a binary eutectic system
/ you-TEK-tik /
Two metals that like each other in the liquid but only partly in the solid. Mix them and something surprising happens: certain mixtures melt at a lower temperature than either pure metal. Eutectic is Greek for easily melted. Tin-lead solder is the classic case, where one specific mix melts lowest, which is exactly why it flows so nicely for joining.
More precisely, a binary eutectic diagram has two solid solutions, alpha rich in one metal and beta rich in the other, with limited mutual solubility, plus a special eutectic point that sits at the lowest liquidus temperature. There a horizontal invariant line, the eutectic isotherm, marks where liquid transforms into both solids at once on cooling. Solvus lines bound the alpha and beta single-phase fields, showing solubility shrinking as temperature falls. The Pb-Sn eutectic is at 61.9 wt% Sn and 183 degrees C.
The shape of this diagram controls the microstructure. An alloy right at the eutectic composition freezes at one temperature into a fine, often lamellar alpha-plus-beta mixture, the eutectic structure. An off-eutectic (hypo- or hyper-eutectic) alloy first grows primary crystals of alpha or beta, and only the leftover liquid, once it reaches the eutectic composition, fills in the gaps with eutectic. All of this is equilibrium; faster cooling changes how fine the structure comes out.
Pb-Sn solder is eutectic at 61.9 wt% Sn and 183 degrees C, its lowest melting point.
The eutectic point is the lowest dip of the liquidus, prized for solders.
The eutectic composition is not a compound; it is simply the lowest-melting mixture, and it freezes into two separate phases, not one.