Local structure of Al-Sr modifier in liquid Al-Si alloys

Sodium has historically been the predominant modifier for Al-Si alloys. However because of its great disadvantages, Al-Sr master alloys is commonly used as modifier for Al-Si Alloys. Many researches have carried out on the relation between the preparation of Al-Sr alloy and modification effect. This paper is about analysis of the clusters in Al-Sr modifier from theoretical point of view. According to the scattering characteristic and hereditary property of Al-Sr modifier in Al-Si alloys, strong chemical interaction between Al and Sr and the existence of Al2Sr and Al4Sr in modification process, Al2Sr and Al4Sr clusters were chosen as models of local structure of Al-Sr modifier. These local structures were optimized by the quantum chemical calculation method at Hartree-Fock level, using the LANL2DZ basis set, which is a double-zeta basis set containing effective core potential (ECP). In order to raise calculated reliability, the density functional theory B3LYP and MP2 methods, which contain the effect of electron correlation, were also used. The stable geometries, electronic state, electronic structure and the electron transformation between AI and Sr were obtained. The structure of Al2Sr cluster is C(2)v and that of Al4Sr cluster is C(4)v. The optimized structures of the two clusters are relatively stable states and are easy to untie. In the modifying temperature range, the energy of Al4Sr is lower than that of Al2Sr in same temperature. Al-Sr modifier existences mainly in the form of Al4Sr in the modifying process, in good agree with experiment. The HOMO and LUMO orbital energy of Al2Sr and Al4Sr are negative. Al2Sr and Al4Sr are easily getting electrons to become negative ions. More positive charge centralizes on atom Sr and electrons shift from Sr to Al in both clusters. The polarity of each is high. Al4Sr have high chemical activity in the modifying process.