Abstract - A relatively new approach to obtaining metal materials containing several principal elements in equiatomic concentrations which look promising for replacing commercially used alloys is proposed. Such materials are called high-entropy alloys (HEAs). Studies show that HEAs tend to form a simple solid-solution structure and can also contain ordered intermetallic phases. Such a method of forming metal materials can be regarded as a background for producing new HEAs with elevated performance characteristics. Most studies focus on the relationship between microstructure and measured properties; significantly less attention is paid to studying and developing new effective methods for creating HEAs. In this paper, we study the possibility of obtaining CoCrFeNiMn–(X) HEAs by centrifugal metallothermic SHS. Chemical and technological modes of modifying cast CoCrFeNiMn alloy during synthesis (in situ) by introducing alloying components into the starting exothermic compositions are tested for the first time. The microstructure and phase composition of NiCrCoFeMn alloys synthesized from mixtures containing Ti–Si–B(C) or Al are characterized. The microstructure of CoCrFeNiMn–(Ti–Si–B(C)) HEAs is found to consist of an HEA-based matrix and new structural inclusions of carbides and borides of titanium. High-Al CoCrFeNiMn–Al HEAs are represented by a composite structure containing NiAl as a basis and dispersion nanoprecipitates (~100 nm) of a Cr- and Fe-based solid solution.
INTRODUCTION Developments in the creation of new alloying systems are widely applied for obtaining modern metallic materials operating under extreme conditions (elevated temperatures and loads), such as high-temperature and heat-resistant alloys based on nickel and iron [1, 2]. The performance characteristics possessed by such alloys were achieved namely by a multicomponent alloying. However, the possibilities of traditional approaches to the production of metallic materials by selecting alloying elements to improve the desired characteristics of a one-component alloy have largely been exhausted and no longer lead to a significant increase in properties.
In 2004, a fundamentally new concept in alloying to produce metallic materials containing several principal elements in an equal atomic percentage was proposed. Such materials were called high-entropy alloys (HEAs) [4–6]. Initial studies supposed that, because of the high configurational entropy of mixing, it would be preferable to form disordered substitutional solid solutions rather than ordered (intermetallic) phases in HEAs, and, in this way, the HEAs should possess both high strength and sufficient plasticity.
However, the authors of [7–9] showed no clear correlation between the calculated values of configurational entropy and the phase composition of obtained experimental multicomponent alloys. The phase composition was found to depend on characteristics of atoms of elements contained in HEAs instead of their number.
The high-entropy alloys belong to a new class of multicomponent alloys in which the concentration of alloying components corresponds to the central regions of phase diagrams. Studies showed that HEAs tend to form a solid-solution structure and can also contain ordered phases [8], and it is possible to obtain structures not typical of ordinary alloys. Thus, a new approach to the formation of metallic materials provides great opportunities for the development of new alloys with elevated performance characteristics. In particular, new HEA-based compositions being developed have great potential to be used as high-temperature materials. In the first step, HEAs containing refractory components (Nb, Mo, Ta, V, and W) were proposed [15–17]. These alloys had a single-phase bcc structure and possessed high-temperature strength (400 MPa at T = 1600°C) [16]. However, their density was significantly higher (>12 g/cm3 ) than that of nickel superalloys. Therefore, one of the most important criteria for choosing alloying components was an increase in the specific strength [18–20]. An increase in the high-temperature strength of alloys can be achieved by forming the desired structure, for example, owing to the solid-solution strengthening and/or the precipitation of secondary phases. This was confirmed experimentally in [21–24]. It is known that the properties of alloys are caused by the combination of phases in the structure and the formation of given structural elements. For example, nickel superalloys have high strength, ensured by the presence of ordered γ' phase (Ni3Al) in a nickel-based matrix.