Review of alloys developed using the entropy approach

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 Composition of high-entropy alloys There are ca. 40 elements in various families of high-entropy alloys. The combination of five to twenty components in alloys makes it possible to obtain a huge number of high-entropy alloys [10, 34]. It is noted that only about five hundred materials related to the HEA family have actually been studied [11, 17]. The strategy for selection of HEA’s composition depends on many factors, the most important of which are related to the operating conditions of the material [26]. Alloys based on 3-d transition elements are the most studied HEAs. Alloys of this group contain at least 4 of the follwing 9 elements: Fe, Co, Cr, Ni, Mn, Al, Ti, Cu, V [2, 3, 15, 35]. Interest in the materials of this family was due to its promising properties such as high hardness, positive temperature hardening coefficient, high wear and corrosion resistance [2, 5, 18, 30, 36, 37]. Awell-known and frequently discussed five-component HEA is the CoCrFeMnNi alloy, which was proposed to the scientific community in 2004 [3]. In the literature, this single-phase material with the structure of a disordered substitution solid solution is sometimes referred to as a “Cantor alloy”. One of the most studied materials belonging to the class of high- entropy alloys isAlCoCrCuFeNi [2, 7, 30, 38, 39]. In addition to the 3-d transition elements, the composition of polymetallic HEAs includes other elements, for example, Zr: AlTiVFeNiZr, AlTiVFeNiZrCoCr, CuTiVFeNiZrCoCr, MoTiVFeNiZrCo, MoTiVFeNiZrCoCr, CuTiVFeNiZrCo, etc. [1]. A large amount of research is related to alloys based on refractory metals (Hf, Ta, Mo, Nb, V, W, Cr, Zr, Ti). The third group of HEAs, which is given special attention by specialists, includes light alloys (Al, Sn, Be, Li, Mg, Ti, Sc, Si, Zn). All these multicomponent materials include aluminum and titanium [17]. The fourth HEAs family is based on 4f transition metals (rare earth elements Gd, Dy, Lu, Tm, Tb, Y). There are also the fifth (Cu, Al, Mn, Ni, Zn, Sn) and sixth (Au, Ag, Cu, Co, Cr, Ni, Pt, Pd, Ru, Rh) families of HEAs. It can also include such elements as Fe, Co, Ni, Si, B, Zr, Hf, Cu, Be, Mo, Cr. There is no doubt that the HEAS nomenclature will expand. High-entropy metal glasses are considered as an independent family of HEAs. Besides, in recent years, the attention of specialists has been paid to the development of high-entropy metal-like compounds [11, 17]. These include ceramic materials in the form of carbides [40], oxides [41], borides [42], and nitrides [43]. Examples of compounds of this kind are (TiZrNbHfTa)C, (AlCrTaTiZr)O x , (T i0.2 Hf 0.2 Zr 0.2 Nb 0.2 Ta 0.2 ) B 2 , (AlCrTaTiZr)N x . Speaking about oxide materials [44], we note simple oxide systems formed by divalent and trivalent metals, as well as high-entropy systems with the structure of perovskite [45] and spinel [46]. The literature provides examples of compositions in which the matrix materials are HEAs obtained by mechanical fusion of pure components (CoCrFeNiMn, CoCrFeNiV, CoCrFeNiCu, CoCrFeNiAl), and the hardening is provided by ceramic particles (WC, TiB 2 , Ti(C, N)) [17, 47–49]. Considering the main components, HEAs are sometimes conditionally divided into several groups [17]. For example, there are high-entropy steels, heat-resistant alloys, light alloys, rare-earth alloys, bronzes and brasses, and catalysts. It is emphasized, however, that this type of classification is not generally accepted. Methods for producing high-entropy alloys Various technical solutions were proposed to obtain alloys having high entropy of mixing, namely, melting, powder metallurgy (mechanical alloying of powders), welding, melt spinning, splat-cooling, self- propagating high-temperature synthesis, magnetron sputtering, cladding of powder mixtures on a metal base. In most cases, high-entropy alloys are obtained by melting the materials followed by crystallization [17]. Methods of electric arc melting in vacuum [2, 5, 6, 50-52], argon-arc [53] and induction [3, 54] melting, melting in resistance furnaces can also be used for HEAs fabrication. One of the most common methods for producing HEAs is vacuum arc melting [55, 56]. The initial material is a mixture of metals with a purity of more than 99 %. The alloys are smelted in an inert gas atmosphere, which prevents the material’s oxidation. In order to increase the homogeneity of ingots, it is remelted several times. The melting point of the material