Review of alloys developed using the entropy approach

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 properties of solid solutions with BCC and FCC lattices [27]. The averaged electron concentration (e/at.) was found to be the main factor determining the phase formation in equiatomic high-entropy alloys. The authors of the study formulated the conditions for the formation of high-entropy chemical compounds (Laves phase, s -phase, m -phase). It was noted that a 100% s -phase forms in those alloys in which this phase is formed for all pairs of its constituent elements. The second prerequisite is the value of the electron concentration in the range from 6.7 to 7.3 e/at. The 100% high-entropy Laves phase, according to the aforementioned study, arises when the total negative enthalpy of mixing of the alloys is equal to or less than -7 kJ/mol. In this case, the alloy should contain two elements with the enthalpy of mixing less than -30 kJ/mol, and the average electron concentration should be in the range from 6 to 7 e/at. It is noted that the nuclei of a solid phase in a high-entropy alloy are formed on the basis of the most refractory element [27]. In study of Firstov et al., the relationship between the electron concentration and the type of crystal lattice of a solid solution was analyzed for several HEAs [23]. It was noted that at a concentration of up to 4 e/at. a ductile HCP-based solid solution is formed. The concentration range from 4.25 to 7.2 e/at. corresponds to one or several types of BCC-based solid solutions. Two-phase solid solutions with BCC and FCC structures are formed in the electron concentration range of 7.2-8.3 e/at. Alloys with an FCC lattice correspond to an electron concentration above 8.4 e/at. Such alloys possess high ductility. The results of Firstov et al. indicate that brittle HEAs have mainly a BCC structure and correspond to the electronic concentration range of ca. 5.5-7.5 e/at. [23]. There exist ambiguity of opinions about the role of various factors on the formation of high-entropy alloys and their structural state. Based on the analysis of atomic radii, valence values, electronegativity, types of crystal structures of single-element metal components, and enthalpy, Rogachev concluded that the main criterion for the formation of high-entropy alloys is the proximity of the sizes of its constituent atoms [17]. The role of other factors, in his opinion, is less significant, which is consistent with the conclusions made earlier in a number of other works. Thereby, numerous attempts to identify the factors that determine the phase composition and the structure of high-entropy alloys have not led to the formulation of reliable, well-founded conclusions. Using the criteria discussed in the literature, it is not possible to accurately predict the structure of newly developed HEAs and the degree of their stability under thermal and thermoplastic impact. Most of the conclusions on the HEAs structure are based on the experimental studies. For instance, using the CoCrFeNi system it was shown that the structure of alloys obtained by adding manganese, aluminum, or vanadium to this alloy is significantly different. For example, the addition of aluminum into this alloy leads to the formation of a multiphase structure [28]. Under certain conditions, the CoCrFeNiV system is also characterized by the presence of several phases [29]. It should be emphasized that the information about the structure of the currently developed HEAs is constantly updated and supplemented with new data. In review [17], Rogachev notes that bulk amorphous alloys (bulk amorphous alloys, metallic glasses) were predecessor of high-entropy alloys, because they also contain several components. Both these materials have fundamental difference in the degree of their structural stability. Metallic glass is a metastable phase, the atoms of which do not have enough time to rearrange and form a crystalline structure during solidification of melt. The metastability of the amorphous phase becomes obvious when the material is heated. At certain temperature and holding time the atoms are rearranged and form a crystalline structure. In single-phase HEAs, dissimilar atoms occupy random positions in the crystal lattice and thus form a disordered solid substitution solution (HCP, BCC, or FCC). Considering the degree of stability, the HEAs having significantly distorted latices due to the proximity of atoms of different sizes, occupy an intermediate position between metallic glasses and stable phases, which have a low density of defects [17]. Currently, the multicomponent high-entropy alloys having a complex multiphase structure are actively studied [30]. It has been experimentally established that more than six phases can be formed in the CrFeNiCoAlCu alloy [31], some of which are nanoscale. These group of HEAs also includes alloys containing an amorphous phase [32], as well as mixtures of intermetallic phases [33].