Observations of grain-boundary phase transformations in an elemental metal

The theory of grain boundary (the interface between crystallites, GB) structure has a long history¹ and the concept of GBs undergoing phase transformations was proposed 50 years ago^2,3. The underlying assumption was that multiple stable and metastable states exist for different GB orientations^4-6. The terminology 'complexion' was recently proposed to distinguish between interfacial states that differ in any equilibrium thermodynamic property⁷. Different types of complexion and transitions between complexions have been characterized, mostly in binary or multicomponent systems^8-19. Simulations have provided insight into the phase behaviour of interfaces and shown that GB transitions can occur in many material systems^20-24. However, the direct experimental observation and transformation kinetics of GBs in an elemental metal have remained elusive. Here we demonstrate atomic-scale GB phase coexistence and transformations at symmetric and asymmetric [Formula: see text] tilt GBs in elemental copper. Atomic-resolution imaging reveals the coexistence of two different structures at Σ19b GBs (where Σ19 is the density of coincident sites and b is a GB variant), in agreement with evolutionary GB structure search and clustering analysis^21,25,26. We also use finite-temperature molecular dynamics simulations to explore the coexistence and transformation kinetics of these GB phases. Our results demonstrate how GB phases can be kinetically trapped, enabling atomic-scale room-temperature observations. Our work paves the way for atomic-scale in situ studies of metallic GB phase transformations, which were previously detected only indirectly^9,15,27-29, through their influence on abnormal grain growth, non-Arrhenius-type diffusion or liquid metal embrittlement.