A eutectic system or eutectic mixture (/juːˈtɛktɪk/yoo-TEK-tik)[1] is a
homogeneous mixture that has a
melting point lower than those of the constituents.[2] The lowest possible melting point over all of the
mixing ratios of the constituents is called the eutectic temperature. On a
phase 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's
lattice will melt at a lower temperature than the other's. Conversely, as a non-eutectic mixture cools down, each of its components
solidifies into a lattice at a different temperature, until the entire mass is solid.
Not all
binary alloys have eutectic points, since the
valence electrons of the component species are not always compatible[clarification needed] in any mixing 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 as
eutectic bonding, where
silicon chips are bonded to gold-plated substrates with
ultrasound, 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 DOHC
Cadillac Blackwing V8 introduced in 2018.
The term eutectic was coined in 1884 by British physicist and chemist
Frederick Guthrie (1833–1886). The word originates from
Greekεὐ- (eû) 'well', and τῆξῐς (têxis) 'melting'.[2]
Eutectic phase transition
The eutectic solidification is defined as follows:[5]
This type of reaction is an invariant reaction, because it is in
thermal equilibrium; another way to define this is the change in
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
phase change during which the temperature of the system does not change.[5]
The resulting solid
macrostructure from 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 a
lamellar structure, but other possible structures include rodlike, globular, and
acicular.[6]
Non-eutectic compositions
Compositions of eutectic systems that are not at the eutectic point can be classified as hypoeutectic or hypereutectic:
Hypoeutectic compositions are those with a greater composition of species α and a smaller percent composition of species β than the eutectic composition (E)
Hypereutectic compositions 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 a proeutectoid phase of species β whereas a hypoeutectic solution will have a proeutectic α phase.[5]
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. 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 for
electrical protection of 3-phase motors for pumps, fans, conveyors, and other factory process equipment.[7]
Eutectic alloys for
soldering, both traditional alloys composed of
lead (Pb) and
tin (Sn), sometimes with additional
silver (Ag) or
gold (Au) — especially
Sn63Pb37 and Sn62Pb36Ag2 alloy formula for electronics - and newer lead-free soldering alloys, in particular ones composed of
tin (Sn),
silver (Ag), and
copper (Cu) such as Sn96.5Ag3.5.
Sodium chloride and
water form a eutectic mixture whose eutectic point is −21.2 °C[8] and 23.3% salt by mass.[9] 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).
Ethanol–water has an unusually biased eutectic point, i.e. it is close to pure ethanol, which sets the maximum proof obtainable by
fractional freezing.
"Solar salt", 60% NaNO3 and 40% KNO3, forms a eutectic molten salt mixture which is used for
thermal energy storage in
concentrated solar power plants.[10] To reduce the eutectic melting point in the solar molten salts,
calcium nitrate is used in the following proportion: 42% Ca(NO3)2, 43% KNO3, and 15% NaNO3.
Menthol and
camphor, 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.[11]
Some inks are eutectic mixtures, allowing
inkjet printers to operate at lower temperatures.[13]
Choline chloride produces eutectic mixtures with many natural products such as
citric acid,
malic acid and
sugars. These liquid mixtures can be used, for example, to obtain antioxidant and antidiabetic extracts from
natural products.[14]
Strengthening mechanisms
Alloys
The primary strengthening mechanism of the eutectic structure in metals is
composite strengthening (See
strengthening 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.[15] 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.[16] On the micro-scale, the additional boundary area acts as a barrier to
dislocations further 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.[16] 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:[17]
Where is is the
surface energy of the two-phase boundary, is the
molar volume of the eutectic phase, is the solidification temperature of the eutectic phase, is the
enthalpy of formation of the eutectic phase, and is 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 (see
creep 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.[18]
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 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 0.76 wt% carbon.[19]
Peritectoid
A peritectoid transformation is a type of
isothermalreversible reaction that has two solid
phases reacting with each other upon cooling of a binary, ternary, ..., n-ary
alloy to create a completely different and single solid phase.[20] The reaction plays a key role in the order and
decomposition of
quasicrystalline phases in several alloy types.[21] A similar structural transition is also predicted for
rotating columnar crystals.
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.
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 the
lever rule. In the
Al-Au phase diagram, for example, it can be seen that only two of the phases melt congruently,
AuAl2 and
Au2Al, while the rest peritectically decompose.
Eutectic calculation
The composition and temperature of a eutectic can be calculated from enthalpy and entropy of fusion of each components.[22]
The Gibbs free energy G depends on its own differential:
Thus, the G/T derivative at constant pressure is calculated by the following equation:
The chemical potential is calculated if we assume that the activity is equal to the concentration:
^Muldrew, Ken; Locksley E. McGann (1997).
"Phase Diagrams". Cryobiology—A Short Course. University of Calgary. Archived from
the original on 2006-06-15. Retrieved 2006-04-29.
^Fichter, Lynn S. (2000).
"Igneous Phase Diagrams". Igneous Rocks. James Madison University. Archived from
the original on 2011-06-28. Retrieved 2006-04-29.
^US 5298062A, Davies, Nicholas A. & Nicholas, Beatrice M., "Eutectic compositions for hot melt jet inks", published 1994-03-29, issued 1994-03-29
^Brunet, Luc E.; Caillard, Jean; André, Pascal (June 2004). "Thermodynamic Calculation of n-component Eutectic Mixtures". International Journal of Modern Physics C. 15 (5). World Scientific: 675–687.
Bibcode:
2004IJMPC..15..675B.
doi:
10.1142/S0129183104006121.
Bibliography
Smith, William F.; Hashemi, Javad (2006), Foundations of Materials Science and Engineering (4th ed.), McGraw-Hill,
ISBN978-0-07-295358-9.
Further reading
Look up eutectic in Wiktionary, the free dictionary.
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.