Eutectic system
![](https://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Eutectic_system_phase_diagram.svg/310px-Eutectic_system_phase_diagram.svg.png)
Aeutectic systemoreutectic mixture(/juːˈtɛktɪk/yoo-TEK-tik)[1]is ahomogeneous mixturethat has amelting pointlower than those of the constituents.[2]The lowest possible melting point over all of themi xing ratiosof the constituents is called theeutectic temperature.On aphase diagram,the eutectic temperature is seen as the eutectic point (see plot on the right).[3]
Non-eutectic mixture ratios have different melting temperatures for their different constituents, since one component'slatticewill melt at a lower temperature than the other's. Conversely, as a non-eutectic mixture cools down, each of its componentssolidifiesinto a lattice at a different temperature, until the entire mass is solid.
Not allbinary alloyshave eutectic points, since thevalence electronsof the component species are not always compatible[clarification needed]in any mi xing 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".[4]
In the real world, eutectic properties can be used to advantage in such processes aseutectic bonding,wheresilicon chipsare bonded to gold-plated substrates withultrasound,and eutectic alloys prove valuable in such diverse applications as soldering, brazing, metal casting, electrical protection, fire sprinkler systems, and nontoxic mercury substitutes. By managing phase transformation during solidification, a suitable eutectic alloy can be made stronger than any of its individual components, a valuable property in an extreme application such as the hypereutectic cast aluminum pistons used in the high-revving 550 hp (410 kW) twin-turbo intercooled DOHCCadillac Blackwing V8introduced in 2018.
The termeutecticwas coined in 1884 by British physicist and chemistFrederick Guthrie(1833–1886). The word originates fromGreekεὐ- (eû)'well' andτῆξῐς(têxis)'melting'.[2]Before his studies, chemists assumed "that the alloy of minimum fusing point must have its constituents in some simple atomic proportions", which was indeed proven to be not the case.[5]
Eutectic phase transition
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/a/a3/Various_eutectic_structures.png/220px-Various_eutectic_structures.png)
The eutectic solidification is defined as follows:[6]
This type of reaction is an invariant reaction, because it is inthermal equilibrium;another way to define this is the change inGibbs free energyequals zero. Tangibly, this means the liquid and twosolid solutionsall coexist at the same time and are inchemical equilibrium.There is also athermal arrestfor the duration of thephase changeduring which the temperature of the system does not change.[6]
The resulting solidmacrostructurefrom a eutectic reaction depends on a few factors, with the most important factor being how the two solid solutions nucleate and grow. The most common structure is alamellar structure,but other possible structures include rodlike, globular, andacicular.[7]
Non-eutectic compositions
[edit]Compositions of eutectic systems that are not at the eutectic point can be classified ashypoeutecticorhypereutectic:
- Hypoeutecticcompositions are those with a greater composition of species α and a smaller percent composition of species β than the eutectic composition (E)
- Hypereutecticcompositions 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 aproeutectoidphase of species β whereas a hypoeutectic solution will have aproeutecticα phase.[6]
Types
[edit]Alloys
[edit]Eutecticalloyshave 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 for eutectic alloys include:
- NEMA Eutectic Alloy Overload Relays forelectrical protectionof 3-phase motors for pumps, fans, conveyors, and other factory process equipment.[8]
- Eutectic alloys forsoldering,both traditional alloys composed oflead(Pb) andtin(Sn), sometimes with additionalsilver(Ag) orgold(Au) — especiallySn63Pb37and Sn62Pb36Ag2alloy formula for electronics - and newer lead-free soldering alloys, in particular ones composed oftin(Sn),silver(Ag), andcopper(Cu) such as Sn96.5Ag3.5.
