Since high-entropy alloys are likely utilized in high temperature environment, the thermal stability is very important for designing HEA. This is especially critical for nano-crystalline where extra driving force exists for grain growth. Two aspects need to be considered for nano-crystalline HEAs: the stability of phases formed, which is dominated by the thermodynamics mechanism (see alloy design), and the retention of nanocrystallinity. The stability of nano-crystalline HEAs are controlled by many factors, including grain boundary diffusion, presence of oxide etc.