Structural Transition of Vacancy-Solute Complexes in Al-Mg-Si Alloys

To theoretically examine the structural transition of vacancy-solute complexes in Al-Mg-Si alloys, we performed first-principles calculations for layered vacancy-solute complexes with additional Mg atoms. The central Mg atom in the additional Mg layer shifted to the Si layer with the increase in the number of Mg atoms to weaken the repulsive Mg-Mg interaction and to form Mg-Si bonds. When five Mg atoms were added to the layered vacancy-solute complex, the central Mg atom completely shifted to the Si layer, and a Mg vacancy was formed in the Mg layer, which indicated that the beta ''-eye is formed upon the addition of Mg atoms. We reproduced beta ''-eye formation from a solid solution with a vacancy using first-principles-based Monte Carlo simulations. Once the beta ''-eye was formed on the layered vacancy-solute complex, the process can be repeated by the formation of alternate Mg and Si layers along (010) beta ''. These results clearly indicate that the layered vacancy-solute complex plays an important role in beta ''-eye formation.