- Casting alloys, such asaluminium-siliconandcast iron(at the composition of 4.3% carbon in iron producing anaustenite-cementiteeutectic)
- Silicon chipsareeutectic bondedto gold-plated substrates through a silicon-gold eutectic by the application ofultrasonicenergy 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, e.g.,Wood's metalandField's metalforfire sprinklers
- Non-toxicmercuryreplacements, such asgalinstan
- Experimentalglassy metals,with extremely high strength andcorrosionresistance
- Eutectic alloys ofsodiumandpotassium(NaK) that are liquid at room temperature and used ascoolantin experimentalfast neutron nuclear reactors.
Others
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/Phase_diagram_ethanol_water_s_l_en.svg/290px-Phase_diagram_ethanol_water_s_l_en.svg.png)
- Sodium chlorideandwaterform a eutectic mixture whose eutectic point is −21.2 °C[9]and 23.3% salt by mass.[10]The eutectic nature of salt and water is exploited when salt is spread on roads to aidsnow removal,or mixed with ice to produce low temperatures (for example, in traditionalice creammaking).
- Ethanol–water has an unusually biased eutectic point, i.e. it is close to pure ethanol, which sets the maximum proof obtainable byfractional freezing.
- "Solar salt", 60% NaNO3and 40% KNO3,forms a eutectic molten salt mixture which is used forthermal energy storageinconcentrated solar powerplants.[11]To reduce the eutectic melting point in the solar molten salts,calcium nitrateis used in the following proportion: 42% Ca(NO3)2,43% KNO3,and 15% NaNO3.
- Lidocaineandprilocaine—both are solids at room temperature—form a eutectic that is an oil with a 16 °C (61 °F) melting point that is used ineutectic mixture of local anesthetic(EMLA) preparations.
- Mentholandcamphor,both solids at room temperature, form a eutectic that is a liquid at room temperature in the following proportions: 8:2, 7:3, 6:4, and 5:5. Both substances are common ingredients in pharmacy extemporaneous preparations.[12]
- Mineralsmay form eutectic mixtures inigneousrocks, giving rise to characteristicintergrowth texturesexhibited, for example, bygranophyre.[13]
- Some inks are eutectic mixtures, allowinginkjet printersto operate at lower temperatures.[14]
- Cholinechloride produces eutectic mixtures with many natural products such ascitric acid,malic acidandsugars.These liquid mixtures can be used, for example, to obtain antioxidant and antidiabetic extracts fromnatural products.[15]
Strengthening mechanisms
[edit]Alloys
[edit]The primary strengthening mechanism of the eutectic structure in metals iscompositestrengthening (Seestrengthening mechanisms of materials). This deformation mechanism works through load transfer between the two constituent phases where the more compliant phase transfers stress to the stiffer phase.[16]By taking advantage of the strength of the stiff phase and the ductility of the compliant phase, the overall toughness of the material increases. As the composition is varied to either hypoeutectic or hypereutectic formations, the load transfer mechanism becomes more complex as there is a load transfer between the eutectic phase and the secondary phase as well as the load transfer within the eutectic phase itself.
A second tunable strengthening mechanism of eutectic structures is the spacing of the secondary phase. By changing the spacing of the secondary phase, the fraction of contact between the two phases through shared phase boundaries is also changed. By decreasing the spacing of the eutectic phase, creating a fine eutectic structure, more surface area is shared between the two constituent phases resulting in more effective load transfer.[17]On the micro-scale, the additional boundary area acts as a barrier todislocationsfurther strengthening the material. As a result of this strengthening mechanism, coarse eutectic structures tend to be less stiff but more ductile while fine eutectic structures are stiffer but more brittle.[17]The spacing of the eutectic phase can be controlled during processing as it is directly related to the cooling rate during solidification of the eutectic structure. For example, for a simple lamellar eutectic structure, the minimal lamellae spacing is:[18]
Where isis thesurface energyof the two-phase boundary,is themolar volumeof the eutectic phase,is the solidification temperature of the eutectic phase,is theenthalpy of formationof the eutectic phase, andis the undercooling of the material. So, by altering the undercooling, and by extension the cooling rate, the minimal achievable spacing of the secondary phase is controlled.
