eutectic

A eutectic refers to a mixture of two or more components that melts at a single, sharply defined temperature, which is the lowest melting point achievable for that combination. In a binary phase diagram, the eutectic point marks where the liquid phase can transform directly into two solid phases upon cooling, without a solid–liquid coexistence interval. The eutectic reaction is written as L → α + β at the eutectic composition and temperature.

At the eutectic composition, cooling from the liquid leads to simultaneous crystallization of two solid phases. The resulting microstructure is typically a fine, interleaved two-phase mixture, often appearing as lamellae (layered) or rod-like structures, formed by cooperative growth of both phases.

Common eutectic systems and temperatures include tin–lead solder, with about 61.9% Sn and 38.1% Pb melting at approximately 183°C; aluminum–silicon around 12.6% Si melting near 577°C; and the iron–carbon system’s ledeburite at about 4.3% C and 1147°C. Another well-known example is silver–copper, which has a eutectic composition near 71.9% Ag and 28.1% Cu at about 779°C. There are many other eutectic mixtures, including some salt systems that melt below the freezing points of their components.

Eutectic alloys are important in practical applications such as solders, low-melting alloys for fusing or casting, and as phase-change materials. Understanding eutectic behavior helps control melting, solidification, and the resulting microstructure in metallurgical and materials engineering contexts.