Review of alloys developed using the entropy approach

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 Properties of high-entropy alloys The main factor determining the interest of specialists in the developed materials is the nature of its behavior in various environmental conditions. The set of parameters recorded at the initial stages of HEAs development, as well as expectations based on theoretical ideas about the possible manifestations of multi- component systems, allow us to conclude that its use in various industries is promising. Given that the high- entropy alloys analyzed in the literature are a family of materials that differ significantly in composition, it is necessary to evaluate the complex of properties corresponding to its purpose and operating conditions in each specific case. The properties of HEAs are determined by its components, structural condition and production technology. The literature contains a wide range of ideas (sometimes contradictory) about the properties of high- entropy alloys. As important indicators of HEAs, its hardness, strength, and wear resistance [2, 7, 102, 103], increased ductility at low temperatures, corrosion resistance, thermal stability [17], resistance to ionizing radiation [22, 104] are noted. It should be emphasized that this type of characteristic is generalized and cannot be applied to all types of HEAs. For structural materials, the combination of strength properties, ductility, crack resistance, and a number of other characteristics that determine the behavior of alloys under severe external loading conditions is of particular importance. The unique properties of multicomponent HEAs are due to the manifestation of four effects [17, 9-13]. One of it is associated with the high entropy of the alloy, the second – with distortions of the crystal lattice, the third – with the delayed diffusion of the alloy components. The fourth was called the “cocktail effect”. The effect of high entropy, from which the name of the alloys analyzed in the work is derived, is deter- mined by the level of configuration entropy. In accordance with one of the classifications of the analyzed al - loys, low- entropy alloys include those with D Smix ≤ 0.69 R , where R is the universal gas constant. Medium entropy alloys are described by the ratio 0.69 R ≤ D Smix 1.61 R . The high-entropy alloys include ones with D Smix ≥ 1.61 R [4]. The search for compositions of stable high-entropy alloys is complicated by a number of circumstances [17]. One of it is that the applicability of the dependences that reliably describe the stabil- ity conditions of ideal solutions in relation to real solid solutions is not obvious [11]. The second of the main effects of high-entropy alloys is the effect of crystal lattice distortions. It is caused by the difference in the size of the atoms that make up the multicomponent system. The degree of distortion is minimal in the neighborhood of atoms that are close in size. In alloys consisting of atoms that differ significantly in size, larger voids – internodes – are formed. Embedded atoms can be located in these voids, forming a region of local stresses [17]. The sources of distortion are also large atoms located in the nodes of the crystal lattice and surrounded by smaller atoms, which position also corresponds to the nodes of the lattice. Distortions of the crystal lattice largely determine the level of the HEAs strength properties. Delayed diffusion is the third effect that determines the stability of the structure and the complex properties of high-entropy alloys. The favorable effect of the low diffusion rate is reflected in the increase in the thermal and chemical stability of the HEAs [105, 106]. Distortions of the crystal lattice and other features characteristic of HEAs are noted as factors that explain the inhibition of diffusion processes[4, 6, 37, 107]. The contradictions revealed in a number of studies allow us to conclude that the confirmation or refutation of the delayed diffusion effect in HEAs requires additional research [17]. The term “cocktail effect” implies that a complex system manifests a mixing effect that is not available to each of its components individually. In fact, this term is synonymous with such a concept as a synergistic effect. Out of the whole set of mechanical properties of HEAs, the specialists paid more attention to the strength indicators. At the same time, the question of the brittleness of high-entropy alloys, the degree of which is determined by the ductility, impact strength and crack resistance of materials, is of fundamental importance. It should be emphasized that the absence of embrittling phases is one of the distinctive requirements for HEAs, which determine its technological properties, in particular, deformability during cold pressure treatment. Taking into account these ideas, it is believed that the high entropy of mixing, preventing the formation of intermetallic phases in the HEAs and contributing to the formation of mainly disordered