Natural aging-induced nanoprecipitation and its impact on tensile properties of Al–Si–Cu–Mg cast alloy

The nanoprecipitation of naturally aging Al–5.70Si–2.06Cu–0.29Mg (wt.%) cast alloy and its impact on tensile properties was investigated based on atomic-scale characterization by transmission electron microscopy and atom probe tomographic analyses. Solution treatment led to the dissolution of θ-Al₂Cu and Q-Al₅Cu₂Mg₈Si₆ phases, along with the spheroidization of the eutectic Si phase of the cast alloy. Upon quenching of the solution-treated alloy, nanoscale Si precipitates with equivalent radii of ∼1.77 nm were formed. Subsequent natural aging resulted in the formation of coherent solute clusters (0.66 ± 0.11 nm in radius), rich in Mg/Si or Cu, which correspond to the precursor phases Q-Al₅Cu₂Mg₈Si₆ and θ-Al₂Cu. Microhardness of the matrix and the strength of the alloy significantly increased due to the natural aging-induced solute clustering. Analysis of the strengthening mechanism indicated that the enhanced strength of the naturally aged alloy was primarily due to the coherency and modulus strengthening of the dislocation-shearable solute clusters. These clusters suppressed the dynamic strain aging, whereas no significant changes were observed in the strain-hardening behavior. However, natural aging led to the degradation of the alloy ductility due to the presence of shearable solute clusters, which could create slip bands and promote failure in regions with a high fraction of secondary phases within the cast alloy.