How it Works
A binary eutectic phase diagram plots temperature (vertical axis) against composition (horizontal axis, % of component B), running from pure A at 0% to pure B at 100%. Two liquidus lines descend from each pure component's melting point and meet at the eutectic point — the lowest-melting composition in the entire system. Below the eutectic temperature, everything is solid; the horizontal eutectic line marks where the last liquid transforms simultaneously into two solid phases.
As you cool an alloy of overall composition C₀, the state point moves straight down the vertical composition line. Above the liquidus it is fully liquid. Between the liquidus and the eutectic temperature, primary solid grains nucleate and grow in the remaining liquid; a horizontal tie line connects the current liquid composition to the solid composition, and the lever rule converts that geometry into exact phase fractions. Once the eutectic temperature is reached, the remaining liquid (now at the eutectic composition) freezes into a fine lamellar structure of alternating solid phases.
Liquidus (hypereutectic): T = Tm,B + (T_eu − Tm,B)·((100 − C) / (100 − C_eu))
Lever rule: fraction solid = (C_liquid − C₀) / (C_liquid − C_solid)
Lever rule: fraction liquid = (C₀ − C_solid) / (C_liquid − C_solid)
Frequently Asked Questions
What is a eutectic point, and why does it have the lowest melting point in the system?
The eutectic point is the single composition and temperature at which a liquid alloy solidifies directly into two solid phases simultaneously, without passing through a mushy two-phase region. Mixing two components lowers the freezing point of each, similar to freezing-point depression, so the eutectic composition sits at the minimum of the liquidus curve — it melts and freezes at a lower temperature than either pure component or any other mixture ratio.
What do the liquidus and solidus lines represent?
The liquidus marks the highest temperature at which solid can exist for a given composition; above it the alloy is fully liquid. The solidus marks the lowest temperature at which liquid can exist; below it the alloy is fully solid. Between the two lies the mushy zone. In this simplified system, the solidus collapses to the horizontal eutectic line plus the vertical lines at the pure components.
How does the lever rule work, and what does it calculate?
In a two-phase region, a horizontal tie line connects the liquid composition (C_liquid) and the solid composition (C_solid) at the current temperature. The lever rule treats the tie line like a balance beam pivoted at the overall composition C₀: fraction solid = (C_liquid − C₀)/(C_liquid − C_solid). It gives the exact relative amounts of each phase present.
What does eutectic (lamellar) microstructure look like, and why does it form?
Below the eutectic temperature, the remaining eutectic liquid solidifies into fine alternating layers (lamellae) of the two solid phases growing side by side. Each phase rejects the component it does not want into the liquid right next to it, feeding growth of the neighboring lamella — a self-organizing diffusion process producing a very fine, regular striped structure.
What is the difference between eutectic and off-eutectic (hypo/hypereutectic) alloys?
An alloy at the exact eutectic composition freezes at a single sharp temperature, like a pure metal. Off-eutectic alloys freeze over a temperature range: primary grains of one solid phase grow first as the liquid cools through the mushy zone, and only the remaining liquid transforms to eutectic solid once it reaches the eutectic temperature.
What are some real-world examples of eutectic systems?
Classic examples include Pb-Sn solder (eutectic near 61.9% Sn, melting at 183°C), Wood's metal (a low-melting Bi-Pb-Sn-Cd eutectic used in fuses and fire sprinklers), and salt-water or salt-ice mixtures, whose eutectic freezing-point depression is why road salt melts ice.
Why was eutectic solder historically preferred for electronics?
Eutectic solder freezes at one sharp temperature instead of over a range, avoiding time spent in a partially liquid mushy state while cooling. Off-eutectic solders can be disturbed by vibration during that mushy interval, forming cracked or grainy cold joints; eutectic solder solidifies almost instantly and cleanly.
How is this related to freezing-point depression and colligative properties?
Just as dissolving a solute in a solvent lowers the solvent's freezing point, a colligative effect, mixing two components in a binary alloy lowers the freezing point along each pure component's liquidus curve. The eutectic point is where both lowered liquidus curves meet.
What happens to the microstructure as the overall composition moves away from the eutectic point?
Moving away from the eutectic composition increases the fraction of primary solid that forms before the eutectic reaction begins, and decreases the fraction of fine eutectic microstructure left over. At the pure-component limits, no eutectic structure forms at all.