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High-entropy alloy
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High-entropy alloy
High-entropy alloys (HEAs) are alloys that are formed by mixing equal or relatively large proportions of (usually) five or more elements. Prior to the synthesis of these substances, typical metal alloys comprised one or two major components with smaller amounts of other elements. For example, additional elements can be added to iron to improve its properties, thereby creating an iron-based alloy, but typically in fairly low proportions, such as the proportions of carbon, manganese, and others in various steels. Hence, high-entropy alloys are a novel class of materials. The term "high-entropy alloys" was coined by Taiwanese scientist Jien-Wei Yeh because the entropy increase of mixing is substantially higher when there is a larger number of elements in the mix, and their proportions are more nearly equal. Some alternative names, such as multi-component alloys, compositionally complex alloys and multi-principal-element alloys are also suggested by other researchers. Compositionally complex alloys (CCAs) are an up-and-coming group of materials due to their unique mechanical properties. They have high strength and toughness, the ability to operate at higher temperatures than current alloys, and have superior ductility. Material ductility is important because it quantifies the permanent deformation a material can withstand before failure, a key consideration in designing safe and reliable materials. Due to their enhanced properties, CCAs show promise in extreme environments. An extreme environment presents significant challenges for a material to perform to its intended use within designated safety limits. CCAs can be used in several applications such as aerospace propulsion systems, land-based gas turbines, heat exchangers, and the chemical process industry.
These alloys are currently the focus of significant attention in materials science and engineering because they have potentially desirable properties.
Furthermore, research indicates that some HEAs have considerably better strength-to-weight ratios, with a higher degree of fracture resistance, tensile strength, and corrosion and oxidation resistance than conventional alloys. Although HEAs have been studied since the 1980s, research substantially accelerated in the 2010s.
Although HEAs were considered from a theoretical standpoint as early as 1981 and 1996, and throughout the 1980s, in 1995 Taiwanese scientist Jien-Wei Yeh came up with his idea for ways of actually creating high-entropy alloys, while driving through the Hsinchu, Taiwan, countryside. Soon after, he decided to begin creating these special alloys in his lab, being in the only region researching these alloys for over a decade. Most countries in Europe, the United States, and other parts of the world lagged behind in the development of HEAs. Significant research interest from other countries did not develop until after 2004 when Yeh and his team of scientists built the world's first high-entropy alloys to withstand extremely high temperatures and pressures. Potential applications include use in state-of-the-art race cars, spacecraft, submarines, nuclear reactors, jet aircraft, nuclear weapons, long range hypersonic missiles, and so on.
A few months later, after the publication of Yeh's paper, another independent paper on high-entropy alloys was published by a team from the United Kingdom composed of Brian Cantor, I. T. H. Chang, P. Knight, and A. J. B. Vincent. Yeh was also the first to coin the term "high-entropy alloy" when he attributed the high configurational entropy as the mechanism stabilizing the solid solution phase. Cantor did the first work in the field in the late 1970s and early 1980s, though he did not publish until 2004. Unaware of Yeh's work, he did not describe his new materials as "high-entropy" alloys, preferring the term "multicomponent alloys". The base alloy he developed, equiatomic CrMnFeCoNi, has been the subject of considerable work in the field, and is known as the "Cantor alloy", with similar derivatives known as Cantor alloys. It was one of the first HEAs to be reported to form a single-phase FCC (face-centred cubic crystal structure) solid solution.
Before the classification of high-entropy alloys and multi-component systems as a separate class of materials, nuclear scientists had already studied a system that can now be classified as a high-entropy alloy: within nuclear fuels Mo-Pd-Rh-Ru-Tc particles form at grain boundaries and at fission gas bubbles. Understanding the behavior of these "five-metal particles" was of specific interest to the medical industry because Tc-99m is an important medical imaging isotope.
There is no universally agreed-upon definition of a HEA. The originally defined HEAs as alloys containing at least 5 elements with concentrations between 5 and 35 atomic percent. Later research, however, suggested that this definition could be expanded. Otto et al. suggested that only alloys that form a solid solution with no intermetallic phases should be considered true high-entropy alloys, because the formation of ordered phases decreases the entropy of the system. Some authors have described four-component alloys as high-entropy alloys while others have suggested that alloys meeting the other requirements of HEAs, but with only 2–4 elements or a mixing entropy between R and 1.5R should be considered "medium-entropy" alloys.
Due to their multi-component composition, HEAs exhibit different basic effects than other traditional alloys that are based only on one or two elements. Those different effects are called "the four core effects of HEAs" and are behind a lot of the particular microstructure and properties of HEAs. The four core effects are high entropy, severe lattice distortion, sluggish diffusion, and cocktail effects.
The high entropy effect is the most important effect because it can enhance the formation of solid solutions and makes the microstructure much simpler than expected. Prior knowledge expected multi component alloys to have many different interactions among elements and thus form many different kinds of binary, ternary, and quaternary compounds and/or segregated phases. Thus, such alloys would possess complicated structures, brittle by nature. This expectation in fact neglects the effect of high entropy. Indeed, according to the second law of thermodynamics, the state having the lowest mixing Gibbs free energy among all possible states would be the equilibrium state. Elemental phases based on one major element have small enthalpy of mixing () and a small entropy of mixing (), and compound phases have large but small ; on the other hand, solid-solution phases containing multiple elements have medium and high . As a result, solid-solution phases become highly competitive for equilibrium state and more stable especially at high temperatures.
