Nanocrystalline tungsten at high radiation exposure
Swelling and microstructural evolution of nanocrystalline (NC) tungsten are investigated by atomic scale simulations exploring the low temperature, high radiation exposure limit. Statistical analysis of microstructures containing at least a million atoms, with the grain size varying from 5 nm to 20 nm, suggests that their evolution is dominated by the spatially fluctuating stress fields of point and extended defects generated by irradiation, and their interaction with structural distortions associated with grain boundaries. Smaller grain size samples exhibit greater resistance to swelling due to the combined effect of a higher integral volume of grain boundary defect-denuded zones and the greater initial excess volume, where the latter decreases as a function of dose and partially compensates the effect of volumetric expansion due to the accumulation of radiation defects. Grain boundaries do not act as static sinks for defects, but evolve and annihilate defects through the rearrangement of their local atomic configurations. The assessment of grain coarsening appears sensitive to the visibility criterion applied to the identification of small grains, where the grain size distribution broadens as a function of dose. Variations of crystal structure, dislocation density, and stress distribution suggest that irradiated NC materials approach the asymptotic dynamic steady state configuration at a higher radiation exposure than the structurally simpler single crystalline materials. A significant feature of evolution of NC materials under irradiation is that due to the spatial limitations imposed by the size of the grains, the formation of an extended dislocation network is suppressed, and only individual dislocation loops can be identified in the microstructure.