Abstract: The strength-ductility inversion relationship of alloys is a persistent challenge in advanced materials de-sign. Al-Cu series cast aluminum alloys that are considered as an exceptionally high-strength light alloy are not exclusive in structural applications due to their inherently poor plasticity. In this work, we em-ployed a squeeze casting technique and Ca microalloying strategy for microstructure modulation to ef-fectively address this difficulty. The addition of low concentrations of Ca (0.5 wt.% and 1 wt.%) elements to the as-cast Al-5Cu-0.5Mn alloy significantly enhances its plasticity by threefold at room temperature. Unexpectedly, even after T6 treatment, which typically compromises ductility for increased strength, the low-Ca micro-alloyed Al-5Cu-0.5Mn exhibited a further increase in its strength without sacrificing its ductility. The low-Ca addition to the alloy generates an ultrafine eutectic colony with a complex "core-shell" structure, which can serve as a carrier for localized stress transfer, effectively distributing the strain uniformly to more grains. Precipitation hardening of α-Al grains and spheroidization of lamellar ultrafine eutectic phases were simultaneously realized in the low-Ca alloy after T6 heat treatment, which resulted in comparable hardness of α-Al grains and eutectic colonies. The synergistic coordination of external strains through extensive strain-hardening induced by slip line and substantial microcrack generation by ultrafine eutectic colonies is evidenced by a series of in situ characterizations of the low-Ca alloys. There-fore, the uniform spreading deformation due to the transfer of strain-hardening effect and the alternating plastic deformation of α-Al grains and ultrafine eutectic colonies are the critical keys to overcoming the strength-plasticity paradox in low-Ca alloys. This study provides a perspective route for Al-Cu system cast aluminum alloys to be utilized as high-strength and tough structural materials.

Key words: Al-Cu-Mn-Ca alloy, Squeeze casting, Plasticity mechanism, Cracking mechanism