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Nanocrystalline materials are expected to have improved radiation resistance as the high density of grain boundary area is thought to act as an effective sink for radiation-induced defects. However, continued absorption of defects can alter the structure of grain boundaries and/or enhance their mobility, eventually leading to microstructural degradation in the form of grain coarsening, thus negating their initial radiation tolerance. Hence, an ideal microstructure might be one with a mix of boundaries that are effective sinks and limit grain coarsening. We show through in situ electron irradiation experiments, however, that this is an insufficient condition. Our observations indicate that even a high density of low energy coherent twin boundaries, supposedly stabilizing the microstructure against grain coarsening, can be a detriment in that it biases the mobility of vacancies accumulating during irradiation thereby resulting in preferential void nucleation near twin boundaries. These observations highlight the fact that radiation induced grain boundary migration depends greatly on the topology of the grain boundary network and that the migration of high-angle grain boundaries can be hindered when coordinated at triple junctions composed of at least two low-energy boundaries, e.g., coincidence site lattice boundaries.

The dynamic behavior of nanoscale prismatic dislocation loops can significantly affect the microstructural variation in crystalline materials upon processes such as plastic deformation and high-energy particle irradiation. Using in situ transmission electron microscopy, this study experimentally demonstrates a reaction which follows the collision between two loops with different Burgers vectors in α-iron. Even after the formation of the junction, the reaction progresses further, unlike conventional reactions between dislocations of macroscopic length, and the larger loop finally absorbs the smaller one.

Transmission electron microscopy of high temperature annealing of pure tungsten irradiated by self-ions was conducted to elucidate microstructural and defect evolution in temperature ranges relevant to fusion reactor applications (500–1200 °C). Bulk isochronal and isothermal annealing of ion irradiated pure tungsten (2 MeV W+ ions, 500 °C, 1014 W+/cm2) with temperatures of 800, 950, 1100 and 1400 °C, from 0.5 to 8 h, was followed by ex situ characterisation of defect size, number density, Burgers vector and nature. Loops with diameters larger than 2–3 nm were considered for detailed analysis, among which all loops had b = 1 2 〈 1 1 1 〉 and were predominantly of interstitial nature. In situ annealing experiments from 300 up to 1200 °C were also carried out, including dynamic temperature ramp-ups. These confirmed an acceleration of loop loss above 900 °C. At different temperatures within this range, dislocations exhibited behaviour such as initial isolated loop hopping followed by large-scale rearrangements into loop chains, coalescence and finally line–loop interactions and widespread absorption by free-surfaces at increasing temperatures. An activation energy for the annealing of dislocation length was derived, finding E a = 1.34 ± 0.2 eV for the 700–1100 °C range.