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^4,5,6. 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^{8,9,10,11,12,13,14,15,16,17,18,19}. Simulations have provided insight into the phase behaviour of interfaces and shown that GB transitions can occur in many material systems^{20,21,22,23,24}. 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 \([11\bar{1}]\) 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^{9,15,27,28,29}, through their influence on abnormal grain growth, non-Arrhenius-type diffusion or liquid metal embrittlement.

晶界（微晶之间的界面，GB）结构理论具有悠久的历史¹，GB 经历相变的概念是在 50 年前提出的^2,3。^{基本假设是对于不同的 GB 方向4,5,6}存在多个稳定和亚稳态。最近提出了术语“肤色”来区分在任何平衡热力学性质上不同的界面状态⁷。不同类型的肤色和肤色之间的过渡已被表征，主要在二元或多组分系统中^{8,9,10,11,12,13,14,15,16,17,18,19}. 模拟提供了对界面相行为的洞察，并表明 GB 转变可以发生在许多材料系统中^{20,21,22,23,24}。然而，GBs 在元素金属中的直接实验观察和转变动力学仍然难以捉摸。在这里，我们展示了元素铜中对称和不对称\([11\bar{1}]\)倾斜 GBs 的原子尺度 GB 相共存和转换。原子分辨率成像揭示了两种不同结构在 Σ19b GB 的共存（其中 Σ19 是重合位点的密度，b 是 GB 变体），与进化 GB 结构搜索和聚类分析^{一致 21,25,26}. 我们还使用有限温度分子动力学模拟来探索这些 GB 相的共存和转变动力学。我们的结果证明了 GB 相如何被动力学捕获，从而实现原子级室温观察。我们的工作为金属 GB 相变的原子尺度原位研究铺平了道路，这些相变以前仅通过它们对异常晶粒生长、非阿伦尼乌斯型扩散或液体的影响而间接检测到^{9、15、27、28、29}金属脆化。