Structure and stability of vacancy–solute complexes in Al–Mg–Si alloys

Abstract

This study investigates the structure and stability of vacancy–solute complexes in Al–Mg–Si alloys using first-principles calculations along with the prediction of initial structures using multiple linear regression analysis. In Al–Mg and Al–Si binary alloys, vacancy–solute complexes become unstable when more than five solute atoms are combined with a vacancy, due to the repulsive interactions between adjacent atom pairs. In Al–Mg–Si alloys, the formation energy of the vacancy–solute complexes continues to decrease as the number of solute atoms increase. The stability of vacancy–solute complexes mainly arises from the Mg–Si bonds formed with the increasing number of solute atoms. The combination of inward displacement of the Mg atoms and outward displacement of the Si atoms accommodates the increase in the number of Mg–Si bonds. Owing to short range repulsive interatomic interactions, Mg–Mg bonds cannot be formed in vacancy–solute complexes of Al–Mg–Si alloys. However, Si–Si bonds are a preferable alternative to enhance stability in vacancy–solute complexes, despite their decreased strength compared with Mg–Si, Al–Mg, and Al–Si bonds. The concentration of Si atoms in the vacancy–solute complexes is always larger than that of Mg atoms at finite temperatures. The stability of the Si-rich composition originates from the difference between the Si–Si and Mg–Mg interactions in the vacancy–solute complexes.