Eutectic system

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File:Eutectic system phase diagram.svg

A phase diagram for a fictitious binary chemical mixture (with the two components denoted by α and β) used to depict the eutectic composition, temperature, and point. (L denotes the liquid state.)

A eutectic system is a mixture of chemical compounds or elements that has a single chemical composition that solidifies at a lower temperature than any other composition. This composition is known as the eutectic composition and the temperature is known as the eutectic temperature. On a phase diagram the intersection of the eutectic temperature and the eutectic composition gives the eutectic point. Not all binary alloys have a eutectic point; for example, in the silver-gold system the melt temperature (liquidus) and freeze temperature (solidus) both increase monotonically as the mix changes from pure silver to pure gold.

Eutectic reaction

File:Various eutectic structures.png

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

The eutectic reaction is defined as follows:

\[\text{Liquid} \xrightarrow[\text{cooling}]{\text{eutectic temperature}} \alpha \,\, \text{solid solution} + \beta \,\, \text{solid solution}\]

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 reaction.

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.

Non-eutectic compositions

Compositions of eutectic systems that are not the eutectic composition are commonly defined to be hypoeutectic or hypereutectic. Hypoeutectic composition are composition to the left of the eutectic composition and hypereutectic composition are compositions to the right.

Types

Alloys

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. A eutectic alloy solidifies at a single, sharp temperature. The phase transformations that occur while solidifying a given alloy can be understood by drawing a vertical line from the liquid phase to the solid phase on a phase diagram.

Some uses include:

• eutectic alloys for soldering, composed of tin (Sn), lead (Pb) and sometimes silver (Ag) or gold (Au)
• casting alloys, such as aluminium-silicon and cast iron (at the composition for an austenite-cementite eutectic in the iron-carbon system)
• one method used by the semiconductor industry to bond silicon chips to gold-plated substrates is to induce a silicon-gold eutectic through the application of ultrasonic energy to the chip
• brazing, where diffusion can remove alloying elements from the joint, so that eutectic melting is only possible early in the brazing process
• temperature response, i.e. Wood's metal and Field's metal for fire sprinklers
• non-toxic mercury replacements, such as galinstan
• experimental glassy metals, with extremely high strength and corrosion resistance
• eutectic alloys of sodium and potassium (NaK) that are liquid at room temperature and used as coolant in experimental fast neutron nuclear reactors.

Others

Sodium chloride and water form a eutectic mixture. It has a eutectic point of −21.2 C and 23.3% salt by mass. The eutectic nature of salt and water is exploited when salt is spread on roads to aid snow removal, or mixed with ice to produce low temperatures (for example, in traditional ice cream making).

60% NaNO₃ and 40% KNO₃ forms a eutectic mixture which is used in solar molten salt technology.

Lidocaine and prilocaine, both solids at room temperature, form a eutectic that is an oil with a 16 °C (61 °F) melting point, used in eutectic mixture of local anesthetic (EMLA) preparations.

Minerals may form eutectic mixtures in igneous rocks, giving rise to characteristic intergrowth textures such as that of granophyre.

Some inks are eutectic mixtures, allowing inkjet printers to operate at lower temperatures.

Other critical points

Eutectoid

File:Phase diag iron carbon.PNG

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 723 °C (1,333 °F) and about 0.83% carbon.

Peritectoid

A peritectoid transformation is a type of isothermal reversible reaction that have two solid phases reacting with each other upon cooling of a binary, ternary, ... , \(n\!\) alloy to create a completely different and single solid phase. The reaction plays a key role in the order and decomposition of quasicrystalline phases in several alloy types.

Peritectic

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.

See also

• Azeotrope
• Freezing point depression

References

Bibliography

• Smith, William F.; Hashemi, Javad (2006), Foundations of Materials Science and Engineering (4th ed.), McGraw-Hill, ISBN 0-07-295358-6.

Further reading

• Sadoway, Donald (2004). "Phase Equilibria and Phase Diagrams" (pdf). 3.091 Introduction to Solid State Chemistry, Fall 2004. MIT Open Courseware. Archived from the original on 2005-10-20. http://web.archive.org/web/20051020162911/http://ocw.mit.edu/NR/rdonlyres/Materials-Science-and-Engineering/3-091Fall-2004/6ECFB930-9D59-4DD5-A872-FE48002587B0/0/notes_10.pdf. Retrieved 2006-04-12.ca:Mescla eutèctica