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High-entropy alloy
High-entropy alloys (HEAs) are alloys that are formed by mixing equal or relatively large proportions of (usually) five or more elements. Prior to the synthesis of these substances, typical metal alloys comprised one or two major components with smaller amounts of other elements. For example, additional elements can be added to iron to improve its properties, thereby creating an iron-based alloy, but typically in fairly low proportions, such as the proportions of carbon, manganese, and others in various steels. Hence, high-entropy alloys are a novel class of materials. The term "high-entropy alloys" was coined by Taiwanese scientist Jien-Wei Yeh because the entropy increase of mixing is substantially higher when there is a larger number of elements in the mix, and their proportions are more nearly equal. Some alternative names, such as multi-component alloys, compositionally complex alloys and multi-principal-element alloys are also suggested by other researchers. Compositionally complex alloys (CCAs) are an up-and-coming group of materials due to their unique mechanical properties. They have high strength and toughness, the ability to operate at higher temperatures than current alloys, and have superior ductility. Material ductility is important because it quantifies the permanent deformation a material can withstand before failure, a key consideration in designing safe and reliable materials. Due to their enhanced properties, CCAs show promise in extreme environments. An extreme environment presents significant challenges for a material to perform to its intended use within designated safety limits. CCAs can be used in several applications such as aerospace propulsion systems, land-based gas turbines, heat exchangers, and the chemical process industry.
These alloys are currently the focus of significant attention in materials science and engineering because they have potentially desirable properties.
Furthermore, research indicates that some HEAs have considerably better strength-to-weight ratios, with a higher degree of fracture resistance, tensile strength, and corrosion and oxidation resistance than conventional alloys. Although HEAs have been studied since the 1980s, research substantially accelerated in the 2010s.
Although HEAs were considered from a theoretical standpoint as early as 1981 and 1996, and throughout the 1980s, in 1995 Taiwanese scientist Jien-Wei Yeh came up with his idea for ways of actually creating high-entropy alloys, while driving through the Hsinchu, Taiwan, countryside. Soon after, he decided to begin creating these special alloys in his lab, being in the only region researching these alloys for over a decade. Most countries in Europe, the United States, and other parts of the world lagged behind in the development of HEAs. Significant research interest from other countries did not develop until after 2004 when Yeh and his team of scientists built the world's first high-entropy alloys to withstand extremely high temperatures and pressures. Potential applications include use in state-of-the-art race cars, spacecraft, submarines, nuclear reactors, jet aircraft, nuclear weapons, long range hypersonic missiles, and so on.
A few months later, after the publication of Yeh's paper, another independent paper on high-entropy alloys was published by a team from the United Kingdom composed of Brian Cantor, I. T. H. Chang, P. Knight, and A. J. B. Vincent. Yeh was also the first to coin the term "high-entropy alloy" when he attributed the high configurational entropy as the mechanism stabilizing the solid solution phase. Cantor did the first work in the field in the late 1970s and early 1980s, though he did not publish until 2004. Unaware of Yeh's work, he did not describe his new materials as "high-entropy" alloys, preferring the term "multicomponent alloys". The base alloy he developed, equiatomic CrMnFeCoNi, has been the subject of considerable work in the field, and is known as the "Cantor alloy", with similar derivatives known as Cantor alloys. It was one of the first HEAs to be reported to form a single-phase FCC (face-centred cubic crystal structure) solid solution.
Before the classification of high-entropy alloys and multi-component systems as a separate class of materials, nuclear scientists had already studied a system that can now be classified as a high-entropy alloy: within nuclear fuels Mo-Pd-Rh-Ru-Tc particles form at grain boundaries and at fission gas bubbles. Understanding the behavior of these "five-metal particles" was of specific interest to the medical industry because Tc-99m is an important medical imaging isotope.
There is no universally agreed-upon definition of a HEA. The originally defined HEAs as alloys containing at least 5 elements with concentrations between 5 and 35 atomic percent. Later research, however, suggested that this definition could be expanded. Otto et al. suggested that only alloys that form a solid solution with no intermetallic phases should be considered true high-entropy alloys, because the formation of ordered phases decreases the entropy of the system. Some authors have described four-component alloys as high-entropy alloys while others have suggested that alloys meeting the other requirements of HEAs, but with only 2–4 elements or a mixing entropy between R and 1.5R should be considered "medium-entropy" alloys.
Due to their multi-component composition, HEAs exhibit different basic effects than other traditional alloys that are based only on one or two elements. Those different effects are called "the four core effects of HEAs" and are behind a lot of the particular microstructure and properties of HEAs. The four core effects are high entropy, severe lattice distortion, sluggish diffusion, and cocktail effects.
The high entropy effect is the most important effect because it can enhance the formation of solid solutions and makes the microstructure much simpler than expected. Prior knowledge expected multi component alloys to have many different interactions among elements and thus form many different kinds of binary, ternary, and quaternary compounds and/or segregated phases. Thus, such alloys would possess complicated structures, brittle by nature. This expectation in fact neglects the effect of high entropy. Indeed, according to the second law of thermodynamics, the state having the lowest mixing Gibbs free energy among all possible states would be the equilibrium state. Elemental phases based on one major element have small enthalpy of mixing () and a small entropy of mixing (), and compound phases have large but small ; on the other hand, solid-solution phases containing multiple elements have medium and high . As a result, solid-solution phases become highly competitive for equilibrium state and more stable especially at high temperatures.
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