  Do high-entropy alloys and ceramics have their grain boundary (GB)
counterparts? As the concept of high-entropy grain boundaries (HEGBs) was
initially proposed in 2016, this article provides the first complete and
rigorous discussion of the underlying interfacial thermodynamics. A simplified
segregation model can illustrate both GB and bulk high-entropy effects, which
reduce GB energy with increasing temperature for saturated multicomponent
(conventional and high-entropy) alloys. HEGBs can be utilized to stabilize
nanocrystalline alloys at high temperatures via thermodynamic and kinetic
effects. GB structural disordering and transitions offer further opportunities
to attain higher effective GB entropies. Future perspective is discussed.
