Place: SFB 986 Materials Science Colloquium, Technical University Hamburg, Germany
Modern structural materials are seldom single crystals, but exhibit a polycrystalline, multiphase, often hierarchical microstructure. The thus occurring interfaces in the microstructure have significant influence on the macroscopic properties. Nowadays even tailored microstructures, containing certain arrangements of grain boundaries with specific properties that can be tuned by segregation engineering, are within experimental reach. This gives additional impetus to the development of predictive material models that bridge between the atomistic details of grain boundaries and the effective properties of the microstructure, and can help to identify microstructures with optimized mechanical properties. Numerical simulation methods, that either allow the study of relevant processes on their characteristic length scale, or can be used to pass on information from finer to coarser length scales, are common tools in this respect. In the presentation some examples of atomistic studies of grain boundaries will be given that illustrate current developments and the challenges that one has to face when trying to extract effective mechanical behavior and to link it to fundamental physical and geometrical properties of the interfaces. The focus will be on lamellar TiAl alloys, in which the high density of interfaces can rule the overall mechanical behavior. High resolution experimental methods exist to analyze the underlying atomistic processes. However, since these processes are not independent, often several of them occur at the same time. To isolate the intrinsic deformation mechanisms of grain boundaries we have carried out molecular statics and molecular dynamics simulations of bicrystal shear at different boundaries. Four distinct mechanisms could be identified, namely rigid grain sliding, grain boundary migration, coupled sliding and migration, and dislocation nucleation and emission – that could be related to structural features of the grain boundaries as well as physical properties of the material. Their relevance for some of the experimentally observed phenomena will be discussed.