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

Highlights: • Sc containing L12 dispersoids are found to form during solidification in the presence of Mg and Si. These dispersoids do not form in alloys free of Mg and Si in the same casting conditions. • The as-cast dispersoids are found to be rich in Si which substitute for ~25% of the Al in the L12 structure. Mg is found to partition at the α–Al/Al3Sc interface. We show that the Si content inside the dispersoids is a function of aging time rather than bulk alloy composition. • The presence of solidification micro-segregation in Al-Mg-Si-Sc-Zr alloys is quantified and modelled using a one dimensional Scheil model. The formation in MgSi precipitates is found to occur in the inter-dendritic regions. These precipitate are nucleated on the dispersoids. The Mg-to-Si ratio in these precipitates varies from ~1.5 to ~2 as a function of precipitate size. • Most of the Sc and Zr is found to supersaturate despite the presence of Si. This suggests that it may be possible to develop aging treatments that will precipitate Sc/Zr dispersoids from the as-cast condition with no need for prior solution treatment. -- Abstract: 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.