hypoeutectic

Hypoeutectic is a term used in alloy metallurgy to describe a composition that lies below the eutectic composition in a binary alloy system that forms a eutectic upon solidification. In such alloys, solidification begins with the formation of primary solid solution phase, and only after reaching the eutectic temperature does the remaining liquid transform into a mixture of two phases (the eutectic) in a characteristic lamellar or rod-like morphology.

Microstructure of hypoeutectic alloys typically consists of a continuous phase of primary solid solution (the solvent phase) along with a surrounding or interwoven eutectic region consisting of the two phases. In the aluminum–silicon system, for example, a hypoeutectic alloy first solidifies primary alpha-Al dendrites, and as cooling continues the remaining liquid becomes the eutectic Al–Si, forming a network of silicon within the aluminum matrix. The exact morphology depends on cooling rate and alloying elements.

Common examples and ranges: the aluminum–silicon system has a eutectic composition of about 12.6 weight percent silicon at roughly 577°C. Alloys with silicon contents below this threshold are called hypoeutectic; those above are hypereutectic. The concept also applies to other eutectic systems, though Al–Si is one of the best-known examples.

Properties and applications: hypoeutectic alloys often exhibit good castability and fluidity due to a lower tendency to form excessive interdendritic eutectic networks, while the presence of the primary phase can influence strength and machinability. Hypoeutectic aluminum–silicon alloys are widely used in cast components such as automotive and aerospace parts where a balance of castability, wear resistance, and strength is desirable.