Grain boundaries (GBs) can be treated as interface phases (also called “complexions”) with different thermodynamic excess properties. It was theoretically predicted that different GB phases can exist for the same macroscopic GB parameters, but experimental evidence of this has been scarce until recently.

Direct evidence of GB phase transitions in pure copper could be obtained with combined atomic-resolution scanning transmission electron microscopy (STEM) imaging and atomistic computer simulations [1,2]. In a Σ37c ⟨111⟩ \{1 10 11\} tilt GB, for example, we found that a low-temperature GB phase (dubbed “domino phase”) transitions to a high-temperature phase (“pearl phase”) at approximately 460 K [2]. It remains an open question if such phases and phase transitions are specific to this copper GB, or if they are a more general feature of fcc metals across different misorientations. Using molecular dynamics computer simulations, we investigated Σ13b, Σ7, Σ19b, and Σ37c tilt GBs in a range of fcc metals. Recurring structural motifs of both pearl-like and domino-like GB phases appear in all systems and we find good matches to STEM investigations of the atomic structure of several GBs in Cu and Al thin films. However, free-energy calculations of different fcc metals under external stress and at finite temperature (using the quasi-harmonic approximation) show that the phase stability depends on the actual material. The free-energy differences between the GB phases are generally small and the phase stability has no clear correlation with the properties of the bulk material, such as cohesive energy, stacking-fault energy, or elastic constants. There is, however, some dependence on the misorientation. Together, this indicates that the GB free energy is strongly influenced by the local, defective bonding environment.

Even though the GB free energy might depend on the material, the pearl-like and domino-like phases themselves are structurally similar across materials. This raises the question if the atomic arrangements are simply the result of sphere-packing geometry. We thus also considered two types of model systems: One with a medium-range Lennard-Jones pair potential and one with next-neighbor pair potentials. With this data, we determine which GB excess properties are universal, which require long-range interatomic interactions, and which are controlled by the physics of the specific material.

Acknowledgment: This result is part of a project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 787446; GB-CORRELATE).

[1] Meiners et al., Nature 579 (2020)
[2] Frommeyer, Brink, et al., arXiv:2109.15192 [cond-mat.mtrl-sci] (2021)