In situ observation of atomic-scale processes accomplishing grain rotation at mixed grain boundaries

A detailed monitoring of atomic-scale processes accomplishing grain rotation is important for understanding the deformation mechanisms in nanocrystalline metals. However, direct observations have been rare thus far, such that our knowledge about grain rotation has to rely heavily on hypothetical models and simulations. Here, we present in situ atomic-resolution evidence, indicating that the atomic processes accomplishing grain rotation in nanocrystalline Pt depend on the type of grain boundary (GB) separating the rotating grains. The grains around general (mixed) GBs rotate via the Frank–Bilby dislocation activities together with atomic shuffling and disconnection activities, the former being the generation, climb, glide and reaction of GB dislocations, whereas the latter leading to GB migration accompanying the grain rotation. While for the grains with a tilt GB in between: their rotation is accomplished almost entirely via the Frank–Bilby dislocation activities. We also discover that the GB dislocation climb, glide, and reaction often involve the formation and destruction of Lomer-like dislocations.