  Grain boundary structural transitions can lead to significant changes in the
properties and performance of materials. In multi-principal element alloys,
understanding these transitions becomes complex due to phenomena such as local
chemical ordering and multi-component segregation. Using atomistic simulations,
we explore a metastable-to-equilibrium grain boundary structural transition in
NbMoTaW. The transition, characterized by structural disordering and reduced
free volume, shows high sensitivity to its local chemical environment. Most
notably, the transition temperature range of the alloy is more than twice that
of a pure metal. Differences in composition between coexisting metastable and
equilibrium structures highlight the change in local site availability due to
structural relaxation. Further examination of grain boundaries with fixed
chemical states at varying temperatures reveals that the amount of segregation
significantly influences the onset temperature yet has minimal effect on the
transition width. These insights underscore the profound effects of chemical
complexity and ordering on grain boundary transitions in complex concentrated
alloys, marking a meaningful advancement in our understanding of grain boundary
behavior at the atomic level.
