Introduction to some Multifunctional High Entropy Alloys

Entropy-related phase stabilization can allow compositionally complex solid solutions of multiple principal elements. The massive mixing approach was originally introduced for metals and has recently been extended to ionic, semiconductor, polymer and low-dimensional materials. Multielement mixing can leverage new types of random, weakly ordered clustering and precipitation states in bulk materials as well as at interfaces and dislocations. The many possible atomic configurations offer opportunities to discover and exploit new functionalities, as well as to create new local symmetry features, ordering phenomena and interstitial configurations. This opens up a huge chemical and structural space in which uncharted phase states, defect chemistries, mechanisms and properties, some previously thought to be mutually exclusive, can be reconciled in one material. Earlier research concentrated on mechanical properties such as strength, toughness, fatigue and ductility. This Review shifts the focus towards multifunctional property profiles, including electronic, electrochemical, mechanical, magnetic, catalytic, hydrogen-related, Invar and caloric characteristics. Disruptive design opportunities lie in combining several of these features, rendering high-entropy materials multifunctional without sacrificing their unique mechanical properties.
Materials have always played a pivotal role in the development of human
society. The range of accessible phase states, kinetics, transformation
phenomena and properties, however, has been constrained by the fact
that many materials used today are mostly based on one or two principal
elements and typically use further elements only in low fractions.
Compositionally complex and high-entropy alloys (HEAs)1–4,
consisting of multiple principal elements, open up this rather limited
chemical composition space. The original idea consists of stabilizing
equimolar solid solutions of five or more chemical elements through enhanced configurational entropy. Today, this approach is embraced more broadly and also encompasses materials that are not (only) entropy-stabilized, targeting compositionally complex materials that have large solid solution ranges in the centre regions of multicomponent phase diagrams5–8. This is because, first, only a few fully random and thermodynamically stable solid solution HEAs have been identified so far, and second, some compositionally complex materials are enthalpy-stabilized rather than entropy-stabilized, for example some ionic materials. Furthermore, most of these materials are metastable and are prone to decompose into several stable phases. Beneficial properties have, in part, emerged from random solid solution states (such as high distortions and atomic-scale symmetry breaking), ordering effects and precipitation. These features allow the introduction of kinetics, microstructure and processing as additional degrees of freedom for material design. It is also understood today that HEAs do not need to be equimolar in their composition, provided
that no single matrix element prevails, making the design approach much more versatile. It is further important to note that the design approach works for the bulk and for internal interfaces and surfaces. Interfaces can be as important as the bulk for certain materials, such as catalysts, hard magnets, topological materials or coatings. The two are chemically connected under near-equilibrium conditions because the partitioning and mixing states of adjacent regions depend on each other, as stated by the Gibbs adsorption isotherm.
These examples of ‘relaxed-constraints’ design opportunities for
multicomponent materials thus give access to a wide range of continuously variable chemical compositions and properties and bring a large variety of additional microstructural phenomena into play9. In the
latter context, kinetics, non-equilibrium phase transformations, and many chemical ordering and decoration phenomena produce a rich underlying lattice defect cosmos (point defects, dislocations, stacking faults, interfaces, surfaces and so on), providing an additional versatile
material design toolbox10–13. The resulting microstructures can differ
profoundly from those in conventional alloys because the lattice
defects can be chemically highly decorated, which can be used to alter
their kinetic, thermodynamic and functional features14,15.