Atomic scale characterization of precipitates in an Al-Si-Mg alloy containing excess Si and trace amounts of Cu

The precipitation kinetics of a foundry Al-Si-Mg-(Cu) alloy containing excess Si and impurity levels of Cu were investigated on the atomic scale. A combination of APT, STEM and DSC analysis was used to characterize the evolution of chemical composition and crystal structure of the precipitates during artificial aging. It was observed that even at trace levels (0.02 wt. %), Cu has the capability to alter the precipitation sequence of the Al-Si-Mg alloy, where instead of pure β″, hybrid phases containing unit cells of β″, B′ and Q′ form during age hardening of the alloy in large number densities. APT investigation confirmed the incorporation of Cu in precipitates during peak-aging and its enrichment to maximum values during over-aging. Observance of more Q′ unit cells in the hybrid phases after over-aging was related to this Cu enrichment. Coarse Cu-free U1 phases nucleating predominantly on grain-boundaries were observed in the over-aged condition. Interfacial energies were calculated to explain the large number densities of the hybrid B′/Q′ precipitates and coarsening of the U1 phases.