Insight into defect cluster annihilation at grain boundaries in an irradiated nanocrystalline iron

•Defect clusters dynamic were examined using in-situ irradiation TEM.•One-dimensional loop hop to GB was revealed.•Burgers-vector rotation mechanism near GB was revealed.•Cluster dynamics marked effects on the morphology of the irradiated microstructure. The design of radiation-tolerant polycrystalline materials has been mainly based on the control and manipulation of grain boundaries (GBs) that leads to annihilation of point defects at grain-boundary neighborhoods thus resulting in the formation of defect denuded zones. Nanocrystalline materials are potential candidates providing large density defect sinks for individual point defects and small highly mobile defect clusters (DCs). Using the in-situ irradiation transmission electron microscopy (TEM) technique, this study not only experimentally revealed the coalescence of small glissile DCs at/near GB and their subsequent annihilation at grain-boundary, but also provide insight into defect cluster dynamics. The small DCs were found to be transported to grain-boundary neighborhoods where they can annihilate by the one-dimensional loop hop or Burgers-vector rotation mechanism to GBs, considered as the main contribution to the long-range flux of interstitials to GB sinks. This process had marked effects on the morphology of the irradiated microstructure in nanocrystalline iron, limiting the length of DC strings and reducing the coalescence of DCs into large clusters (dislocation loops). [Display omitted]