Structure, Stability, and Properties of High-Entropy Alloys
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CTURE, PHASE TRANSFORMATIONS, AND DIFFUSION
Structure, Stability, and Properties of High-Entropy Alloys A. S. Rogachev* Merzhanov Institute of Structural Macrokinetics and Problems of Materials Science, Russian Academy of Sciences (ISMAN), Chernogolovka, 142432 Russia *e-mail: [email protected] Received March 16, 2020; revised March 24, 2020; accepted March 25, 2020
Abstract—This critical review considers the fundamental concept, methods of obtaining, stability, and properties of a new class of materials—high-entropy alloys and compounds. An analysis of factors that determine the structure and stability of multicomponent disordered solid solutions has been carried out. Particular attention has been given to results obtained since 2017. The concept of temperature regions within stable and metastable phases, which explains the available experimental data, has been proposed. Keywords: high-entropy alloys, equiatomic multicomponent alloys, high-entropy compounds, crystallization of melts, mechanical alloying, crystal structure, metastability, properties DOI: 10.1134/S0031918X20080098
number of degrees of freedom cannot be less than zero, the number of phases in the system is determined by the expression
p ≤ n + 2,
(2)
(1) f = n – p + 2, where f is the number of degrees of freedom that the system has in the equilibrium state, n is the number of components, and p is the number of phases. Since the 733
15 10 5
United States
20
Israel Spain Czech Republic Netherlands Iran Switzerland Romania Austria Canada Hungary Poland Italy Australia Ukraine France Russia Sweden England Korea Japan Taiwan India Germany
25
0 Countries Fig. 1. The fractions of different countries in publications on HEAs indexed according to WoS.
China
which takes into account all solid, liquid, and gaseous phases; the temperature and the pressure are considered as the degrees of freedom. But the expressions (1) and (2) limit only the maximum possible number of phases in the equilibrium state and do not impose restrictions on the minimum number of equilibrium phases. Thus, the formation of a single-phase stable alloy in systems with as many components as possible does not contradict the phase rule based on the laws of thermodynam-
Fraction of publications, %
INTRODUCTION The concept of high-entropy alloys (HEAs) appeared a little more than 15 years ago [1–3], which is a vanishingly small period on the scale of the history of metallurgy. However, thousands of papers on this topic (more than 5000 on the Web of Science basis) have already been published, and research on the topic remains ongoing in all economically developed countries (Fig. 1). The emergence of HEAs is characterized in many publications as a large step in the development of metallic alloys [4, 5]. The idea of a HEA is based on the assumption that five or more metallic components taken in equal or close molar fractions can form a single-phase crystalline alloy. The same amounts of components distinguish the HEAs from traditional alloys, in which one component is usually the basi
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