Aggregation or dispersion of Si atoms in Al-Si alloys? from the view point of energetics

Al-Si binary system forms eutectic alloys with about 12% Si and their mechanical and physical properties are strongly related to the distribution of eutectic silicon in the microstructure. This paper studies the structures of Al32-nSin (n = 2, 4, 6, 8) crystals by using global evolutionary algorithm combined with density functional theories. The lowest energy structures are determined and the bonding strengths of the Al-Al, Al-Si and Si-Si bonds are evaluated. The binding energies of the alloy crystals increase considerably with the silicon content and the dispersion of silicon atoms is favorable for the stability. At given composition, the dispersion of Si atoms forms two Al-Si bonds at the expense of one Al-Al and one Si-Si bonds and the energy gain is 0.022 eV. The mechanical properties are investigated and the calculated moduli show that the shear modulus and Young's modulus are related obviously to the silicon composition and distributions while the bulk moduli are quite close for all of the alloy structures. The moduli values are largest for Al28Si4 (its silicon content is close to the eutectic) and its Young's modulus is 31% larger than the pure aluminum.