N.V. Luzginova

Helium implanted RAFM steels studied by positron beam Doppler Broadening and Thermal Desorption Spectroscopy

Reduced Activation Ferritic/Martensitic steels are being extensively studied because of their foreseen application in fusion and Generation IV fission reactors. To mimic neutron irradiation conditions, Eurofer97 samples were implanted with helium ions at energies of 500 keV and 2 MeV and doses of 5x1015-1016 He /cm2, creating atomic displacements in the range 0.07–0.08 dpa. The implantation induced defects were characterized by positron beam Doppler Broadening (DB) and Thermal Desorption Spectroscopy (TDS). The DB data could be fitted with one or two layers of material, depending on the He implantation energy. The S and W values obtained for the implanted regions suggest the presence of not only vacancy clusters but also positron traps of the type present in a sub-surface region found on the reference sample. The traps found in the implanted layers are expected to be HenVm clusters. For the 2 MeV, 1016 He/cm2 implanted sample, three temperature regions can be observed in the TDS data. Peaks below 450 K can be ascribed to He released from vacancies in the neighbourhood of the surface, the phase transition is found at 1180 K and the peak at 1350 K is likely caused by the migration of bubbles.

Dose effects in He implanted Eurofer97 steel

Reduced Activation Ferritic/Martensitic steels are being extensively studied because of their foreseen application in fusion reactors. To reproduce neutron irradiation conditions, Eurofer97 samples were implanted at room temperature with helium ions at energies of 500 keV and 2 MeV and doses of 1x1015-1017 He /cm2. The implantation induced defects were characterized by positron beam Doppler Broadening (DB). The samples were annealed in the range 300 – 1500 K, in 100 K steps. As the temperature increases, the annealing of vacancies and vacancy clusters is noticed and followed by the coalescence of HenVm clusters. At temperatures around 1200 K HeV pairs dissociate and bubbles are formed. Above 1300 K the helium release from bubbles is observed. The S-W graphs reveals that the samples have similar positron traps up to 1200 K. At 1200 K helium bubbles are noticed and the S,W pair shows a clearly distinct behaviour from the S,W values of vacancy type defects. As the temperature increases and helium is release, the S,W pairs shift towards the S,W’s of vacancies.

Ageing Behaviour of Neutron/Irradiated Eurofer97

Eurofer97 is a candidate structural material for nuclear fusion reactors. To better understand the ageing effects due to radiation during long-term use in real fusion conditions, Eurofer97 was neutron irradiated at the High Flux Reactor in The Netherlands. TEM images of post-irradiation Eurofer97 reveal a high density of irradiation-induced dislocation loops, fine precipitates and agglomeration of point defects. Microscopy results are related to post-irradiation tensile mechanical tests done at room temperature and at 300 ℃. An increase of yield and ultimate tensile strength combined with a decrease of total elongation is observed in both tests and correlated with the presence of radiation damage.