Formation process of solute clusters in Al-Zn-Mg alloys from first-principles calculations

The formation of solute clusters in Al-Zn-Mg alloys was investigated using first-principles-based Monte Carlo simulations. After the formation of the truncated cube octahedron (TCO) shell composed of eight Mg atoms and thirty Zn atoms, the Zn atom at the face-center site moved to the octahedral site, resulting in an interstitial Zn and vacancy pair in the TCO shell. If a vacancy was introduced in the Monte Carlo simulation, at the nearest neighbor of the vacancy, the number of Zn atoms increased first, followed by the number of Mg atoms. The most stable vacancy-solute cluster is V- Mg₄Zn₈. This result suggests that thermal equilibrium vacancies were quenched and embedded in the vacancy-solute clusters in water-cooled specimens in Al-Zn-Mg alloys.