- A Homo-Heterogeneous Mechanism (HHM) for the precipitation of silicon atoms in aluminum is proposed. The main point of HHM is that the formation of critical nucleus consists of two steps. At the first step the pre-critical nucleus is formed. It plays a role of a preferable site for the further hetero-nucleation and then for the growth of Si precipitates on these nuclei. The reported results are based on ab initio calculations of total energy for equilibrium configurations of silicon clusters embedded into aluminum lattice. HHM explains the results of experimental studies of silicon precipitation in quenched Si–Al alloy obtained in this work and by other authors.