Metal Intermetallics in Selected High Temperature Alloys
The high-temperature alloys used in aerospace and many other industries are not simply composed of a single type of metal but of a complex alloy system. These systems may vary considerably with respect to their composition and microstructure in order to provide the desired properties of the end-product. It is well known that one of the most important constituents in high-temperature alloys is the intermetallic phase which is typically composed of the alloy’s elements together in a specific atomic arrangement.
Intermetallics are versatile materials that have a wide range of mechanical and physical properties, such as high strength, hardness, wear resistance and oxidation resistance at high temperatures. In some cases, these materials may even be superplastic (able to form thin sheets). This unique combination of properties is what gives these materials their important role in high temperature alloys and makes them attractive for many applications.
The general classification of intermetallics can be divided into three main categories: ordered intermetallic phases, disordered intermetallic phases and solid-solution intermetallics. The ordered phases in high-temperature alloys typically refer to concentrated solid solutions of the alloy’s elements, while the disordered intermetallics are further divided into intermetallic compounds and amorphous intermetallics. The ordered intermetallic phases are generally characterized by their high strength, ductility and corrosion resistance, while the disordered intermetallics are usually highly brittle and prone to corrosion. The solid-solution intermetallics are comprised of a mix of the ordered and disordered phases, providing a balance of the properties of each.
Intermetallics have been studied extensively in recent years, and new understanding has been developed on their structure, composition, and properties. In the case of high temperature alloys, research has revealed that the intermetallic phase can play an important role in determining the performance of the alloy system. Specifically, some of the more widely studied intermetallics in high temperature alloys include gamma prime (Γ’), Sigma (Σ), Laves phase (L), and precipitate phase (P). Gamma prime (Γ’) is an ordered phase that predominantly consists of one element, such as nickel in an inverse hafnium-nickel system, while Sigma (Σ) is an intermetallic phase composed of two elements, such as the hafnium-iron-nickel system. The Laves phase is an ordered intermetallic that can contain multiple elements, such as a niobium-molybdenum-tungsten system, while the precipitate phase is a disordered intermetallic that generally contains two elements, such as a cobalt-titanium system.
These various intermetallic phases have a wide range of important physical and mechanical characteristics that influence the performance of a given high temperature alloy. For example, gamma prime (Γ’) typically provides a large amount of strength and hardness and is resistant to corrosion, while the Sigma (Σ) phases are generally less hard but can provide good oxidation resistance. The Laves phases typically possess high wear and oxidation resistance, while the precipitate phases can provide an increased strength and ductility. In addition to these characteristics, the presence of the various intermetallic phases can also influence the microstructure and performance of the alloy.
In conclusion, the intermetallic phases are the predominant constituent of high-temperature alloys and can be divided into ordered and disordered phases. Each of these intermetallics possess a unique combination of properties that enable them to significantly influence the properties and performance of the high temperature alloy. As the properties of these intermetallics are further studied and understood, it is expected that the use of high temperature alloys for aerospace and other applications will continue to grow and benefit from the superior performance of intermetallics.
