Eutectic Definition and Examples

This binary phase diagram indicates the eutectic composition, eutectic temperature, and the eutectic point

Wizard191 / Wikimedia Commons /CC-SA 3.0

A eutectic system is a homogeneous, solid mixture of two or more substances that form a super-lattice; the mixture either melts or solidifies at a lower temperature than the melting point of any of the individual substances. The phrase most commonly refers to a mixture of alloys. A eutectic system only forms when there is a specific ratio between the components. The word comes from the Greek words "eu," meaning "good" or "well" and "tecsis," meaning "melting."

Examples of Eutectic Systems

Several examples of eutectic systems or eutectoids exist, in metallurgy and in various other fields. These mixtures typically have useful properties that are not possessed by any single constituent substance:

  • Sodium chloride and water form a eutectoid when the mixture is 23.3% salt by mass with a eutectic point at -21.2 degrees Celsius. The system is used to make ice cream and to melt ice and snow.
  • The eutectic point of the mixture of ethanol and water is nearly pure ethanol. The value means there is a maximum proof or purity of alcohol that can be obtained using distillation.
  • Eutectic alloys are often used for soldering. A typical composition is 63% tin and 37% lead by mass.
  • Eutectoid glassy metals exhibit extreme corrosion resistance and strength.
  • Inkjet printer ink is a eutectic mixture, permitting printing at a relatively low temperature.
  • Galinstan is a liquid metal alloy (composed of gallium, indium, and tin) used as a low-toxicity replacement for mercury.

Related Terms

Concepts and terms related to eutectic systems include:

  • Eutectoid: Eutectoid refers to a homogeneous solid mixture that forms from cooling two or more melted metals to a certain temperature.
  • Eutectic Temperature or Eutectic Point: The eutectic temperature is the lowest possible melting temperature for all of the mixing ratios of the component substances in a eutectoid. At this temperature, the super-lattice will release all of its components and the eutectic system will melt into a liquid as a whole. Contrast this with a non-eutectic mixture, in which each component will solidify into a lattice at its own specific temperature until the whole material eventually becomes solid.
  • Eutectic Alloy: A eutectic alloy is an alloy formed from two or more components that exhibits eutectic behavior. A eutectic alloy melts at a distinct temperature. Not all binary alloys form eutectic alloys. For example, gold-silver does not form a eutectoid, as the valence electrons are not compatible with super-lattice formation.
  • Eutectic Percentage Ratio: This is defined as the relative composition of the components of a eutectic mixture. The composition, particularly for binary mixtures, is often shown on a phase diagram.
  • Hypoeutectic and Hypereutectic: These terms apply to compositions that could form a eutectoid, but do not have the appropriate ratio of component substances. A hypoeutectic system has a smaller percentage of β and a greater percentage of α than a eutectic composition, while a hypereutectic system has a greater percentage of α and a lower percentage of β than a eutectic composition.