Microalloying-modulated strength-ductility trade-offs in as-cast Al–Mg–Si–Cu alloys

This work presents detailed and quantitative strengthening effects from various kinds of secondary phases in as-cast Al–Mg–Si–Cu alloys for the first time, through changing Ge and Sb content. Three typical kinds of strength-ductility trade-off relations are elaborated by deformation, embrittling and strengthening mechanisms, as reflected by solidification experiments, multi-mechanistic models and first-principles calculations. Microalloying-modulated strength-ductility trade-offs are strongly associated with their special microstructures, where the size and volume fraction of coarse primary phases increase and fine precipitates are modified. As compared to the secondary phases contributing up to 81.9% of the total strength, strengthening effects from solid-solution atoms (6.5–8.7%), grain boundaries (1.8–2.5%) and coarse primary phases are negligible. Therefore, the increased strength of modified alloys is modulated by modifying the fine precipitates, including Mg₂Si (β) plates, Al₄Cu₂Mg₈Si₇ (Q) needles, Al₂Cu (θ) dispersoids, (Ge, Si) rods/particles and nano-sized Si particles. On the other hand, the decreased ductility of modified alloys is determined by the secondary phases, especially the coarse primary phases like β, Q, Mg₃Sb₂, Mg₂(Ge, Si) and (Ge, Si) structures, which also implies that the ductility cannot be predicted by the Peierls’ ductility D derived from simulating α-Al solid solutions by first-principles methods.