Elements for structural utilization in space, the energy sector, and aircraft engines require materials exhibiting high strength, good ductility, high fracture toughness, corrosion resistance under some severe environments, etc. Intensive study in the past eight decades has seen the development of superalloys, largely nickel-based superalloys, satisfying these specifications for applications. Currently, the enhancement in the properties of nickelbased superalloys has reached almost saturation. Recently, researchers have recognized high-entropy alloys (HEAs) as a novel class of alloys with excellent properties, which are promising to use for a wide range of applications due to their excellent thermal stability, high hardness, high strength at room and elevated temperature, superior wear resistance, unique electrical and magnetic characteristics, as well as many other notable properties. [1] HEAs are multielement alloys of at least five principal elements, with atomic percentage of each between 5 and 35, and minor elements, if any, less than 5 atomic percentage. [2] The concept of HEAs has raised interest in design and processing approaches as well as the characterization of new materials for room-and hightemperature applications. [2][3][4][5][6] HEAs are multiple-principal-element systems, which, in spite of consisting of multiple elements with different crystal structures, generally exist as a crystalline single phase. [2] This is attributed to the high configurational entropy during mixing, which lowers the total Gibbs' free energy of the HEAs and thus stabilizes the alloy in a single-phase solid-solution form compared with conventional alloys, where a number of intermetallic phases crystallize in the matrix. In HEAs, as multiple principal elements are present, the concept of solvent and solute does not exist; thereby, the criteria for the formation of solid-solution phases only based on Hume-Rothery's principles may not be sufficient. Furthermore, in equiatomic HEAs, only high mixing entropy is not the factor that controls solid-solution formation. An extensive investigation for identifying the conditions for delineating the formation of bulk metallic glasses (BMGs) or else solid solutions in HEAs reveals some important parameters such as 1) enthalpy of mixing (ΔH mix ), 2) mixing entropy (ΔS mix ), and 3) radius ratio (δ) between the elements. [7,8] The solid-solution phases are expected when À22 kJ mol À1 ≤ΔH mix ≤ 7 kJ mol À1 , 11 J mol À1 K À1 ≤ΔS mix ≤ 19.5 J mol À1 K À1 , and 0 ≤δ ≤ 8.5. [8] The key factors responsible for excellent properties exhibited by HEAs are attributed to 1) high mixing entropy, 2) severe lattice distortion, 3) sluggish diffusion, and 4) cocktail effect. [9] At very high temperatures, HEAs have high yield strength compared with BMGs. In distinction, BMGs can preserve their high strength only up to their glass transition temperature. As the mechanism of strengthening and microstructural stability in HEAs has not been fully recognized, extensive analysis is required for 1) recognizin...
