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Introduction
Materials development has always been at the forefront of technological innovations, which have governed the growth of human civilizations since ancient times. Alloys have been an integral part of materials evolution, since their first accidental discovery (arsenic bronze, 3000 BCE) in the primeval cave fires [1]. The conventional alloy design strategy involved adding one or more elements to a parent metal to achieve enhancement in desired properties. A plethora of currently used alloys, like steels, bronzes, aluminum alloys, and magnesium alloys, were developed using this traditional approach. It followed that the exploration of phase diagrams was largely limited to three dimensions and terminal composition range. However, a shift in this paradigm occurred more than a decade ago when parallel works of Cantor et al. [2] and Yeh et al. [3] marked the beginning of a new class of multicomponent alloys termed high-entropy alloys (HEAs).
HEAs are defined as multicomponent alloys containing 5 or more constituents in equiatomic or near equiatomic ratio [1]. They tend to exhibit solid solution structures, instead of complex phases, stabilized by their high configurational entropy of mixing. HEAs have shown some fascinating properties, like high strength [4], high fracture toughness at cryogenic temperatures [5], enhanced thermal stability [6], superior oxidation [7, 8], and corrosion resistance [9]. The advent of HEAs has also generated a lot of fundamental curiosities and led to the investigation of hitherto unexamined questions: atomic occupancy in a multicomponent lattice, visualization of higher dimensional phase diagrams, entropy contribution to phase stability, and atomic transport in multielement matrices being some of them. HEAs have been produced by melting and casting route, powder metallurgical synthesis, and deposition techniques [1]. While the bulk HEAs form the major share of HEAs synthesized so far, nanocrystalline HEAs have also been consistently developed. Nanocrystallinity adds another exciting facet to the vast compositional space and complex dimensions of HEAs. Nanostructured HEAs have shown improved mechanical properties [10], excellent magnetic behavior [11], and high thermal stability [12].
Mechanical alloying (MA) is a well-known solid-state, non-equilibrium, top-down approach to produce nanocrystalline materials. It involves milling of elemental...