Competitive roles of dislocations on blister formation in polycrystalline pure tungsten

Dislocation in tungsten is a significant contributing factor to blister formation in terms of both the nucleation and growth of bubbles under high-flux deuterium (D) ion irradiation. Unfortunately, the focus of previous publications was limited to only extreme cases which was either dislocation-rich or recrystallized. Therefore, this study aims to further improve the understanding of the microstructure dependence of blister formation in pure tungsten based on a more detailed comparison. The modification of the surface structure after D exposure has been investigated using various tungsten specimens with individual dislocation densities introduced by different annealing processes. The experimental results demonstrated that the number density of blisters and D retention increased with decreasing dislocation density, but decreased again when the tungsten material was fully recrystallized. Such an unexpected appearance of blisters was understood based on the competitive roles of intrinsic dislocations in blister formation by three aspects: trapping site, diffusion channel, and trigger of local plastic deformation necessary for blister growth. It was finally concluded that the dislocation mainly acted as a diffusion channel under the present exposure conditions wherein the irradiation temperature was quite high and the exposed materials possessed the recovered dislocation structure, thereby resulting in the acceleration of blister formation with decreasing dislocation density due to the lowered out-diffusion rate of D. However, in recrystallized tungsten, blister formation was restrained by the poor plasticity as well as the complete lack of trap sites.