In situ atomic observation of transformation twinning in nanocrystals

The formation of twinning in nanocrystals impacts their physical and chemical properties. However, the pathways of transformation twinning triggered by an external stimulus is not well-understood. Here, we investigated the transformation twinning of face-centered cubic (FCC) metal nanocrystals under electron beam irradiation, utilizing transmission electron microscopy (TEM) with high spatiotemporal resolution. We found that a bi-crystal twin can form through swap motion as well as via a structural destruction-repair mechanism. The destruction-repair of crystal structure during twinning involves structure disruption, defects formation, and subsequent recrystallization. Regarding twinning through swap motion, nanocrystals maintain a relatively intact crystal structure, while crystal defects form and propagate during twin nucleation. For the transformation twinning evolving from a bi-crystal twin to fivefold twins (or the reverse processes in detwinning), twinning is accompanied by the decomposition of the twin boundaries, interface migration, and strain modulations. Our results provide new mechanistic understandings of transformation twinning in nanocrystals.