Eutectic Phase Diagrams: Why a Mixture Melts Lower Than Either Metal

A mixture that melts lower than either pure ingredient

Mix two metals (or any two mutually soluble-in-the-liquid, insoluble-or-partially-soluble-in-the-solid substances) and, at one particular composition, the mixture melts at a lower temperature than either pure component melts on its own. That composition is the eutectic point, from the Greek for "easily melted," and the temperature-vs-composition map showing it is a eutectic phase diagram — one of the most-used diagrams in metallurgy and materials science.

Reading the diagram: liquidus, solidus, and the eutectic point

The liquidus line marks the temperature above which the alloy is fully liquid at a given composition; the solidus marks the temperature below which it is fully solid. Between them is a two-phase region where liquid and solid coexist in equilibrium. In a simple eutectic system the liquidus forms a V shape (two branches meeting at the eutectic point), and the eutectic point is where the liquidus reaches its global minimum — the single lowest-melting composition in the whole system, sitting at the eutectic temperature TE.

The lever rule: how much of each phase is present

At any temperature inside a two-phase region, the diagram not only says which phases are present but, via the lever rule, exactly what fraction of the material is in each phase. Draw a horizontal tie line at the temperature of interest; it crosses the liquidus at one composition and the solidus (or solvus) at another. The fraction of solid is the length of the segment from the overall composition to the liquid boundary, divided by the total tie-line length — literally a lever balanced at the overall composition, with the two phase compositions as the fulcrum points:

fraction_solid  = (C0 - C_liquid) / (C_solid - C_liquid)
fraction_liquid = (C_solid - C0)  / (C_solid - C_liquid)
// C0: overall alloy composition, C_liquid and C_solid: the tie-line's endpoint compositions

Primary phase vs eutectic microstructure

Cool an alloy whose overall composition is not exactly the eutectic composition and solidification happens in two stages. First, as the liquidus is crossed, primary (proeutectic) crystals of whichever phase the composition favours nucleate and grow as coarse grains, and the remaining liquid's composition is dragged along the liquidus toward the eutectic point as it does. Once the remaining liquid finally reaches the eutectic composition at the eutectic temperature, it solidifies all at once into the characteristic eutectic microstructure — fine, alternating lamellae (or rods) of the two solid phases growing cooperatively from the same interface, because at that single composition and temperature both solid phases are in equilibrium with the liquid simultaneously.

An alloy cooled from exactly the eutectic composition skips the primary-phase stage entirely: the whole liquid transforms straight into fine lamellar eutectic structure at TE, with no coarse primary grains at all. This is metallurgically useful — eutectic alloys solidify at a single sharp temperature rather than over a range, which is part of why eutectic solder (historically tin-lead near 63/37 by weight, close to the Sn-Pb eutectic) was preferred for a clean, fast solidification with minimal segregation.

Real alloys: Pb-Sn solder and Al-Si castings

The Pb-Sn phase diagram is the classroom standard specifically because its eutectic point (about 61.9% tin by weight, 183 degC) is well below the melting points of both pure lead (327 degC) and pure tin (232 degC) — a large, clearly visible melting-point depression. Al-Si alloys are the industrial workhorse example: aluminium-silicon castings near the eutectic composition (about 12.6% silicon) solidify with excellent fluidity and minimal shrinkage, which is exactly what a casting process needs, and this is why eutectic or near-eutectic Al-Si alloys dominate cast automotive engine components.

Frequently asked questions

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