A eutectic system is a mixture of two metals, A and B, whose melting behaviour is mapped on a phase diagram: temperature on the vertical axis, composition on the horizontal axis. Two liquidus lines slope down from each pure metal's melting point and meet at the eutectic point — the single composition and temperature at which the alloy freezes all at once, like a pure metal, instead of over a range.
Solder, cast iron and many aluminium casting alloys are engineered close to their eutectic composition because it gives the lowest possible melting point and the finest, strongest microstructure of any mixing ratio.
A 3D binary phase diagram sits beside a molten crucible: adjust the composition and cooling rate to watch dendrites form on the liquidus line and the remaining melt freeze into lamellar eutectic bands.
Off-eutectic alloys cross the liquidus and grow primary dendrites of the majority metal while the leftover liquid drifts toward the eutectic composition; at the eutectic point, alloys of any starting mix freeze in one step into alternating solid layers.
Set the melt's composition and cooling rate, then let cooling run. Watch the marker slide down the liquidus toward the eutectic point while atoms in the crucible lock into dendrites and lamellae. Reheat to try another mix.
Eutectic alloys such as solder and many casting aluminium alloys are chosen precisely because their single, sharp freezing point gives predictable, fine-grained castings instead of a mushy, slow-freezing range.