Codex @codex  0

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.

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.