Eutectic system

"Eutectic" redirects here. For the sports mascot, see St. Louis College of Pharmacy § Mascot.
A phase diagram for a fictitious binary chemical mixture (with the two components denoted by A and B) used to depict the eutectic composition, temperature, and point. (L denotes the liquid state.)

A eutectic system (US dict: yü-'tek-tik)[1] from the Greek "ευ" (eu = easy) and "Τήξις" (teksis = melting) describes a homogeneous solid mix of atomic and/or chemical species, to form a joint super-lattice, by striking a unique atomic percentage ratio between the components — as each pure component has its own distinct bulk lattice arrangement. It is only in this atomic/molecular ratio that the eutectic system melts as a whole, at a specific temperature (the eutectic temperature) the super-lattice releasing at once all its components into a liquid mixture. The eutectic temperature is the lowest possible melting temperature over all of the mixing ratios for the involved component species.

Upon heating any other mixture ratio, and reaching the eutectic temperature — see the phase diagram to the right — one component's lattice will melt first, while the temperature of the mixture has to further increase for (all) the other component lattice(s) to melt. Conversely, as a non-eutectic mixture cools down, each mixture's component will solidify (form its lattice) at a distinct temperature, until all material is solid.

The coordinates defining a eutectic point on a phase diagram are the eutectic percentage ratio (on the atomic/molecular ratio axis of the diagram) and the eutectic temperature (on the temperature axis of the diagram).[2]

Not all binary alloys have eutectic points because the valence electrons of the component species are not always compatible, in any mixing ratio, to form a new type of joint crystal lattice. For example, in the silver-gold system the melt temperature (liquidus) and freeze temperature (solidus) "meet at the pure element endpoints of the atomic ratio axis while slightly separating in the mixture region of this axis".[3]

Eutectic reaction

Four eutectic structures: A) lamellar B) rod-like C) globular D) acicular.

The eutectic reaction is defined as follows:[4]

This type of reaction is an invariant reaction, because it is in thermal equilibrium; another way to define this is the Gibbs free energy equals zero. Tangibly, this means the liquid and two solid solutions all coexist at the same time and are in chemical equilibrium. There is also a thermal arrest for the duration of the change of phase during which the temperature of the system does not change.[4]

The resulting solid macrostructure from a eutectic reaction depends on a few factors. The most important factor is how the two solid solutions nucleate and grow. The most common structure is a lamellar structure, but other possible structures include rodlike, globular, and acicular.[5]

Non-eutectic compositions

Compositions of eutectic systems that are not at the eutectic composition can be classified as hypoeutectic or hypereutectic. Hypoeutectic compositions are those with a smaller percent composition of species β and a greater composition of species α than the eutectic composition (E) while hypereutectic solutions are characterized as those with a higher composition of species β and a lower composition of species α than the eutectic composition. As the temperature of a non-eutectic composition is lowered the liquid mixture will precipitate one component of the mixture before the other. In a hypereutectic solution, there will be a proeutectoid phase of species β whereas a hypoeutectic solution will have a proeutectic α phase.[4]



Eutectic alloys have two or more materials and have a eutectic composition. When a non-eutectic alloy solidifies, its components solidify at different temperatures, exhibiting a plastic melting range. Conversely, when a well-mixed, eutectic alloy melts, it does so at a single, sharp temperature. The various phase transformations that occur during the solidification of a particular alloy composition can be understood by drawing a vertical line from the liquid phase to the solid phase on the phase diagram for that alloy.

Some uses include:


Other critical points


Iron-carbon phase diagram, showing the eutectoid transformation between austenite (γ) and pearlite.

When the solution above the transformation point is solid, rather than liquid, an analogous eutectoid transformation can occur. For instance, in the iron-carbon system, the austenite phase can undergo a eutectoid transformation to produce ferrite and cementite, often in lamellar structures such as pearlite and bainite. This eutectoid point occurs at 727 °C (1,341 °F) and about 0.76% carbon.[12]


A peritectoid transformation is a type of isothermal reversible reaction that has two solid phases reacting with each other upon cooling of a binary, ternary, ..., ary alloy to create a completely different and single solid phase.[13] The reaction plays a key role in the order and decomposition of quasicrystalline phases in several alloy types.[14]


Peritectic transformations are also similar to eutectic reactions. Here, a liquid and solid phase of fixed proportions react at a fixed temperature to yield a single solid phase. Since the solid product forms at the interface between the two reactants, it can form a diffusion barrier and generally causes such reactions to proceed much more slowly than eutectic or eutectoid transformations. Because of this, when a peritectic composition solidifies it does not show the lamellar structure that is found with eutectic solidification.

Such a transformation exists in the iron-carbon system, as seen near the upper-left corner of the figure. It resembles an inverted eutectic, with the δ phase combining with the liquid to produce pure austenite at 1,495 °C (2,723 °F) and 0.17% carbon.

Gold-aluminium phase diagram (German). Top axis title reads "Weight-percent Gold", lower axis title reads "Atomic-percent Gold"

At the peritectic decomposition temperature the compound, rather than melting, decomposes into another solid compound and a liquid. The proportion of each is determined by the lever rule. In the Al-Au phase diagram, for example, it can be seen that only two of the phases melt congruently, AuAl2 and Au2Al , while the rest peritectically decompose.

Eutectic calculation

The composition and temperature of a eutectic can be calculated from enthalpy and entropy of fusion of each components.[15]

The Gibbs free energy, G, depends on its own differential

Thus, the G/T derivative at constant pressure is calculated by the following equation

The chemical potential is calculated if we assume the activity is equal to the concentration.

At the equilibrium, , thus is obtained by:

Using and integrating gives

The integration constant K may be determined for a pure component with a melting temperature and an enthalpy of fusion Eq.

We obtain a relation that determines the molar fraction as a function of the temperature for each component.

The mixture of n components is described by the system

that can be solved by

See also


  1. "eutectic". Merriam-Webster Dictionary.
  2. Smith & Hashemi 2006, pp. 326–327
  4. 1 2 3 Smith & Hashemi 2006, p. 327.
  5. Smith & Hashemi 2006, pp. 332–333.
  6. "Operation of the Overloads". Retrieved 2015-08-05.
  7. Muldrew, Ken; Locksley E. McGann (1997). "Phase Diagrams". Cryobiology—A Short Course. University of Calgary. Retrieved 2006-04-29.
  8. Senese, Fred (1999). "Does salt water expand as much as fresh water does when it freezes?". Solutions: Frequently asked questions. Department of Chemistry, Frostburg State University. Retrieved 2006-04-29.
  9. "Molten salts properties". Archimede Solar Plant Specs.
  10. Fichter, Lynn S. (2000). "Igneous Phase Diagrams". Igneous Rocks. James Madison University. Retrieved 2006-04-29.
  11. Davies, Nicholas A.; Beatrice M. Nicholas (1992). "Eutectic compositions for hot melt jet inks". US Patent & Trademark Office, Patent Full Text and Image Database. United States Patent and Trademark Office. Retrieved 2006-04-29.
  12. Iron-Iron Carbide Phase Diagram Example
  13. IUPAC Compendium of Chemical Terminology, Electronic version. "Peritectoid Reaction" Retrieved May 22, 2007.
  14. Numerical Model of Peritectoid Transformation. Peritectoid Transformation Retrieved May 22, 2007. Archived September 28, 2007, at the Wayback Machine.
  15. International Journal of Modern Physics C, Vol. 15, No. 5. (2004), pp. 675-687


Further reading

Look up eutectic in Wiktionary, the free dictionary.
This article is issued from Wikipedia - version of the 11/14/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.