The influence of grain size on the displacement damage creation, D retention and D transport in tungsten

In a future fusion reactor neutron irradiation will create displacement damage which influences material properties, such as material strain and strength. One of the options to improve the behaviour of the material is by changing the microstructure of the material. Here we have studied how bulk microstructure, i.e. grain size in the material, influences the accumulation of radiation damage in tungsten (W). One of the hypotheses is that possibly less damage could be created due to defect annihilation at grain boundaries. To create displacement damage, we have used high energy W ions which are a good proxy for neutron irradiation, excluding transmutation, helium production and most importantly activation of the material. The amount of created defects was accessed by performing hydrogen isotope (HI) retention measurements, since lattice defects act as trapping sites for HIs with high de-trapping energy as compared to the energy of HI diffusion between solute interstitial sites. Therefore, the hydrogen isotope concentration can be treated as a measure of defect content present in the material. The effect of microstructure on the generation of radiation damage was studied previously with W single crystal and polycrystalline W with grain sizes between 1 and 50 µm [1]. There, no significant effect was observed, with all samples showing similar D concentration in the damage zone. By using a laser-deposited, nano-crystalline W layer on a W substrate we proceeded with the study and went down with the grain size to the nanometer scale. The as-deposited layer had a grain size of few nanometers. By tempering, grain sizes of a few hundred nm up to few µm were adjusted. Samples were irradiated at 290 K by 20 MeV W-ions to create displacement damage down to 2.3 µm and with a maximum damage dose of 0.23 dpa. The concentration of defects was assessed by exposing samples to deuterium (D) atoms with an energy of 0.3 eV at 600 K and to 300 eV D ions at 450 K. In both cases D populates the created and existing defects. D retention and D depth profiles were measured by nuclear reaction analysis utilizing D(3He,p)4He nuclear reaction. In the nanograined samples D populated the damaged region more than three times faster than in samples with grain size of hundred nm and few micrometer size grains. The concentration of defects was assessed by the final D concentration in the samples. Samples with smaller grain size showed larger D concentration in the irradiated area. However, large D concentration in the non-irradiated sample showed that defect density was already high in the initial material. Samples were also analysed by transmission electron microscopy to analyse the damage distribution in the material where nanometer-size voids were observed.