Review of alloys developed using the entropy approach

This paper provides a review of studies on the development and characterization of high-entropy alloys (HEAs). It is structured in the following way. Alloys’ design strategy based on entropy approach. Expectations and modern perceptions. This section describes the initial principles of multicomponent alloys design which provide stable structure and mechanical properties. It is noted that the role of high mixing entropy in the formation of disordered solid solutions and the suppression of the brittle intermetallic phases formation have been significantly reconsidered over time. Currently, obtaining a single-phase solid solution structure is not the main requirement for HEAs. The composition of HEAs. This section describes some typical multicomponent alloys having different elemental compositions. It is shown, that at present time the most studied alloys are based on 3-d transition elements. Using alloys of this group the possibility of providing both high and low values of strength and ductility is shown. Fabrication methods of HEAs. This section describes the methods for the fabrication of high-entropy alloys. It is noted that the most commonly used methods are based on the melting of the initial materials and its subsequent crystallization. Such methods of HEAs fabrication as powder metallurgy, magnetron sputtering, self-propagating high-temperature synthesis, melt spinning, and diffusion welding are also discussed. Structure of HEAs. This section provides the data on HEAs possessing multiphase structure and containing fine nanosized precipitates. Besides, the studies in which HEAs have been obtained in the form of metallic glasses, carbides, oxides, and borides are reviewed. The factors that can affect the structural state of the multicomponent alloys are discussed. The ambiguity of opinions of different research groups is noted. Properties of HEAs. This section mainly concentrates on the mechanical properties of HEAs. However, some other promising properties of HEAs like high wear resistance and reduced diffusivity are also discussed. Plastic deformation of HEAs. This section describes the evolution of the structure and properties of HEAs caused by thermal and mechanical processing. Characterization methods of HEAs. This section lists the characterization techniques, which are most frequently used to study HEAs. The structure of these alloys is mainly studied by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and optical microscopy. The methods for properties measurements are also briefly reviewed. Application of HEAs. This section describes the promising fields of HEAs application. It can be utilized in the aerospace, aircraft, and nuclear industries as well as for car manufacturing, austoelectronics, and in the design of microwave devices. Russian-language publications on HEAs. This section lists the studies, published in the Russian language as well as the thesis, done in Russian universities.