Eutectic
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A eutectic or eutectic mixture is a mixture of two or more elements which has a lower melting point than any of its constituents. The proper ratios of components to obtain a eutectic alloy is identified by the eutectic point on a phase diagram. The term comes from the Greek 'eutektos', meaning 'easily melted.'
Binary_phase_diagram.PNG
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The phase diagram at right displays a simple binary system composed of two components, α and β, which has a eutectic point. The phase diagram plots relative concentrations of α and β along the X-axis, and temperature along the Y-axis. The eutectic point is the point at which the liquid phase borders directly on the solid α + β phase, representing the minimum melting temperature of any possible alloy of α and β. Not all binary system alloys have a eutectic point.
Solid products of a eutectic transformation 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 solid if pushed to an extreme.
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Metallic eutectics
The term is often used in metallurgy to describe the 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. 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).
- casting alloys, such as aluminum-silicon and cast iron (an iron-carbon eutectic).
- 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 for fire sprinklers.
- non-toxic mercury replacements, such as galinstan.
- experimental metallic glasses, with extremely high strength and corrosion resistance.
Other eutectic mixtures
Sodium chloride and water form a eutectic mixture. It has a eutectic point of -21.2 C[1] (http://www.ucalgary.ca/~kmuldrew/cryo_course/cryo_chap6_1.html) and 23.3%[2] (http://antoine.frostburg.edu/chem/senese/101/solutions/faq/saltwater-ice-volume.shtml) 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).
Minerals may form eutectic mixtures in igneous rocks[3] (http://csmres.jmu.edu/geollab/Fichter/IgnRx/Phasdgrm.html).
Some inks are eutectic mixtures, allowing inkjet printers to operate at lower temperatures[4] (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).
Other critical points: eutectoid and peritectic
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 (iron carbide), often in lamellar structures such as pearlite and bainite. This eutectoid point is at about 0.6% carbon; alloys of nearly this composition are called high-carbon steel, while those which do not undergo eutectoid transformation 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.