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A two-solid microstructure formed by the coupled transformation of liquid at a eutectic point. Lamellae and rods are common morphologies. A hypoeutectic alloy can contain both primary crystals and a eutectic microstructure; those microconstituents are not the same accounting categories as its total phase fractions.

A recognizable constituent of a microstructure, which can itself contain more than one thermodynamic phase. Primary Bi-rich crystals and a two-phase eutectic microstructure are different microconstituents; counting them differs from counting the total Bi-rich and Sn-rich phase amounts.

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.

(Bi) and $(\beta \mathrm{Sn})$ are Bi-rich and Sn-rich terminal solid solutions, not intermetallic compounds. At the eutectic composition, cooling through $138{}^{\circ }\mathrm{C}$ givesThe two solids grow together as a fine eutectic microstructure, commonly lamellar or rod-like. Below about $13{}^{\circ }\mathrm{C}$, the tin-rich component has an equilibrium tendency to transform from metallic tetragonal white tin to diamond-cubic grey tin, with a substantial volume increase. This is tin pest, which can disrupt the microstructure mechanically. The dashed transition does not transform the Bi-rich phase into tin. Actual transformation can be slow and depend strongly on impurities and nucleation; crossing the line does not guarantee immediate disintegration.

At $125{}^{\circ }\mathrm{C}$, primary Bi-rich crystals are surrounded by a transformed two-phase eutectic microstructure. Reading approximately $x_B=0.026$ and $x_S=0.94$ gives phase compositions $1.49$ and $89.90wt\%$ Sn. Consequently,These are total phase mass fractions, including both primary and eutectic material. They differ from the microconstituent fractions: just before the eutectic reaction, roughly $39\%$ by mass is liquid and subsequently becomes the eutectic constituent, with roughly $61\%$ primary Bi-rich material. Further slow cooling changes the phase compositions and allows secondary precipitation. Faster cooling usually refines the primary crystals and eutectic spacing and increases retained segregation; the equilibrium diagram does not specify the resulting nonequilibrium fractions uniquely.