Ductility fracture mechanisms of Al-7Si-Mg casting alloys considering Si particles: A combined experimental and crystal plasticity study

Al-7Si-Mg casting alloys are widely utilized for their excellent formability, though they commonly exhibit poor ductility. This study comprehensively investigates the factors influencing the ductility of Al-7Si-Mg casting alloys, proposing a fracture mechanism and developing a model for ductile fracture of the alloy. Tensile tests were conducted on samples subjected to different aging times, with analysis of the alloy's microstructure and tensile fracture surfaces. The findings reveal that fracture initiation occurs at clusters formed by the aggregation of Si particles, followed by the coalescence of cracks within these clusters, leading to macroscopic fracture. Different tensile strains were applied to specimens subjected to under-aging and over-aging treatments. The distribution of geometrically necessary dislocations (GNDs) in stretched samples indicates that the Si cluster area undergoes greater localized plastic deformation compared to the Al matrix, and this disparity increases with the applied strain. It has been observed that the reduced elongation observed in over-aged alloys is due to the coalescence of Si clusters under lower strain conditions. Representative volume elements with the characteristics of Si cluster is proposed, and the crystal plasticity finite element method is used for tensile simulations. The simulation results show that the proposed model is more consistent with experimental findings pertaining to elongation than with the results of a conventional void model. A longer aging time reduces ductility owing to a more intense strain localization under same tensile strain.