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A eutectic system is a homogeneous mixture of two or more components that, in the solid state, form a single super lattice whose main characteristic is that it has a lower melting point than that of the individual components. Most eutectic systems are binary systems (formed by only two phases or components), although there are examples of certain alloys forming ternary eutectic systems.
The word eutectic comes from the ancient Greek term eutektos , which is a combination of the terms eu , meaning “well,” and teko , meaning to melt. Therefore, eutectic literally means “well melting”, referring to the fact that eutectics are easier to melt than their individual components due to their lower melting point.
How are eutectic systems formed?
A eutectic system is formed only when the components or solid phases that make up the mixture are in a specific proportion called the eutectic composition. This composition is characteristic of each eutectic system. Furthermore, eutectics generally form between compounds that are similar or chemically related to one another. Such is the case of some eutectic alloys formed by two or more metals.
By heating and melting a heterogeneous mixture of these two phases in the appropriate proportion, a homogeneous liquid mixture is formed which, when cooled, crystallises again, forming a new crystalline structure in which both substances are part of the same cell or lattice. This is the so-called super lattice or super cell, which is repeated in all directions to create a completely homogeneous crystal in which neither of the two original phases can be distinguished. In other words, the phases of the system co-crystallize to form a new solid.
types of eutectics
Eutectic systems can be classified in different ways. Two common forms are according to its composition and according to the crystallinity of the solid.
Depending on the composition, eutectics can be classified as:
- Inorganic eutectics: are those formed by inorganic compounds such as metals and salts. In the latter case, they are generally hydrated salts. These are the most common eutectic systems.
- Organic Eutectics: Many organic compounds form eutectics with each other. In this case they are called organic eutectics.
- Organic / inorganic eutectics: are those formed by an organic and an inorganic phase, such as the mixture of water and ethanol.
In addition to this classification, we can distinguish three classes of eutectics depending on the crystallinity of the solid, that is, depending on its microstructure. In general terms, this microstructure can be of two types: faceted and non-faceted. They are also often called, respectively, glassy or amorphous microstructure. In binary systems, three different combinations of this type of microstructures can occur, giving rise to three different classes of eutectics:
- Non-faceted eutectics – non-faceted (NN): These are the most common and consist of a non-faceted or amorphous phase embedded in another amorphous phase. These eutectics show a very regular microstructure.
- Faceted – non-faceted (NF) eutectics: In these eutectics, one of the phases is amorphous or non-faceted while the other is faceted. The microstructure of these eutectics is usually between regular and complex, or it can even become completely irregular, depending on the particular characteristics of each phase.
- Faceted Eutectics – Faceted (FF): FF eutectics are rare and usually form between two intermetallic compounds. These eutectics often possess unique mechanical properties such as high hardness by forming long-range crystal structures with strong metallic bonds.
Examples of eutectic systems
Aluminum-silicon alloy
Aluminum and silicon form an inorganic eutectic alloy of the FN (faceted – non-faceted) type when the mixture contains 13% silicon by mass. In this system, aluminum forms the amorphous phase (called the alpha phase), while silicon forms the crystalline or faceted phase of the system. This alloy is of great importance for the manufacture of cast aluminum parts.
Iron-carbon alloy (carbon steel)
Carbon steel is a eutectic system known for hundreds of years. It consists of an iron matrix with carbon atoms embedded in the structure. These elements form a eutectic system with a composition of 4.30% carbon and the rest iron. The melting point of the system (the eutectic temperature) is 1,147 °C and it consists of a mixture of γ-austenite with iron carbide or cementite. Cementite is present in crystalline form embedded in an amorphous austenite matrix, making this eutectic system another example of FN systems.
lead-tin alloy
The eutectic system formed between lead and tin is a system that contains 62% tin by mass. This mixture melts at just 183°C, which is 50°C below the melting point of tin, which is 232°C, and almost 205°C below the melting point of pure lead, which is 327.5 °C
Camphor-naphthalene alloy
Naphthalene and camphor are both aromatic organic compounds that form a eutectic system. Therefore, this is an example of an organic eutectic system. A system similar to this is formed between naphthalene and benzene.
Galinstan
This is an example of a ternary eutectic system. It consists of an alloy containing 68.5% gallium, 21.5% indium and 10% tin. The melting point of this system is just -19 °C, so the mixture is liquid at room temperature. This fact makes galinstan a non-toxic substitute for mercury.
Nickel-silicon alloy
The nickel-silicon eutectic system is an example of an FF eutectic, that is, one in which both phases are in a crystalline state, forming faceted solids embedded within each other. The eutectic composition is 84% nickel and 16% silicon. This system is characterized by being extremely hard, resistant to fatigue and wear due to adhesion.
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