Intermetallic Compound Definition
Intermetallic compounds are substances composed of two or more elemental metals in definite proportions rather than variable proportions in the case of solid solutions. The properties and the crystal structure of the intermetallic compound thus formed are different from its constituents.
Intermetallic compounds include electron compounds, size packing phases and zintl phases. Also, post-transition metals such as aluminium, galium etc and metalloids such as silicon, arsenic are included. However, a homogenous and heterogenous solid solution of metals and interstitial compounds are excluded under the definition of Intermetallic compounds.
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Intermetallic Alloy
Intermetallics, Intermetallic compound, Intermetallic alloy or ordered intermetallic alloy are substances created with an ordered mixture of two or more metallic elements. Intermetallics are generally hard and brittle with desirable mechanical properties.
Alloy - An alloy is an admixture of two or more metals that is to obtain a substance with the desirable properties of its components. Intermetallic materials are alloyed to obtain more desirable properties. Due to limited independent slip systems required for plastic deformation, intermetallics have cleavage or intergranular fracture modes. But intermetallic with ductile fracture modes are also present such as 15Al-40Ti. For other intermetallic alloys, boron is added to improve ductility.
Metallic alloys are possible replacements for plasmonic materials due to their large free electron densities. Gold and silver continue to be used as metallic plasmonic materials as intermetallics with more atoms in a unit cell suffer from higher optical losses. Due to the strong plasmonic performance by noble metals, their interband transitions can be shifted to another unimportant part of the spectrum by alloying two or more elements to create unique band structures. But optical properties usually get worse than get better in these cases but still alloys of noble metals have received significant attention.
Applications of Intermetallics
Intermetallic compounds are termed exotic materials or advanced materials owing to their superior mechanical, chemical, electrical, magnetic, optical and semiconducting properties. More than 25000 distinct intermetallics are known. Some are produced in large quantities for a wide range of applications while others are produced, although not in large quantities but still essential to many billion-dollar markets. The various industry application of intermetallic compounds are as follows:
Intermetallics have given rise to the development of various novel materials such as alnico and hydrogen storage materials used in nickel-metal hydride batteries.
Nickel-base superalloys and other titanium aluminides are also being used in turbine-blade applications.
Titanium aluminides are also being used in very small quantities for the grain refinement of titanium alloys.
In microelectronics, Silicides are being used as barrier and contact layer.
NiAl alloys have many desirable properties such as oxidation, carburization, nitridation resistance, high thermal and electrical conductivity which makes them an attractive choice for metallizations in advanced semiconductor, surface catalysts and high vacuum circuit breakers.
Conclusion
Intermetallics are substances made up of two or more metals with a distinctive crystal structure where the metallic elements are in a fixed abundance ratio. Deviations due to precise stoichiometry are common on both sides necessitating or introducing vacancies. Usually, metal-metalloid compounds are also added in this category as they resemble the metal-metal compound properties. Intermetallics are widely used compounds and have a wide range of applications in various industries.
FAQs on Intermetallic Compound
1. What is an intermetallic compound?
An intermetallic compound is a distinct chemical compound formed between two or more metallic elements. Unlike an alloy, which is a solid solution (a physical mixture), an intermetallic compound has a defined chemical formula and an ordered crystal lattice structure that is different from its constituent metals. This fixed stoichiometry and ordered arrangement give them unique properties.
2. What are the key properties of intermetallic compounds?
Intermetallic compounds exhibit a unique combination of properties that differ significantly from their parent metals. Key characteristics include:
High Melting Points: Their strong atomic bonding results in higher melting points compared to the constituent metals.
Hardness and Brittleness: They are typically very hard but brittle, especially at room temperature, due to their complex crystal structures.
Corrosion Resistance: Many intermetallics show excellent resistance to corrosion and oxidation at high temperatures.
Defined Stoichiometry: They form in fixed atomic ratios, such as Ni₃Al or Fe₃C.
Unique Physical Properties: They can possess desirable magnetic, superconducting, or thermoelectric properties not found in the original metals.
3. How do intermetallic compounds differ from alloys?
The primary difference lies in their atomic structure and composition. An alloy is a solid solution where atoms of different metals are randomly mixed within a crystal lattice, and their composition can vary over a wide range. In contrast, an intermetallic compound has a highly ordered, unique crystal structure with atoms in specific lattice sites and a fixed stoichiometric ratio. This makes an intermetallic a distinct chemical compound, whereas an alloy is more like a solid-state mixture.
4. What are some common examples of intermetallic compounds?
There are many important intermetallic compounds used in various fields. Some well-known examples include:
Nickel Aluminide (Ni₃Al): A key component in high-temperature superalloys for jet engines.
Titanium Aluminide (TiAl): Used in aerospace for its low density and high-temperature strength.
Cementite (Fe₃C): A hard and brittle compound found in steel that significantly influences its properties.
Copper-Tin compounds (Cu₆Sn₅, Cu₃Sn): Formed in solder joints, crucial for electronic device reliability.
Magnesium Silicide (Mg₂Si): Used to strengthen certain aluminium alloys.
5. What is the difference between stoichiometric and non-stoichiometric intermetallic compounds?
The distinction is based on whether they follow the law of definite proportions. Stoichiometric compounds (or Daltonides) have a fixed chemical composition represented by small whole numbers, such as Mg₂Ni. In contrast, non-stoichiometric compounds (or Berthollides) can exist over a narrow range of compositions. This is due to defects in their crystal lattice, meaning their chemical formula might have fractional subscripts, like Ni₀.₉₈O.
6. What type of chemical bonding is found in intermetallic compounds?
The bonding in intermetallic compounds is complex and not purely of one type. It is a hybrid of metallic, ionic, and covalent bonding. While the presence of metal atoms provides metallic character, the ordered structure and fixed ratios introduce significant ionic and covalent characteristics. This mixed bonding is responsible for their unique combination of properties, such as high melting points (covalent/ionic nature) and brittleness (directional bonding).
7. How are intermetallic compounds different from solid solutions?
An intermetallic compound and a solid solution are both solid-state mixtures of metals, but they differ fundamentally in structure. A solid solution involves one type of atom (solute) dissolving in the crystal lattice of another (solvent) without changing the basic crystal structure; the solute atoms are distributed randomly. An intermetallic compound is not a solution but a new phase with its own distinct and ordered crystal structure, which is different from either of the parent metals.
8. Why are intermetallic compounds important in industrial applications like solder joints?
In electronics, the reliability of a solder joint depends on the formation of a thin, uniform intermetallic layer (e.g., Cu₆Sn₅) at the interface between the solder and the copper pad. This layer creates a strong metallurgical bond, ensuring good mechanical and electrical connection. The controlled formation of this compound is critical; if the layer grows too thick or becomes irregular, it can lead to brittleness and premature failure of the electronic device.
9. How are intermetallic compounds identified on a phase diagram?
On a binary phase diagram, which maps the phases of a two-component system at different temperatures and compositions, intermetallic compounds are typically represented by narrow, vertical lines or very narrow single-phase regions. This indicates that the compound is stable only at or very near a specific stoichiometric composition. Often, these compounds show a congruent melting point, which appears as a distinct peak on the liquidus line of the diagram.
