial structures and energetics, in particular of semicoherent interfaces. Previous first-principles work in the field has often excluded the effect of misfit. Therefore, we present a simple model combining the interfacial interaction from first-principles methods with a continuum description to account for the elastic displacements. The accuracy of the model has been satisfactorily tested against atomistic modelling. We investigate the effect of misfit on interfaces in two classes of materials: steels and cemented carbides. In the first case, we study the interface between Fe and a VN precipitate. The results show that even a small misfit (2%) has a large influence on the interface energy. In the second case, we apply the method to the semicoherent Sigma=2 twist boundary found in both tungsten carbide powder and sintered cemented carbides. The results are discussed in the context of grain boundary evolution during sintering.