Eutectic point




The alloy) depends on the relative proportions of its ingredients. A eutectic or eutectic mixture is a mixture at such proportions that the melting point is as low as possible, and that furthermore all the constituents crystallize simultaneously at this temperature from molten liquid solution. Such a simultaneous crystallization of a eutectic mixture is known as a eutectic reaction, the temperature at which it takes place is the eutectic temperature, and the composition and temperature at which it takes place is called the eutectic point.

The term comes from the Greek eutektos, meaning 'easily melted.'

  The equilibrium liquid phase L borders directly on the solid phase α + β (a homogeneous composed of both A and B), representing the minimum melting temperature of any possible alloy of A and B.

Not all binary system alloys have a eutectic point: those that form a silver system, have no eutectic. An alloy system that has a eutectic is often referred to as a eutectic system, or eutectic alloy.

Solid products of a eutectic reaction can often be identified by their amorphous solid if pushed to an extreme.

Metallic eutectics

The term is often used in alloy of two or more component materials having the relative concentrations specified at the eutectic point. When a non-eutectic alloy freezes, one component of the alloy crystallizes at one temperature and the other at a different temperature. With a eutectic alloy, the mixture freezes as one at a single temperature. A eutectic alloy therefore has a sharp melting point, and a non-eutectic alloy exhibits a plastic melting range. The phase transformations that occur while freezing a given alloy can be understood using the phase diagram by drawing a vertical line from the liquid phase to the solid phase on a phase diagram; each point along the line describes the composition at a given temperature.

Some uses include:

  • eutectic alloys for soldering, composed of gold (Au).
  • casting alloys, such as aluminum-carbon system).
  • 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 corrosion resistance.
  • eutectic alloys of potassium (NaK) that are liquid at room temperature and used as coolant in experimental fast neutron nuclear reactors.

Other eutectic mixtures

Sodium chloride and water form a eutectic mixture. It has a eutectic point of −21.2 C[1] and 23.3%[2] salt by weight. 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).

EMLA (Eutectic Mixture of Local Anesthetic) preparations.

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Some inks are eutectic mixtures, allowing inkjet printers to operate at lower temperatures.[4]

Other critical points

Eutectoid

When the solution above the transformation point is solid, rather than liquid, an analogous eutectoid transformation can occur. For instance, in the martensitic transformation.

 

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 you find with eutectic freezing.

Such a transformation exists in the austenite at 1495 °C and 0.17 mass percent carbon.

References

  1. ^ Muldrew, Ken; Locksley E. McGann (1997). Phase Diagrams. Cryobiology—A Short Course. University of Calgary. Retrieved on 2006-04-29.
  2. ^ 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 on 2006-04-29.
  3. ^ Fichter, Lynn S. (2000). Igneous Phase Diagrams. Igneous Rocks. James Madison University. Retrieved on 2006-04-29.
  4. ^ 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 on 2006-04-29.
  • Sadoway, Donald (2004). Phase Equilibria and Phase Diagrams (pdf). 3.091 Introduction to Solid State Chemistry, Fall 2004. MIT Open Courseware. Retrieved on 2006-04-12.

Bibliography

  • Mortimer, Robert G. (2000). Physical Chemistry. Academic Press. ISBN 0-12-508345-9. 
  • Reed-Hill, R.E.; Reza Abbaschian (1992). Physical Metallurgy Principles. Thomson-Engineering. ISBN 0-534-92173-6. 
  • Easterling, Edward (1992). Phase Transformations in Metals and Alloys. CRC. ISBN 0-7487-5741-4. 
  • Askeland, Donald R.; Pradeep P. Phule (2005). The Science and Engineering of Materials. Thomson-Engineering. ISBN 0-534-55396-6. 

See also
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Eutectic_point". A list of authors is available in Wikipedia.