Grain boundaries (GBs) are important structural elements in polycrystalline materials, significantly influencing their mechanical, physical, and chemical properties.
Methods
Experiments: Copper (Cu) thin films were grown on sapphire substrates using molecular beam epitaxy. GB structures were characterized using high-resolution scanning transmission electron microscopy (STEM).
Computations: Evolutionary algorithms were employed to predict GB structures, and molecular dynamics simulations were used to investigate the kinetics of GB phase transitions.
Key Findings
Two distinct GB structures, termed "pearl" and "domino" structures, were observed to coexist at symmetric and asymmetric [111] tilt GBs.
The pearl structure was identified as the ground state, while the domino structure was found to be metastable.
Transitions between the pearl and domino structures were observed, indicating a kinetically limited phase transformation.
The coexistence of both structures at room temperature is attributed to the suppression of the GB phase transition kinetics at low temperatures.
Provides the first experimental observation of GB phase transitions in an elemental metal.
Reveals the kinetic nature of GB phase transitions.
Offers new insights into the influence of GB phase transitions on material properties.
Fig. 1. High-resolution electron microscopy images reveal two types of grain boundary (GB) phase transitions in copper. The transitions occur at both symmetric and asymmetric tilt GBs, involving "domino" and "pearl" structures.
Fig. 2. Evolutionary grain boundary (GB) structure search and clustering analysis successfully predicted the two observed GB phases (pearl and domino) of the Σ19b (178)[111] GBs. The analysis considered GB energy, excess volume, and GB stress components (τ11 and τ22) to identify these distinct phases. The lowest-energy configurations for both phases were found to have the same number of atoms.
Authors
The first author of this work is Dr. Thorsten Meiners from the Max-Planck-Institut für Eisenforschung. Prof. Timofey Frolov from Lawrence Livermore National Laboratory and Prof. Christian H. Liebscher from the Max-Planck-Institut für Eisenforschung are the corresponding authors of this paper.
Meiners, T., Frolov, T., Rudd, R.E. et al. Observations of grain-boundary phase transformations in an elemental metal. Nature 579, 375–378 (2020). DOI: https://doi.org/10.1038/s41586-020-2082-6
Editor: Dr. Jun-Jing He
