- Eutectic point
melting pointof a mixture of two or more solids (such as an 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.'
phase diagramfor a binary system. The eutectic point is the point at which the liquidphase L borders directly on the solidphase α + β.] The equilibrium phase diagramat the right displays a simple binary system composed of two components, A and B, which has a eutectic point. The phase diagram plots relative concentrations of A and B along the horizontal axis, and temperaturealong the vertical axis. The eutectic point is the point at which the liquidphase L borders directly on the solid phase α + β (a homogeneous mixture 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
solid solutionat all concentrations, such as the gold- silversystem, 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
lamellar structure, as opposed to the dendritic structures commonly seen in non-eutectic solidification. The same conditions that force the material to form lamellae can instead form an amorphous solidif pushed to an extreme.
The term is often used in
metallurgyto describe the alloyof 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 tin(Sn), lead(Pb) and sometimes silver(Ag) or gold(Au).
*casting alloys, such as
aluminium- siliconand cast iron(at the composition for an austenite- cementiteeutectic in the iron- carbonsystem).
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 metaland Field's metalfor fire sprinklers.
*non-toxic mercury replacements, such as
*experimental metallic glasses, with extremely high strength and
*eutectic alloys of
sodiumand potassium( NaK) that are liquid at room temperature and used as coolantin experimental fast neutron nuclear reactors.
Other eutectic mixtures
Sodium chlorideand waterform a eutectic mixture. It has a eutectic point of −21.2 C [cite web | last = Muldrew | first = Ken | coauthors = Locksley E. McGann | year = 1997 | url = http://www.ucalgary.ca/~kmuldrew/cryo_course/cryo_chap6_1.html | title = Phase Diagrams | work = Cryobiology—A Short Course | publisher = University of Calgary | accessdate = 2006-04-29] and 23.3% [cite web | last = Senese | first = Fred | year = 1999 | url = http://antoine.frostburg.edu/chem/senese/101/solutions/faq/saltwater-ice-volume.shtml | title = Does salt water expand as much as fresh water does when it freezes? | work = Solutions: Frequently asked questions | publisher = Department of Chemistry, Frostburg State University | accessdate = 2006-04-29] 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 creammaking). Lidocaineand prilocaine, both solids at room temperature, form a eutectic that is an oil with a 16°C melting point, used in EMLA (Eutectic Mixture of Local Anesthetic) preparations. Minerals may form eutectic mixtures in igneousrocks, [cite web | last = Fichter | first = Lynn S. | year = 2000 | url = http://csmres.jmu.edu/geollab/Fichter/IgnRx/Phasdgrm.html | title = Igneous Phase Diagrams | work = Igneous Rocks | publisher = James Madison University | accessdate = 2006-04-29] giving rise to characteristic intergrowth textures such as that of granophyre.
Some inks are eutectic mixtures, allowing
inkjet printers to operate at lower temperatures. [cite web | last = Davies | first = Nicholas A. | coauthors = Beatrice M. Nicholas | year = 1992 | url = http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=/netahtml/search-bool.html&r=1&f=G&l=50&co1=AND&d=ptxt&s1=5298062.WKU.&OS=PN/5298062&RS=PN/5298062 | title = Eutectic compositions for hot melt jet inks | work = US Patent & Trademark Office, Patent Full Text and Image Database | publisher = United States Patent and Trademark Office | accessdate = 2006-04-29]
Other critical points
When the solution above the transformation point is solid, rather than liquid, an analogous eutectoid transformation can occur. For instance, in the
iron- carbonsystem, the austenitephase can undergo a eutectoid transformation to produce ferriteand cementite( iron carbide), often in lamellar structures such as pearliteand bainite. This eutectoid point occurs at 727°C (1340.6 ºF) and about 0.83% carbon [ [http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/examples/kimcon.html Iron-Iron Carbide Phase Diagram Example ] ] ; alloys of nearly this composition are called "high-carbon steel", while those which have less carbon are termed "mild steel". The process analogous to glass formation in this system is the martensitic transformation.
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
iron- carbonsystem, 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 austeniteat 1495 °C and 0.17 mass percent carbon.
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Freezing point depression
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