Strengthening metallic eutectic phases to resist deformation at high temperatures (seecreep deformation) is more convoluted as the primary deformation mechanism changes depending on the level of stress applied. At high temperatures where deformation is dominated by dislocation movement, the strengthening from load transfer and secondary phase spacing remain as they continue to resist dislocation motion. At lower strains where Nabarro-Herring creep is dominant, the shape and size of the eutectic phase structure plays a significant role in material deformation as it affects the available boundary area for vacancy diffusion to occur.[19]
Other critical points
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/8/8a/Iron_carbon_phase_diagram.svg/390px-Iron_carbon_phase_diagram.svg.png)
Eutectoid
[edit]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, theaustenitephase can undergo a eutectoid transformation to produceferriteandcementite,often in lamellar structures such aspearliteandbainite.This eutectoid point occurs at 723 °C (1,333 °F) and 0.76 wt% carbon.[20]
Peritectoid
[edit]Aperitectoidtransformation is a type ofisothermalreversible reactionthat has two solidphasesreacting with each other upon cooling of a binary, ternary,...,n-aryalloyto create a completely different and single solid phase.[21]The reaction plays a key role in the order anddecompositionofquasicrystallinephases in several alloy types.[22]A similar structural transition is also predicted forrotating columnar crystals.
Peritectic
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Phasendiagramm_Gold-Aluminium.svg/280px-Phasendiagramm_Gold-Aluminium.svg.png)
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 thelamellar structurethat 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 pureausteniteat 1,495 °C (2,723 °F) and 0.17% carbon.
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 thelever rule.In theAl-Auphase diagram, for example, it can be seen that only two of the phases melt congruently,AuAl2andAu2Al,while the rest peritectically decompose.
Eutectic calculation
[edit]The composition and temperature of a eutectic can be calculated from enthalpy and entropy of fusion of each components.[23]
The Gibbs free energyGdepends on its own differential:
Thus, theG/Tderivative at constant pressure is calculated by the following equation:
The chemical potentialis calculated if we assume that the activity is equal to the concentration:
At the equilibrium,,thusis obtained as
Using[clarification needed]and integrating gives
The integration constantKmay be determined for a pure component with a melting temperatureand an enthalpy of fusion:
We obtain a relation that determines the molar fraction as a function of the temperature for each component:
The mixture ofncomponents is described by the system
which can be solved by
See also
[edit]- Azeotrope,or constant boiling mixture
- Freezing-point depression
- Low melting alloy
References
[edit]- ^"eutectic".Merriam-Webster Dictionary.Merriam-Webster.
- ^abGuthrie, Frederick (June 1884)."LII. On eutexia".The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science.5th series.17(108): 462–482.doi:10.1080/14786448408627543.p. 462:
The main argument of the present communication hinges upon the existence of compound bodies, whose chief characteristic is the lowness of their temperatures of fusion. This property of the bodies may be called Eutexia, the bodies possessing it eutectic bodies or eutectics (εὖ τήκειν).
- ^Smith & Hashemi 2006,pp. 326–327
- ^"Collection of Phase Diagrams".crct.polymtl.ca.
- ^Britain), Chemical Society (Great (1885).Journal of the Chemical Society.Chemical Society.
- ^abcSmith & Hashemi 2006,p. 327.
- ^Smith & Hashemi 2006,pp. 332–333.
- ^"Operation of the Overloads".Retrieved2015-08-05.
- ^Muldrew, Ken; Locksley E. McGann (1997)."Phase Diagrams".Cryobiology—A Short Course.University of Calgary. Archived fromthe originalon 2006-06-15.Retrieved2006-04-29.
- ^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.Retrieved2006-04-29.
- ^"Molten salts properties".Archimede Solar Plant Specs.Archived fromthe originalon 2020-04-20.Retrieved2011-07-15.
