Structural transformations at interfaces are of profound fundamental interest as complex examples of phase transitions in low-dimensional systems. Despite decades of extensive research, no compelling evidence exists for structural transformations in high-angle grain boundaries in elemental systems. Here we show that the critical impediment to observations of such phase transformations in atomistic modelling has been rooted in inadequate simulation methodology. The proposed new methodology allows variations in atomic density inside the grain boundary and reveals multiple grain boundary phases with different atomic structures. Reversible first-order transformations between such phases are observed by varying temperature or injecting point defects into the boundary region. Owing to the presence of multiple metastable phases, grain boundaries can absorb significant amounts of point defects created inside the material by processes such as irradiation. We propose a novel mechanism of radiation damage healing in metals, which may guide further improvements in radiation resistance of metallic materials through grain boundary engineering.

While the theory of grain boundary (GB) structure has a long history 1 and the proposition that grain boundaries can undergo phase transformations was made already 50 years ago 2 , 3 , the search for these GB transitions has been in vain. The underlying assumption was that multiple stable and metastable states exist for different grain boundary orientations 4 , 5 , 6 . The terminology complexion was recently proposed to distinguish between those interfacial states that differ in any equilibrium thermodynamic property 7 . Different types of complexions and transitions between them have been characterized mostly in binary or multicomponent systems 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 . Novel simulation schemes have provided a new perspective on the phase behavior of interfaces and showed that grain boundary transitions can occur in a multitude of material systems 20 , 21 , 22 , 23 , 24 . However, their direct experimental observation and transformation kinetics in an elemental metal remained elusive. Here, we show atomic scale grain boundary phase coexistence and transformations at symmetric and asymmetric [11-1] tilt grain boundaries in elemental copper (Cu). We found the coexistence of two different grain boundary structures at Σ19b grain boundaries by atomic resolution imaging. Evolutionary grain boundary structure search and clustering analysis 21 , 25 , 26 finds the same structures and predicts the properties of these GB phases. Furthermore, finite temperature molecular dynamics simulations explore their coexistence and transformation kinetics. Our results demonstrate how grain boundary phases can be kinetically trapped enabling atomic scale room temperature observations. Our work paves the way for atomic scale in situ studies of grain boundary phase transformations in metallic grain boundaries. In the past, only indirect measurements have indicated the existence of interfacial transitions 9 , 15 , 27 , 28 , 29 . Now, their atomic scale role on the influence of abnormal grain growth, non-Arrhenius type diffusion or liquid metal embrittlement can be explored.

Recent experimental measurements of Ag impurity diffusion in the Σ5 (310) grain boundary (GB) in Cu revealed an unusual non-Arrhenius behavior suggestive of a possible structural transformation [Divinski et al., Phys. Rev. B 85, 144104 (2012)]. On the other hand, atomistic computer simulations have recently discovered phase transformations in high-angle GBs in metals [Frolov et al., Nature Communications, 4, 1899(2013]. In this paper we report on atomistic simulations of Ag diffusion and segregation in two different structural phases of the Cu Σ5 (310) GB which transform to each other with temperature. The obtained excellent agreement with the experimental data validates the hypothesis that the unusual diffusion behavior seen in the experiment was caused by a phase transformation. The simulations also predict that the low-temperature GB phase exhibits a monolayer segregation pattern while the high-temperature phase features a bilayer segregation. Together, the simulations and experiment provide the first convincing evidence for the existence of structural phase transformations in high-angle metallic GBs and demonstrate the possibility of their detection by GB diffusion measurements and atomistic simulations.Motivation. Structural transformations at grain boundaries (GBs) are of fundamental interest and can have a significant impact on microstructure, mechanical behavior and transport properties of polycrystalline materials [1,2]. A number of GB phases have been found in alloys [3] and ceramic materials [4,5], where they often appear in the form of intergranular thin films and are referred to as "complexions" [6]. In metallic alloys, several phases with discrete thickness have been observed, such as the segregated bilayer structure believed to be responsible for the liquid-layer embrittlement effect [7]. However, despite

The study of grain boundary phase transitions is an emerging field until recently dominated by experiments. The major bottleneck in the exploration of this phenomenon with atomistic modeling has been the lack of a robust computational tool that can predict interface structure. Here we develop a computational tool based on evolutionary algorithms that performs efficient grand-canonical grain boundary structure search and we design a clustering analysis that automatically identifies different grain boundary phases. Its application to a model system of symmetric tilt boundaries in Cu uncovers an unexpected rich polymorphism in the grain boundary structures. We find new ground and metastable states by exploring structures with different atomic densities. Our results demonstrate that the grain boundaries within the entire misorientation range have multiple phases and exhibit structural transitions, suggesting that phase behavior of interfaces is likely a general phenomenon.

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.