High-entropy alloys: basic concepts

High-entropy alloys (HEAs) have emerged as an important class of materials since 2004 with immense potential both at the level of fundamental understanding and also in terms of possible applications. This chapter deals with the basic concepts that underpin this new class of alloys. It starts with the definition of HEAs and how they differ from other multicomponent alloys such as Ni-base superalloys. Then, it gives a classification of alloys in terms of the number of components. A notation that is commonly used to represent composition in these alloys, particularly the non-equiatomic alloys with alloying additions, is brought out. Finally, four core effects of HEAs are elucidated, namely, high-entropy, severe lattice distortion, sluggish diffusion, and cocktail effects.