Eutectic system


A eutectic system or eutectic mixture is a type of a homogeneous mixture that has a melting point lower than those of the constituents. 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.
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. A non-eutectic mixture thus does not have a single melting/freezing point temperature at which it changes phase, but rather a temperature at which it changes between liquid and slush and a lower temperature at which it changes between slush and solid.
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.
The term was coined in 1884 by the British physicist and chemist Frederick Guthrie. The word originates. Before his studies, chemists assumed "that the alloy of minimum fusing point must have its constituents in some simple atomic proportions", but he showed that that is not always true.

Eutectic phase transition

The eutectic solidification is defined as follows:
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.
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. Besides, a vermicular microstructure has been observed in a high-entropy alloy.

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
  • 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.

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 three-phase motors for pumps, fans, conveyors, and other factory process equipment.
  • Eutectic alloys for soldering, both traditional alloys composed of lead and tin, sometimes with additional silver or gold — especially SnPb and SnPbAg alloy formula for electronics - and newer lead-free soldering alloys, in particular ones composed of tin, silver, and copper such as SnAg.
  • Casting alloys, such as aluminium-silicon and cast iron
  • Silicon chips are eutectic bonded to gold-plated substrates through a silicon-gold eutectic by the application of ultrasonic energy 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 metal and Field's metal for fire sprinklers
  • Non-toxic mercury replacements, such as galinstan
  • Experimental glassy metals, with extremely high strength and corrosion resistance
  • Eutectic alloys of sodium and potassium that are liquid at room temperature and used as coolant in experimental fast neutron nuclear reactors.

    Others

  • Salts and water form eutectic systems. Magnesium perchlorate has a eutectic point of. Sodium chloride and water form a eutectic mixture whose eutectic point is −21.2 °C and 23.3% salt by mass. 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.
  • 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. To reduce the eutectic melting point in the solar molten salts, calcium nitrate is used in the following proportion: 42% Ca2, 43% KNO3, and 15% NaNO3.
  • Lidocaine and prilocaine—both are solids at room temperature—form a eutectic that is an oil with a melting point that is used in eutectic mixture of local anesthetic preparations.
  • 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.
  • Minerals may form eutectic mixtures in igneous rocks, giving rise to characteristic intergrowth textures exhibited, for example, by granophyre.
  • Some inks are eutectic mixtures, allowing inkjet printers to operate at lower temperatures.
  • 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.

    Strengthening mechanisms

Alloys

The primary strengthening mechanism of the eutectic structure in metals is composite strengthening. This deformation mechanism works through load transfer between the two constituent phases where the more compliant phase transfers stress to the stiffer phase. 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. 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. 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:
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 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.

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 and 0.76 wt% carbon.