- ^Phaechamud, Thawatchai; Tuntarawongsa, Sarun; Charoensuksai, Purin (October 2016)."Evaporation Behavior and Characterization of Eutectic Solvent and Ibuprofen Eutectic Solution".AAPS PharmSciTech.17(5): 1213–1220.doi:10.1208/s12249-015-0459-x.ISSN1530-9932.PMID26669887.
- ^Fichter, Lynn S. (2000)."Igneous Phase Diagrams".Igneous Rocks.James Madison University. Archived fromthe originalon 2011-06-28.Retrieved2006-04-29.
- ^US 5298062A,Davies, Nicholas A. & Nicholas, Beatrice M., "Eutectic compositions for hot melt jet inks", published 1994-03-29, issued 1994-03-29
- ^Socas-Rodriguez, Bárbara; Torres-Cornejo, Mónica Vanesa; Álvarez-Rivera, Gerardo; Mendiola, Jose A. (May 2021)."Deep Eutectic Solvents for the Extraction of Bioactive Compounds from Natural Sources and Agricultural By-Products".Applied Sciences.11(1): 4897.doi:10.3390/app11114897.hdl:10261/253199.ISSN2076-3417.
- ^Courtney, T. H. (1990).Mechanical behavior of materials(2nd ed.). New York: McGraw-Hill.
- ^abCallister, W. D. (2010).Materials science and engineering: an introduction.
- ^Porter, D. A.; Easterling, K. E.; Sherif, M. Y. (2009).Phase transformations in metals and alloys.
- ^Wu, T.; Plotkowski, A.; Shyam, A.; Dunand, D.C. "Microstructure and creep properties of cast near-eutectic Al-Ce-Ni alloys".Mater. Sci. Eng. A.833:12.
- ^Ballentine, Kim (28 April 1996)."Iron-Iron Carbide Phase Diagram Example".Archived fromthe original(PDF)on 16 February 2008.
- ^IUPAC,Compendium of Chemical Terminology,2nd ed. (the "Gold Book" ) (1997). Online corrected version: (2006–) "Peritectoid reaction".doi:10.1351/goldbook.P04501
- ^Das, Amit; Manna, Indranil; Pabi, S. K. (October 1999). "A numerical model of peritectoid transformation".Metallurgical and Materials Transactions A.30(10).The Minerals, Metals & Materials Society,ASM International:2563–2573.doi:10.1007/s11661-999-0295-2.S2CID95279944.
- ^Brunet, Luc E.; Caillard, Jean; André, Pascal (June 2004). "Thermodynamic Calculation ofn-component Eutectic Mixtures ".International Journal of Modern Physics C.15(5). World Scientific: 675–687.Bibcode:2004IJMPC..15..675B.doi:10.1142/S0129183104006121.
Bibliography
[edit]- Smith, William F.; Hashemi, Javad (2006),Foundations of Materials Science and Engineering(4th ed.), McGraw-Hill,ISBN978-0-07-295358-9.
Further reading
[edit]![](https://upload.wikimedia.org/wikipedia/commons/thumb/9/99/Wiktionary-logo-en-v2.svg/40px-Wiktionary-logo-en-v2.svg.png)
- Askeland, Donald R.; Pradeep P. Phule (2005).The Science and Engineering of Materials.Thomson-Engineering.ISBN978-0-534-55396-8.
- Easterling, Edward (1992).Phase Transformations in Metals and Alloys.CRC.ISBN978-0-7487-5741-1.
- Mortimer, Robert G. (2000).Physical Chemistry.Academic Press.ISBN978-0-12-508345-4.
- Reed-Hill, R. E.;Reza Abbaschian(1992).Physical Metallurgy Principles.Thomson-Engineering.ISBN978-0-534-92173-6.
- Sadoway, Donald(2004)."Phase Equilibria and Phase Diagrams"(PDF).3.091 Introduction to Solid State Chemistry, Fall 2004.MIT Open Courseware. Archived fromthe original(PDF)on 2005-10-20.Retrieved2006-04-12.