Micro-segregation and precipitates in as-solidified Al-Sc-Zr-(Mg)-(Si)-(Cu) alloys

In this work, we investigate the as-cast microstructure of three model alloys, an Al-Sc-Zr, an Al-Mg-Si-Sc-Zr and an Al-Mg-Si-Cu-Sc-Zr alloy. Transmission electron microscopy and atom probe tomography are used to investigate the precipitates present in the microstructure after casting. We find that higher Si content results in the formation of dispersoids upon casting. The as-cast dispersoids are found to be rich in Si with Si substituting for ~25% of the Al in the L12 structure. Variations in the local alloy composition and dispersoids size is explained by the presence of solidification micro-segregation which is fitted using a one-dimensional Scheil model. This segregation is found to be responsible for the formation of MgSi rod-like precipitates in the inter-dendritic regions. These precipitates are found to nucleate on the as-cast dispersoids. A closer look at the solid solution reveals that most of the Sc and Zr is supersaturated after casting even in the presence of Mg, Si and Cu. We discuss the impact of these results on the design of a suitable heat treatment for 6xxx-series alloys with Sc.