Abstract:Based on the empirical electron theory of solids and molecules, the valence electron structures of S phase and the S/a interface were calculated, then the relationships between their VESs and precipitation strength and interface properties were analyzed in Al-Cu-Mg alloys in this paper. It is shown the covalent bond distribution of S phase is uniform and its main bond network is built by four stronger covalent bonds connected by Cu atoms. The nature of precipitation strength of S phase lies in its binding force of the strongest covalent bond is bigger 135.14% than that of the matrix a-Al and causes the stronger inhibition to the dislocation movement. The covalent electron density difference of (100)S//(100)a, (010)S// aand (001)S//(021)ais 0.003%, 3.564%and 5.811% respectively, while the covalent electron density of (100)S//(100)ais 10.3915 nm-2 and 10.3918 nm-2 , and that of (010)S// ais 0.0486 nm-2 and 0.0469 nm-2, and that of (001)S//(021)ais 0.0486 nm-2 and 0.0459 nm-2. Compared with (001)S//(021)a和(010)S// a, the binding force and covalent electron density of (100)S//(100)a is the biggest while its covalent electron density difference is the smallest, so that the interface continuity is the best and the interface stress is the smallest.