Review of alloys developed using the entropy approach

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 HEAs is quite obvious. However, characterization of structure of multicomponent equiatomic alloys is as difficult tasks as characterization of structure of convenient metallic alloys. To understand the structure of HEAs it is also necessary to use a wide variety of characterization techniques. The most important are the methods of X-ray diffraction analysis, transmission and scanning electron microscopy. The characterization methods involved in a particular study depends on its goals, the composition of the material, the geometric parameters of the samples and other features of the analyzed HEAs. It is generally believed, that the properties of “classical” single-phase HEAs are related to random loca- tion of dissimilar atoms in the crystalline lattice. It is believed that the atoms located in a multicomponent system, which differs in size, electronic structure, and thermodynamic properties, lead to a significant dis - tortion of the crystal lattice of the solid solution, an increased efficiency of the solid-solution hardening and stabilization of the material properties [23]. This feature significantly distinguishes HEAs from convenient alloys [20]. In [17], using the example of a five-component equiatomic ABCDE alloy, it is graphically shown that in elementary cells of the BCC and FCC types, the long-range order for each type of atom is absent and the resulting phase is a completely disordered substitution solid solution. An equiatomic alloy of the ABCDE type can only be constructed from a set of elementary cells that differ in composition. CoCrFeNi and CoCrFeNiMn alloys are typical representatives of single-phase high-entropy alloys with the structure of a disordered substitution solid solution with an FCC structure. In the thesis of Shaisultanov [82], it was shown that when V or V and Mn are added to the CoCrFeNi system, a tetragonal s -phase is formed in the alloy structure along with the FCC phase. An even more complex structure is formed in an alloy containing, in addition to these four components, Al and Cu (CoCrFeNiAlCu). Four phases are observed in the structure of this alloy, including the disordered BCC phase (which predominant consists of chromium and iron), the ordered B2 phase ((which predominant consists of aluminum and nickel), the ordered L1 2 phase (enriched in copper), and the ordered L1 2 phase (enriched in cobalt, chromium, and iron). The most important characteristic that determines the interest of many specialists in high-entropy alloys is the stability of their structure, and hence stability of their properties. The statement that the structural sta- bility of HEA’s is explained only by high values of the configurational entropy has now lost its relevance. In many works, it has been experimentally shown that in alloys with high values of the mixing entropy, other phases, including intermetallics can appear along with the solid solution. Using the method of anomalous X-ray scattering and neutron diffraction, it was shown in [83] that a two-week exposure at 753 K of a four-component FeCoCrNi alloy obtained by arc melting did not lead to the appearance of the solid solution ordering effect and the formation of a long-range order in it. This stabil- ity of the analyzed alloy is associated with its high configuration entropy. The question of the HEAs stability during thermal and thermoplastic impact remains open. A detailed analysis of this issue is presented in the work of Rogachev [17]. A large amount of research is devoted to the stability of the five-component CoCrFeNiMn alloy (“Cantor alloy”). The diameter of the manganese atoms (0.274 nm) is significantly larger compared to the atoms included in the four-component CoCrFeNi system. For this reason, the maximum distortion of the crystal lattice, localized near the manganese atoms, in the five-component system is significantly higher than in the CoCrFeNi alloy. The analysis of the behavior of the Cantor alloy under various conditions of thermal and thermoplastic action does not give grounds to formulate unambiguous conclusions about its stability. In the literature, there are data on the long-term preservation of the single-phase structure of the material in a wide temperature range (1273-1473 K), which indicates its high stability [29, 84-86]. At the same time, based on the results of experimental studies, it is concluded that plastic deformation and high-temperature exposure are factors leading to precipitation of secondary phases from the CoCrFeNiMn alloy [17, 50, 85, 87, 88], including nanoscale intermetallics such as NiMn, FeCo. An increase in the chromium and manganese content accelerates the formation of secondary phases. At the same time, it is noted [17] that when a secondary phase precipitates from a high-entropy CoCrFeNiMn alloy, its matrix phase remains a solid solution with an FCC structure. The CoCrFeNiAl system, like CoCrFeNiMn, is characterized by metastability. As a result of short annealing of this HEA, several types of structural components appear in it [89].