Characterization of nanostructures in a high pressure die cast Al-Si-Cu alloy

Several interesting nanostructures of high pressure die cast commercial Al-Si-Cu alloy (A383) were characterized via transmission electron microscopy (TEM) and atom probe tomography (APT). The refinement of the eutectic Si, which was caused by Sr addition, was found to be associated with the formation of nano-Al particles (∼5 nm), with Cu partitioning (up to 5 at.%) at the Al/Si interface, within the eutectic Si. Direct observation of Sr segregation (0.4 at.%) at the interface between the eutectic Al and refined eutectic Si was achieved using APT. It is proposed that Sr refines the eutectic Si by restricting the growth of Si. Interestingly, the segregation of Mg (10.2 at.%) and Cu (0.9 at.%) is also present at the interface between the eutectic Al and the refined eutectic Si. Moreover, the presence of nano-Si particles (∼20 nm) in the primary Al was characterized in the primary Al via APT. Segregations of Mg (2.9 at.%) and Cu (2.8 at.%) were also found at the interface between the primary Al and the nano-Si particles. Density functional theory (DFT) was used to assess the driving force for the segregation behavior of Cu and Mg at the Al/Si interface. DFT results also suggest that the segregation of Cu and Mg slightly decreases the cohesive energy of Al/Si interface but is a function of the length scale of the interface.