  Grain boundaries (GBs) can be treated as two-dimensional (2-D) interfacial
phases (also called 'complexions') that can undergo interfacial phase-like
transitions. As bulk phase diagrams and calculation of phase diagram (CALPHAD)
methods are a foundation for modern materials science, we propose to extend
them to GBs to have equally significant impacts. This perspective article
reviews a series of studies to compute the GB counterparts to bulk phase
diagrams. First, a phenomenological interfacial thermodynamic model was
developed to construct GB lambda diagrams to forecast high-temperature GB
disordering and related trends in sintering and other properties for both
metallic and ceramic materials. In parallel, an Ising-type lattice statistical
thermodynamic model was utilized to construct GB adsorption diagrams, which
predicted first-order GB adsorption (a.k.a. segregation) transitions and
critical phenomena. These two simplified thermodynamic models emphasize on the
GB structural (disordering) and chemical (adsorption) aspects, respectively.
Subsequently, hybrid Monte Carlo and molecular dynamics atomistic simulations
were used to compute more rigorous and accurate GB 'phase' diagrams. Computed
GB diagrams of thermodynamic and structural properties were further extended to
include mechanical properties. Moreover, machine learning was combined with
atomistic simulations to predict GB properties as functions of four independent
compositional variables and temperature in a 5-D space for a given GB in
high-entropy alloys or as functions of five GB macroscopic (crystallographic)
degrees of freedom (DOFs) plus temperature and composition for a binary alloy
in a 7-D space. Relevant other studies are examined. Future perspective and
outlook, including two emerging fields of high-entropy grain boundaries (HEGBs)
and electrically (or electrochemically) induced GB transitions, are discussed.
