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

In Al-Mg-Si alloys the stability and structure of vacancy-solute complexes VMg₄Si₈ with additional Mg atoms have been investigated using the first-principles calculations. The layered VMg₄Si₈ complex, which is the second most stable complex of VMg₄Si₈, becomes stable upon the addition of Mg atoms to the adjacent layers because of the formation of Mg-Si bonds. 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 as well as to form Mg-Si bonds. When five Mg atoms were added to the layered VMg₄Si₈ complex, the central Mg atom completely shifted to the Si layer, and a Mg vacancy is formed in the Mg layer. Therefore, the layered VMg₄Si₈ + Mg₅ complex has the same structure as the β"-eye. These results indicate that the layered VMg₄Si₈ complex evolves into the β"-eye upon the addition of Mg atoms. In contrast, the layered Mg-Si with a Mg vacancy, which has a similar structure to the layered VMg₄Si₈ complex, does not evolve into the β"-eye. We confirmed the formation of the β"-eye from a solid solution with a vacancy using first-principles-based Monte Carlo simulations. The β" structure is composed of β"-eyes linked by Si-Si bonds. Therefore, to investigate the evolution from the β"-eye to the β" phase, larger supercells are required in order to include more than one β"-eye, which will be examined in future work.