Sammendrag
Many important properties in the Al-Mg-Si alloy system, like hardness and toughness, is to a large extent determined by the atomistic precipitate interface structure. In this work we investigate the strain field around bulk hardening β'' precipitates and the fracture decohesion at β' grain boundary precipitates, which are exposed to strain localisation. The main tool in this work is density functional theory (DFT) guided by and combined with high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) observations. For the strain field around β'', elastic theory using finite elements is used for the boundary conditions far from the precipitate, while DFT is used within and near the precipitate/matrix interface. Like β'', β' precipitates are needle-shaped with the needle direction along [001]. Due to the large lateral misfit between the bulk β' structure and the aluminium matrix, these precipitates are surrounded by an interface region with an U2-like structure. Based on high quality HAADF-STEM images, realistic atomistic models of this rather complex interface structure along the of these rather complex (130)Al and (110)Al interfaces has been set up and their decohesion energy calculated with DFT. The results show that additional defects, like vacancy clusters, are required for initialisation of cracks.