Abstract <p>Effect of a high pressure (up to 10 GPa) on the structure formation in an Al₉₅Co₅ hypereutectic alloy upon rapid cooling the melt heated to 1500°C is studied. A two-phase structure consisting of an Al(Co) solid solution and cobalt aluminide forms in all samples independent of applied pressure. As the melt solidifies under high pressure, the structure is substantially denser and finer with a higher (by a factor of ~1.1–1.4) microhardness as compared to the sample prepared at normal atmospheric pressure. At pressures higher than 5 GPa, the solidification mechanism changes: the initially hypereutectic alloy solidifies as a hypoeutectic alloy. In this case, an anomalously supersaturated Al(Co) solid solution forms: the cobalt content in aluminum exceeds the equilibrium value by a factor of 100 and is ~3 at % at pressure of 10 GPa. The alloys Al₉₇Co₃, Al₉₆Co₄, Al₉₄Co₆, and Al₉₃Co₇ solidify according to the same mechanism as Al₉₅Co₅ alloy does; in this case, the quantitative proportions of the structural components (α-Al, α-Al + Al₉Co₂ eutectic) in the alloys change.</p>