1. Overview of High-Entropy Alloys

Alloys have evolved from simple to complex compositions depending on the ability of mankind to develop the materials. The resulting improved functions and performances of alloys enable advancements in civilizations. In the last century, significant evolution and progress have led to the invention of special alloys, such as stainless steels, high-speed steels, and superalloys. Although alloys composed of multiple elements have higher mixing entropy than pure metals, the improved properties are mostly due to mixing enthalpy that allows the addition of suitable alloying elements to increase the strength and improve physical and/or chemical properties. Since the turn of the century, more complex compositions with higher mixing entropies have been introduced. Such complex compositions do not necessarily guarantee a complex structure and microstructure, or the accompanied brittleness. Conversely, significantly higher mixing entropy from complex compositions could simplify the structure and microstructure and impart attractive properties to the alloys. Jien-Wei Yeh and Brian Cantor independently announced the feasibility of high-entropy alloys and equi-atomic multicomponent alloys in reports published in 2004. This breakthrough in alloying concepts has accelerated research on these new materials throughout the world over the last decade.