Review of alloys developed using the entropy approach

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 Development of alloys using the entropy approach. Expectations and modern ideas. The first studies related to the fabrication and characterization of high-entropy alloys (HEAs) were carried out at the end of the 20th century. An US patent for this type of materials was registered in 2002 by Taiwanese scientist J.-W. Yeh [1]. In 2004, the first studies of J.W. Yeh et al. [2] and B. Cantor et al. [3] were published, which are now highly cited. Thus, about 20 years ago, a new class of materials appeared, called “high-entropy alloys” [2, 3-6]. These alloys contain from 5 to 13 elements in an approximately equiatomic or equimolar ratio [2, 7]. The content of each element in the HEA is usually in the range from 5% to 35%. The high interest to HEAs is evidenced by the fact that in such a short period of time more than 5000 studies devoted to the analysis of these alloys were published. Among them, several reviews on the synthesis, characterization of structure and properties of HEAs can be recommended [8-17]. The interest to high-entropy alloys was due to its attractive properties, including high strength, ductility, wear resistance, and corrosion resistance [2, 18, 19]. Unlike traditional alloys, for example, steels, brass, bronze, aluminum or titanium alloys, high-entropy alloys do not have a “principal” or “matrix” component. All the elements present in the equiatomic ratios and thus can be considered as a principal, since in a disordered solid solution, each of the elements of the system has the same probability of being found in any of the available atomic positions of the crystal [20]. Therefore, in the case of high-entropy alloys with a solid solution structure, it is impossible to categorize the components as principal and minor (or alloying) ones. In fact, equiatomic alloys are located in the central regions of multicomponent state diagrams. Since none of components presented in significant quantities can be distinguished as a principal, sometimes such multicomponent alloys are called baseless (without a base) or compositionally complex [21]. Insufficient attention devoted in previous years to equiatomic alloys containing five or more compo - nents, and the dominance of materials based on one major element, was explained by the expectation that multicomponent mixtures will consist of brittle intermetallic compounds or complex phases. For this rea- son, the “entropy” approach to the alloys design began to be applied only in the last two decades [22]. One of the main ideas behind the development of the HEAs was to obtain a single-phase structure in the form of a disordered substitutional solid solution. It was assumed that the formation of phases with an ordered structure, including various intermetallics, would lead to embrittlement of the material consisting of mul- tiple components. An obvious feature of HEAs is the high entropy of mixing, which reduces the tendency to form interme- tallic compounds in alloys and promotes the formation of single-phase substitutional solutions with a BCC or FCC structures. The high entropy of mixing is considered as a measure of the probability of preserving the structure and phase composition of alloys, ensuring its thermal stability, and maintaining high mechani- cal, physical, and chemical properties [23, 24]. The entropy of an alloy is determined by the value of four components – the configurational entropy of mixing ( D S conf ), the entropy of atomic vibration ( D S n ), the entropy of electron motion ( D S e ), and the entropy of magnetic moments ( D S m ) [11]. The concept of HEAs development is based on the fact that in multicomponent alloys, the configura - tional entropy is of a high level, which is not typical for traditional materials [2, 25, 26]. In comparison with the configurational entropy, the contribution of the D S n , D S m and D S e components in the HEAs is small. Thus, the name “high-entropy alloys” is associated with an increased configurational component of entropy. The transition of the system from an unstable state to a more stable one is accompanied by a decrease in the Gibbs energy. An increase in the number of elements of the system, leading to an increase in the con- figurational entropy, is a factor contributing to a decrease in the Gibbs energy. This reduces the tendency of the system to form ordered solid solutions and intermetallics and increases the probability of disordered solid solutions formation [26]. A lot of studies were related to the search of criteria that determine the formation of high-entropy alloys and predict their structure and phase state. Gorban et al. analyzed more than 200 high-entropy alloys and derived the relationship between the electron concentration, phase composition, lattice parameters, and