In situ high-resolution transmission electron microscopy is used to study grain boundary structure and kinetics in bicrystalline Au films at elevated temperature. We report the first direct evidence for the existence of cooperative atomic motion in grain boundary migration. Certain nanoregions at grain boundaries, typically involving up to several hundred atoms, are found to switch back and forth between neighboring grains. Reversible structural fluctuations at temperatures near ${0.5T}_m$ and above have been discovered in [110] and [001] tilt, as well as in general grain boundaries.

• Received 3 December 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.225501