Superfunctional high-entropy alloys and ceramics by severe plastic deformation

High-entropy alloys and ceramics containing at least five principal elements have recently received high attention for various mechanical and functional applications. The application of severe plastic deformation (SPD), particularly the high-pressure torsion method, combined with the CALPHAD (calculation of phase diagram) and first-principles calculations resulted in the development of numerous superfunctional high-entropy materials with superior properties compared to the normal functions of engineering materials. This article reviews the recent advances in the application of SPD to developing superfunctional high-entropy materials. These superfunctional properties include (i) ultrahigh hardness levels comparable to the hardness of ceramics in high-entropy alloys, (ii) high yield strength and good hydrogen embrittlement resistance in high-entropy alloys; (iii) high strength, low elastic modulus, and high biocompatibility in high-entropy alloys, (iv) fast and reversible hydrogen storage in high-entropy hydrides, (v) photovoltaic performance and photocurrent generation on high-entropy semiconductors, (vi) photocatalytic oxygen and hydrogen production from water splitting on high-entropy oxides and oxynitrides, and (vii) CO₂ photoreduction on high-entropy ceramics. These findings introduce SPD as not only a processing tool to improve the properties of existing high-entropy materials but also as a synthesis tool to produce novel high-entropy materials with superior properties compared with conventional engineering materials.