The eutectic is a mixture of two or more elements which has a lower melting point than any of its constituents. The proper ratios of components to obtain a eutectic alloy is identified by the eutectic point on a phase diagram.

The phase diagram at right displays a simple binary system composed of two components, α and β, which has a eutectic point. The phase diagram plots relative concentrations of α and β along the X-axis, and temperature along the Y-axis. The eutectic point is the point at which the liquid phase borders directly on the solid α + β phase, representing the minimum melting temperature of any possible alloy of α and β. Not all binary system alloys have a eutectic point.

The term is often used in metallurgy to describe the alloy of two or more component materials having the relative concentrations specified at the eutectic point. When a non-eutectic alloy freezes, one component of the alloy crystallizes at one temperature and the other at a different temperature. With a eutectic alloy, the mixture freezes as one at a single temperature. The phase transformations that occur while freezing a given alloy can be understood using the phase diagram by drawing a vertical line from the liquid phase to the solid phase on a phase diagram; each point along the line describes the composition at a given temperature.

Typical usage: eutectic alloys for soldering in the electronic industry, composed of tin (Sn), lead (Pb) and sometimes silver (Ag).

In steel, iron and carbon react to form iron carbide. Iron and iron carbide, in turn, precipitate out into eutectic crystals. So mild steel is made of grains of iron and of eutectic and harder steel has grains of eutectic and of iron carbide.

See also

• Azeotrope

de:Eutektische Legierung