eutecticum

Eutecticum is a Latin term historically used in chemistry and metallurgy to denote what is now commonly called a eutectic mixture. It refers to a specific composition in a multi-component system that melts at the lowest temperature achievable for that system. At the eutectic temperature, the liquid coexists with solid phases.

In binary phase diagrams, the eutectic point occurs where the liquidus and solidus lines meet. At the eutectic composition, the liquid transforms on cooling into two solid phases simultaneously, and on heating the two solids melt to form a single liquid phase.

Examples include the lead–tin system, with about 61.9% Sn and 38.1% Pb melting at 183°C; the iron–carbon system, spanned by ledeburite at about 4.3 wt% C and 1147°C; and the aluminum–silicon system near 12.6 wt% Si at about 577°C. The salt–water eutectic occurs at about −21°C with roughly 23 wt% NaCl.

Eutectic mixtures are valued for melting and solidifying at a single, well-defined temperature, which is advantageous in soldering, casting, and alloy design. Microstructures often show lamellae or rod-like arrangements of the coexisting solids.

Today, eutecticum is mainly of historical or Latin usage; in modern English, the term eutectic is standard.