High entropy alloys

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 the like in various steels. Hence, high entropy alloys are a novel class of materials. The term “high-entropy alloys” was coined 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.Although HEAs were considered from a theoretical standpoint as early as 1981 and 1996, and throughout the 1980s, in 1995 Jien-Wei Yeh came up with his idea for ways of actually creating high-entropy alloys in 1995, while driving through the Hsinchu, Taiwan, countryside. Soon after he decided to begin creating these special metal alloys in his lab. With Taiwan being the only country researching these alloys for over a decade, most other 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 Jien-Wei Yeh and his team of Taiwanese scientists invented and built the world's first high-entropy alloys that can 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.There is no universally agreed-upon definition of a HEA. Yeh 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, as the formation of ordered phases decreases the entropy of the system. Some authors have described 4-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.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 (4+ components) systems that cannot be easily represented on a 2-dimensional phase diagram, virtually nothing is known.High-entropy alloys are difficult to manufacture using extant techniques as of 2018, and typically require both expensive materials and specialty processing techniques.The atomic-scale complexity presents additional challenges to computational modelling of high-entropy alloys. Thermodynamic modelling using the CALPHAD method requires extrapolating from binary and ternary systems. Most commercial thermodynamic databases are designed for, and may only be valid for, alloys consisting primarily of a single element. Thus, they require experimental verification or additional ab initio calculations such as density functional theory (DFT). However, DFT modeling of complex, random alloys has its own challenges, as the method requires defining a fixed-size cell, which can introduce non-random periodicity. This is commonly overcome using the method of 'special quasirandom structures,' designed to most closely approximate the radial distribution function of a random system, combined with the Vienna Ab-initio Simulation Package. Using this method, it has been shown that results of a 4-component equiatomic alloy begins to converge with a cell as small as 24 atoms. The exact muffin-tin orbital method with the coherent potential approximation has also been employed to model HEAs. Other techniques include the 'multiple randomly populated supercell' approach, which better describes the random population of a true solid solution (although is far more computationally demanding). This method has also been used to model glassy/amorphous (including bulk metallic glasses) systems without a crystal lattice.The crystal structure of HEAs has been found to be the dominant factor in determining the mechanical properties. bcc HEAs typically have high yield strength and low ductility and vice versa for fcc HEAs. Some alloys have been particularly noted for their exceptional mechanical properties. A refractory alloy, VNbMoTaW maintains a high yield strength (>600 MPa (87 ksi)) even at a temperature of 1,400 °C (2,550 °F), significantly outperforming conventional superalloys such as Inconel 718. However, room temperature ductility is poor, less is known about other important high temperature properties such as creep resistance, and the density of the alloy is higher than conventional nickel-based superalloys.