Past exam of the materials science course of the University of Cambridge / 2016 / Paper 1 / 4 / a

Eutectic solidification is the coupled transformation of a liquid into two distinct solid phases at the eutectic temperature and composition, $L\to \alpha +\beta$. At fixed pressure in a binary system the three-phase equilibrium is invariant. An exactly eutectic liquid can therefore finish freezing at one temperature, instead of over a broad liquid-plus-solid interval.

Eutectic compositions are useful in joining alloys, for example silver-copper brazing filler: the comparatively low melting temperature and narrow freezing interval help liquid flow into a joint and then solidify. Alternating growth of the two phases often produces a fine lamellar eutectic microstructure. The eutectic is a two-phase aggregate, not a new single solid phase.