Introduction to High‐Entropy Materials

The advancements of human civilization are largely determined by the materials and tools that were manufactured and mastered by humans. Recently, crystalline multi-principal element materials located at or near the center of the phase diagram with equal or near- equal atomic fraction of constituting elements have been introduced and have attracted increasing attention due to their unique compositions, microstructure, and unexpected properties. This chapter reviews the basic concepts and information about these fascinating materials. It emphasizes that, for both high-entropy alloys (HEAs) and high-entropy ceramics (HECs), the composition and entropy definitions are just guidelines without the guarantee of formation of a single-phase solid solution. In high-entropy materials, cocktail effect is used to emphasize a synergistic effect of the whole properties from the overall contribution of the constituent elements. With the development of HEAs, it is found that HEAs with non-equimolar compositions or multiphase structure prevail in mechanical properties.