the eutectic reaction
/ you-TEK-tik /
At the eutectic point, cooling the liquid does something dramatic and all at once: a single liquid freezes directly into two different solid crystals at the same time, at one fixed temperature. It is as if one fluid suddenly splits into two interwoven solids.
More precisely, the eutectic reaction is Liquid to alpha plus beta on cooling, at the invariant eutectic temperature and composition. By the phase rule, three phases in a binary at fixed pressure give F = 0, so it happens at exactly one temperature while all three phases coexist, and the temperature stays flat until every last bit of liquid is gone. The two solids usually grow as fine alternating lamellae, because the two different compositions must separate sideways by short-range diffusion just ahead of the moving front. For Pb-Sn: L(61.9 Sn) to alpha(19 Sn) plus beta(97.5 Sn) at 183 degrees C.
Eutectic microstructures are fine and can be quite strong, and the low melting point is prized for solders and casting alloys, which fill molds well and freeze at a single temperature. The eutectic is the archetype of a liquid transforming into two solids; its purely solid cousin is the eutectoid reaction, the one that makes pearlite in steel.
Pb-Sn: L to alpha + beta at 183 degrees C, freezing at one flat temperature into fine lamellae.
Three phases at once means zero degrees of freedom, so the temperature is fixed.
The eutectic (liquid to two solids) is not the same as the eutectoid (solid to two solids); they look alike on the diagram but start from a different phase.