1. The finely dispersed structure of unmodified Silumin alloys formed during rapid cooling is a degenerate eutectic due to the independent nucleation and growth of phases resulting from the suppression of diffusion processes with large supercooling. 2. Additions of surface-active elements to Silumin alloys suppress the nucleation centers of eutectic silicon and slow down their growth in the process of further transformations. 3. The leading phase during solidification of the modified eutectic is aluminum, which outstrips the growth of silicon crystals and blocks them from the liquid. Due to the periodic supersaturation of the liquid with silicon, the silicon particles are wedged out at the solidification front and are immediately surrounded by aluminum phase.