Review of alloys developed using the entropy approach

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 and its components is an important factor determining the choice of the method for HEA fabrication. For the production of refractory HEAs, it is rational to use the technology of electric arc melting. In order to form an optimal structure and improve the mechanical properties, high-entropy alloys obtained by casting are usually subjected to thermoplastic processing – cold and hot rolling [54, 57, 58], forging [7], and drawing [59]. As a final technological operation, homogenizing annealing or other methods of heat treatment are usually used to promote the formation of an equilibrium structure [54, 59, 60]. Mechanical alloying of pure powder components in planetary ball mills is also one of the commonly used methods for fabrication of HEAs [26, 61-67]. The mixtures of elementary powders are deformed in high-energy ball mills, which provides their interaction. The processing time is ca. 10 - 20 hours. For example, CoFeNiXY alloys (where X is Cr and Si, and Y is Mn, Al, Ti or Zn) were obtained by mechanical alloying. Hot isostatic pressing is a method of compaction that improves the quality of the HEAs structure [68]. Wear debris left by steel balls during mechanical alloying of powder mixtures containing iron do not affect the quality of the resulting HEAs. In other cases, one can expect that iron will appear in the composition of HEA when it is produced by ball milling. A typical peculiarity of mechanical alloying of highly ductile material is its sticking to the surface of deforming balls, which ultimately affects the quality of the HEA. The powder materials obtained by mechanical alloying require subsequent consolidation. One of the most effective approaches to address this issue is spark plasma sintering [64-66, 69]. In a number of studies, multicomponent high-entropy alloys were obtained using self-propagating high-temperature synthesis (SHS) technologies [70, 71]. High-entropy alloys in the formof thin films andmultilayer coatings are obtained bymagnetron sputtering [9, 11, 17, 43, 72]. A thin-layer HEAs can be produced by spraying a single target, which includes several components. The second approach is based on the simultaneous magnetron sputtering of several targets [73]. High-entropy alloys in the form of nitrides [74], carbides [40] and other compounds are obtained by magnetron sputtering. Alternating targets of different compositions, nanostructures consisting of layers of HEAs and pure metals were also formed by magnetron sputtering [75]. In recent studies the HEAs in the form of metallic glasses attract a lot of attention [17, 76, 77]. The fabrication of glassy HEAs is done by melt spinning. The metal glass thus obtained as an amorphous strip, which is cooled at high rate on a rotating copper cylinder. One of the technologies of rapid solidification of HEAs is based on splat cooling, which consists in the collision of a drop of melt with the inner surface of a copper cylinder rotating at high speed [78]. In[79], thepossibilityofobtainingheat-resistant compositesbasedonFeCoNiMnCr andFeCoNi 2 MnCrCu high-entropy alloys was studied. Layered composites were formed by diffusion welding of aluminum foils and thin layers of high-entropy alloys. The HEAs ingots obtained in a vacuum arc furnace were deformed by rolling to a thickness of 0.4 mm on two-roll mills at room temperature with a degree of ~ 15 % in one pass [79, 80]. Layered billets were welded by the diffusion method in two stages. At the first stage with a duration of 2 hours, the temperature was 600 °C, at the second stage the temperature was 950 °C. Structure of high-entropy alloys The most important parts of studies on HEAs s are related to characterization of their structure, since it determines the properties and the possible applications of the materials. The structure of high-entropy alloys obtained by melt crystallization is largely determined by the cooling rate. According to the data presented in [17], the crystallization processes of HEAs and traditional alloys are identical. Under conditions of rapid cooling of materials, a fine-grained structure is formed. In cases where the melt cools at a low rate of tens of degrees per second, a dendritic-type structure is formed [81]. With rapid cooling of the melt, the tendency to form dendrites disappears. At the beginning of HEAs development it was believed that the high entropy of mixing, being a factor preventing the formation of ordered phases and intermetallic compounds, promotes the formation of disordered substitutional solid solutions. From this point of view, one could expect that the structure of