High-entropy alloy
High-entropy alloys are alloys that are formed by mixing equal or relatively large proportions of 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 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.
Development
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 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.
Definition
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.The four core effects of HEAs
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.High entropy effect
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. However, recent experimental and theory work have revealed the flaw of this framework. The stability of the solid-solution phase should be considered by the change in the Gibbs free energy if a small amount of a new compound phase form from the solid-solution phase. If the formation of the new compound phase can decrease the Gibbs free energy, the solid solution phase is not stable. Compared with the high entropy effect, this new paradigm considers the multiphase microstructure and explains the experimental observation that the single phase high-entropy alloys are actually rare.Severe lattice distortion effect
Because solid solution phases with multi-principal elements are usually found in HEAs, the conventional crystal structure concept is thus extended from a one or two element basis to a multi-element basis. Every atom is surrounded by different kinds of atoms and thus suffers lattice strain and stress mainly due to atomic size difference. Besides the atomic size difference, both different bonding energy and crystal structure tendency among constituent elements are also believed to cause even higher lattice distortion because non-symmetrical bindings and electronic structure exist between an atom and its first neighbours. This distortion is believed to be the source of some of the mechanical, thermal, electrical, optical, and chemical behaviour of HEAs. Thus, overall lattice distortion would be more severe than that in traditional alloys in which most matrix atoms have the same kind of atoms as their surroundings.Sluggish diffusion effect
As explained in the last section, an HEA mainly contains a random solid solution and/or an ordered solid solution. Their matrices could be regarded as whole-solute matrices. In HEAs, those whole-solute matrices' diffusion vacancies are surrounded by different element atoms, and thus have a specific lattice potential energy. This large fluctuation of LPE between lattice sites leads to low-LPE sites, serving as traps and hindering atomic diffusion. This leads to the sluggish diffusion effect.Cocktail effect
The cocktail effect is used to emphasise the enhancement of the alloy's properties by at least five major elements. Because HEAs might have one or more phases, the whole properties are from the overall contribution of the constituent phases. Besides, each phase is a solid solution and can be viewed as a composite with properties coming not only from the basic properties of the constituent, but by the mixture rule also from the interactions among all the constituents and from severe lattice distortion. The cocktail effect takes into account the effect from the atomic-scale multicomponent phases and from the multiple composite phases at the micro scale.Alloy design
In conventional alloy design, one primary element such as iron, copper, or aluminum is chosen for its properties. Then, small amounts of additional elements are added to improve or add properties. Even among binary alloy systems, there are few common cases of both elements being used in nearly-equal proportions such as Pb-Sn solders. Therefore, much is known from experimental results about phases near the edges of binary phase diagrams and the corners of ternary phase diagrams and much less is known about phases near the centers. In higher-order systems that cannot be easily represented on a two-dimensional phase diagram, virtually nothing is known.Early research of HEA was focussed on forming single-phased solid solution, which could maximize the major features of high entropy alloy: high entropy, sluggish diffusion, severe lattice distortion, and cocktail effects. It has been pointed out that most successful materials need some secondary phase to strengthen the material, and that any HEA used in application will have a multiphase microstructure. However, it is still important to form single-phased material since a single-phased sample is essential for understanding the underlying mechanism of HEAs and testing specific microstructures to find structures producing special properties.