Thermo-kinetic synergy and micro-alloying effects of multiphase transformations in an as-cast Al-Mg-Si-Cu alloy

This work explores effects of Ge and Sb on the formation of secondary phases, with an emphasis laid on the thermo-kinetics of phase transformations and the mechanisms determining the micro-alloying effects. The results reveal that the dominating microstructures in an Al-Mg-Si-Cu alloy, including Mg₂Si, Al₄Cu₂Mg₈Si₇ (Q), Si and Al₂Cu (θ) phases, are thermal activated. Compared to elemental Si, intermetallic compounds like Mg₂Si and Q phases have larger critical nucleation radius and phase-transformation energy barrier, and the larger thermodynamic driving force and kinetic migration energy barrier generally result in the finer microstructures. Micro-alloying affects the phase transformations in two ways: the element Sb not only promotes forming Mg₃Sb₂ phases but also facilitates the formation of elemental Si, Q needles and θ dispersoids through the mismatch effect; whereas the element Ge facilitates the formation of (Ge, Si) structures with different shapes and crystal structures by a substitution behavior following the formation energy criterion.