Thermal and athermal nucleation of MgSi co-clusters in Al-Mg-Si alloys

The interplay between thermal and athermal nucleation of MgSi co-clusters during quenching of solution-heat-treated Al-Mg-Si alloys is investigated through computer simulations. Thermal nucleation is typically described by classical nucleation theory, which refers to the formation of supercritical nuclei via the diffusion-controlled attachment of solute atoms to clusters of critical size. In the process of athermal nucleation, pre-existing subcritical nuclei become supercritical due to a decrease in critical size, for instance, as a result of increased undercooling during quenching. In this study, we develop a comprehensive nucleation model that integrates thermal and athermal contributions, offering new insights into the MgSi co-cluster formation in Al-Mg-Si alloys during continuous cooling. The results reveal that athermal nucleation is the predominant nucleation mechanism for MgSi co-clusters during quenching. Furthermore, the dependencies of thermal and athermal nucleation on cooling rate, temperature, and alloy composition are elucidated.