🧪 Binary Alloy Phase Diagram — Eutectic & Solidification

A hypoeutectic alloy (composition to the left of the eutectic) first crosses the liquidus on cooling, nucleating primary alpha-phase crystals. As temperature falls, the remaining liquid becomes progressively richer in component B, tracking the liquidus curve toward the eutectic composition. When the temperature reaches the eutectic isotherm, the remaining liquid — now at the eutectic composition — solidifies simultaneously into a fine lamellar mixture of alpha and beta phases.

Classic examples include lead-tin (Pb-Sn, eutectic at 61.9 wt% Sn, 183 °C — the backbone of traditional electronics solder), silver-copper (Ag-Cu, eutectic at 28.1 wt% Cu, 779 °C — used in silver brazing filler), and bismuth-tin (Bi-Sn, eutectic at 57 wt% Bi, 139 °C — a low-temperature solder used in thermosensitive applications). Modern lead-free SAC305 solder (Sn-Ag-Cu) is a ternary near-eutectic.

Slow equilibrium cooling gives atoms time to diffuse, producing coarse lamellar microstructure with alternating wide plates of alpha and beta. Rapid quenching suppresses diffusion, producing fine lamellar or even amorphous (glassy) structures. Very fast quenching can create supersaturated solid solutions — the basis of rapidly solidified aluminium alloys used in aircraft, where the fine microstructure dramatically improves strength and corrosion resistance.

A hypereutectic alloy has a composition to the right of the eutectic point (more of component B). On cooling it first precipitates primary beta-phase crystals while the liquid moves toward the eutectic composition. Both hypoeutectic and hypereutectic alloys ultimately produce eutectic microstructure in the remaining liquid — the difference is only in which primary phase precipitates first and in what proportion it appears in the final microstructure.

In an isomorphous system (such as copper-nickel) the two components are completely miscible in both liquid and solid phases, so there is no two-solid region and no eutectic point — only a lens-shaped two-phase region between the liquidus and solidus curves. The eutectic type shown here has limited solid solubility; the two components can only partially dissolve in each other, leading to the characteristic V-shaped liquidus and the eutectic reaction.

Yes. Move the eutectic composition slider (x_E) away from 50% to create an asymmetric system where the eutectic lies closer to one pure component. This mimics most real eutectic systems: the Pb-Sn eutectic at 61.9 wt% Sn is strongly asymmetric, as is the Al-Si eutectic at 12.6 wt% Si that is the basis of cast aluminium alloys used in automotive engine blocks.

The orange dashed line traces how the state of a single alloy (at a composition you can set by clicking) evolves as it cools from above the liquidus. In the liquid region the path descends vertically. On crossing the liquidus, the composition of the remaining liquid follows the liquidus curve (moving toward the eutectic), while the overall composition stays fixed — the path therefore appears to bend. At the eutectic isotherm the remaining liquid solidifies completely at constant temperature.