Influence of annealing and keV-ion implantation into tungsten on deuterium retention and permeation: The dependence on bulk and surface microstructure

Abstract

The microstructure of tungsten, a primary candidate for the first wall of Tokamak devices, affects the retention and permeation of hydrogen isotopes. These have a close connection to the tritium inventory. In this work, the tungsten microstructure was tuned by annealing and intense 15,700 displacements per atom (dpa) keV-W-self-ion implantation. The samples were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The results reveal that the annealing treatment has little influence on the crystallite size but decreases the number of defects such as vacancies, which relates to the bulk microstructure. Moreover, ion implantation changes the sample surface from a smooth quasi-mirror to a grey morphology with various voids. The deuterium retention is suppressed by the decrease of bulk defects due to annealing, but does not present a saturation with the displacement damage increase. It is interesting to note that in permeation, the rate limiting step is bulk diffusion while the deuterium permeability is enhanced for samples with a surface modification. The possible permeation mechanisms and their relation with the sample microstructure are discussed in this work.

Keywords

• Tungsten
• Deuterium
• Retention
• Permeation
• Ion implantation
• Annealing