Multifunctional high-entropy materials

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. High-entropy materials leverage phase stabilization through mixing several elements and are primarily known for their mechanical strength and high toughness. This Review explores their use as a platform for multifunctional material design, in which several, even conflicting, properties can be reconciled because of the compositional tolerance inherent in the high-entropy concept, including electronic, magnetic, mechanical, catalytic, thermal expansion and hydrogen storage properties.