  Phase transformations in metallic grain boundaries (GBs) present significant
fundamental interest in the context of thermodynamics of low-dimensional
physical systems. We report on atomistic computer simulations of the Cu-Ag
system that provide direct evidence that GB phase transformations in a
single-component GB can continue to exist in a binary alloy. This gives rise to
segregation-induced phase transformations with varying chemical composition at
a fixed temperature. Furthermore, for such transformations we propose an
approach to calculations of free energy differences between different GB phases
by thermodynamic integration along a segregation isotherm. This opens the
possibility of developing quantitative thermodynamics of GB phases, their
transformations to each other, and critical phenomena in the future.
