GoKawiilAlloys with high proportions of several metals
Atomic structure model of fcc CoCrFeMnNi [ 1 ]
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.[2] Hence, high-entropy alloys are a novel class of materials.[1][2] The term "high-entropy alloys" was coined by Taiwanese scientist Jien-Wei Yeh[3] 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.[4] Some alternative names, such as multi-component alloys, compositionally complex alloys and multi-principal-element alloys are also suggested by other researchers.[5][6] 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.[2] 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.[7][8][9] Although HEAs have been studied since the 1980s, research substantially accelerated in the 2010s.[2][6][10][11][12][13][14]
Development [ edit ]
Early research in high entropy alloys began in the late 1970s and early 1980s by Brian Cantor. Previously recognized as multi-principal element alloys, the earliest works included research on rapidly solidified AI-Cu-Li-Mg-Zr alloys[15] and oxidation behavior of CO58NI10FE5SI11B16.[16] 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.[17] It was one of the first HEAs to be reported to form a single-phase FCC (face-centred cubic crystal structure) solid solution.[18] Today, potential applications include use in state-of-the-art race cars, spacecraft, submarines, nuclear reactors,[19] jet aircraft, nuclear weapons, long range hypersonic missiles, and so on.[20][21]
Two foundational works were produced in 2004 and 2005 that defined the classification of high entropy alloys. One was produced by Brian Cantor, "Microstructural development in equiatomic multicomponent alloys",[22] and "Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes"[23] by Taiwanese scientist Jien-Wei Yeh. The term, 'high entropy alloy', was coined by Jien-Wei Yeh in 2004 as his first work concluded high configurational entropy was the mechanism stabilizing the solid solution phase.[23]
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.[24] 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 [ edit ]
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.[23] 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.[25] Some authors have described four-component alloys as high-entropy alloys[26] while others have suggested that alloys meeting the other requirements of HEAs, but with only 2–4 elements[27] or a mixing entropy between R and 1.5R[28] should be considered "medium-entropy" alloys.[27]
... continue reading