Superior strength-ductility synergy of Al-Si-Cu-Mg alloys achieved by regulating solute clusters and precipitates: Experimental validation and numerical simulation

In this work, a double-stage aging (i.e., pre-aging plus second-aging) strategy has been conducted on an Al-8Si-2Cu-0.5Mg alloy to comprehensively investigate the formation of solute clusters during pre-aging and their impact on the subsequent precipitation behavior during second-aging. Particularly, strengthening and toughening mechanisms for enhanced mechanical properties of the double-stage aged (DA) Al-8Si-2Cu-0.5Mg alloy have been revealed in comparison to the single-stage aged (SA) counterpart. A combination of Cs-corrected transmission electron microscope (TEM), atom probe tomography (APT), first-principles calculations and molecular dynamic (MD) simulations is employed. The results reveal a marked tendency for Mg-Si-Cu cluster formation during pre-aging. This cluster growth is accompanied by preferential Mg enrichment within the clusters, i.e., the Mg:(Si+Cu) ratio of clusters shows an increasing trend during second-aging at 165 °C. This results in a high density of both Mg-Si-Cu clusters and mixed sub-unit precipitates in the peak-aged DA Al-Si-Cu-Mg alloy, which demonstrates a superior synergy of strength and ductility. The yield strength (YS) of both the peak-aged SA and DA alloys are nearly identical (∼295 MPa), while the elongation (EL) of the peak-aged DA alloy (∼14.2 %) is superior to that of the peak-aged SA alloy (∼9.2 %). MD simulations elucidate the toughening mechanism, i.e., Mg-Si-Cu clusters and mixed sub-unit precipitates induce weak stress concentrations, present a viable option for optimizing the strength-ductility balance. This research provides valuable insights into the microstructure evolution of Al-Si-Cu-Mg alloys during aging treatments, offering potential avenues for strength-ductility synergy of Al-Si-Cu-Mg alloys.