Review of alloys developed using the entropy approach

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 Poor deformability at room temperature is a factor limiting the use of some high-entropy alloys [2, 7, 121]. For example, in [7], a low set of mechanical properties of castings made of the AlCoCrCuFeNi alloy is noted. As a solution to this problem, it was proposed to use the method of comprehensive hot forging (a-b-c forging) at a temperature of 950 °C. The alloy was obtained by induction melting, followed by electroslag remelting and casting into a copper cooled mold. Forging was carried out in an isothermal die block on a hydraulic press at a traverse movement speed of 1 mm/s. The total degree of deformation was ~ 1,000 %. In the process of comprehensive forging, the dendritic structure of the cast alloy is eliminated, the structure of the HEAs becomes finer (2.1 microns) and more homogeneous. The result of structural transformations accompanying the hot plastic deformation of the alloy is an increase in the yield strength from 790 MPa to 1170 MPa. The increase in the relative elongation observed under these conditions (from 0.2 % to 1 %) does not allow us to speak of a significant improvement in the plasticity indicators [122]. One of the features of the behavior of the forged AlCoCrCuFeNi alloy, recorded by the authors of the work [7], is the manifestation of the superplasticity effect in the high temperature range (800–1,000 °C). When deformed at a rate of 10 –2 s –1 , the relative elongation of the samples subjected to stretching is 1,240 % [82]. Methods for studying high-entropy alloys The choice of methods for studying the structure and properties of high-entropy alloys is determined by various factors, including the structural features of the materials, its operating conditions, and the size of the samples. One of the most important research methods is related to the X-ray diffraction analysis of alloys, the identification of the phases present, and the determination of the parameters of its crystal lattices. In many works in the field of HEAs, structural studies are carried out using the methods of transmission and scanning electron microscopy, X-ray spectral analysis, and light microscopy. The methods of mechanical and other tests are determined by the purpose of the developed high-entropy alloys. For structural alloys, information about strength properties under uniaxial tension and compression is of the utmost importance. The level of reliability and durability of products made from HEAs is related to such properties of materials as impact strength, static and fatigue crack resistance. In some cases, the corrosion resistance characteristics are important. The tasks associated with the study of nanoscale particles released in HEAs involve the use of high- resolution transmission electron microscopy methods [123]. In [124], the method of small-angle scattering of synchrotron radiation was used to solve such problems in the study of a deformed CoCrFeNiMn alloy. In [125], a method based on neutron diffraction was used to study the structural-phase state of a three- component CoCrFeNi alloy. The experimental results obtained in this way allowed us to make a concluson about the features of the fine structure of the CoCrFeNiMn alloy [84]. Using the method of anomalous X-ray scattering and neutron diffraction, we studied the processes of structural transformations during heating of a four-component FeCoCrNi alloy obtained by arc melting. It was shown [83] that the two-week exposure of the alloy at 753 K did not lead to the manifestation of the ordering effect of the solid solution and the formation of a long-range order in it. One of the most important characteristics of multicomponent alloys is the degree of structure ordering. In the work of M.V. Ivchenko, an optical tomographic atomic probe “Cameca atom probe” (3D-AP) was used for a precision study of the local atomic composition of the six-component AlCoCrCuFeNi alloy [18, 102]. The same method was used to study the structure and properties of the six-component alloy AlCrFeCoNiCu after casting and rapid quenching from the melt [36]. The traditional approach to substantiating the compositions of multicomponent systems and analyzing its properties is associated with a high labor intensity of research, conducting many experiments. One of the tasks typical for the analyzed materials is associated with the need to model phase diagrams of the state. Examples of its solution by the CALPHAD method (CALculations of PHAse Diagrams) are given in [126, 127]. The development of computer technologies and special software in many cases can significantly reduce the cost of developing high-entropy alloys. By methods of mathematical simulation,