Roles of microalloying elements on the cluster formation in the initial stage of phase decomposition of Al-based alloys

In age-hardenable aluminum alloys, nanoscale clusters composed of solute atoms and vacancies are formed and play very important roles in microstructure control and alloy properties. The microalloying elements in the aluminum alloys affect the cluster formation and resultantly precipitate microstructures. The fundamental effects of microalloying elements on Al-Cu, AI-Zn, Al-Li-Cu, and Al-Mg-Si alloys are demonstrated through changes in the electrical resistivity, hardness, differential scanning calorimetry (DSC), and microstructural evolution. In order to understand the complicated effects of microalloying elements, a Monte Carlo simulation model was developed and performed. In the simulation, it was found that some microalloying elements preferentially trap quenched-in excess vacancies to retard cluster formation in the initial stage of aging. The complex clusters of solute/microalloying element/vacancy were found to be formed in the subsequent stage and act as effective nucleation sites for clusters and Guinier-Preston (GP) zones. The roles of microalloying elements are well understood in terms of the ordering parameters and are well predicted based on the ordering parameter maps (OP maps) proposed in this work.