Experimental investigation of high He/dpa microstructural effects in neutron irradiated B-alloyed Eurofer97 steel by means of small angle neutron scattering (SANS) and electron microscopy

Highlights: • small angle neutron scattering (SANS) measurements have been carried out on B-alloyed Eurofer97-1 neutron irradiated HFR JRC-Petten, up 16 dpa at 400 °C/450 °C, with expected helium concentrations, produced via B activation, of 400 appm and 5600 appm. • the results show that increasing the expected helium concentrationfrom 400 appm to 5600 appm an increase of roughly one order of magnitude in the SANS cross section is observed, together with a drastic change of the steel matrix, appearing almost non-magnetic. • under the assumption that the observed SANS effects are mostly due to helium, the obtained helium bubble volume fractions of 0.007 and of 0.038, respectively for 400 appm and 5600 appm helium contents, are consistent with the irradiation conditions; the agreement between SANS obtained volume distributions and TEM histogram is satisfactory. - Abstract: High He/dpa microstructural effects have been investigated, by means of small-angle neutron scattering (SANS) and transmission electron microscopy (TEM), in B-alloyed ferritic/martensitic steel Eurofer97-1 (0.12 C, 9 Cr, 0.2 V, 1.08 W wt%, B contents variable between 10 and 1000 ppm), neutron irradiated at the High Flux Reactor of the JRC-Petten at temperatures between 250 °C and 450 °C, up do a dose level of 16 dpa. Under these irradiation parameters, B activation is expected to produce corresponding helium contents variable between 80 and 5600 appm, with helium bubble distributions relevant for the technological applications. The SANS measurements were carried out under magnetic field to separate nuclear and magnetic SANS components; a reference, un-irradiated sample was also measured to evaluate as accurately as possible the genuine effect of the irradiation on the microstructure. Increasing the estimated helium content from 400 to 5600 appm, the analysis of the SANS cross-sections yields an increase in the volume fraction, attributed to helium bubbles, of almost one order of magnitude (from 0.007 to 0.038); furthermore, the difference between nuclear and magnetic SANS components is strongly reduced. These results are discussed in correlation with TEM observations of the same samples and are tentatively attributed to the effect of drastic microstructural changes in Eurofer97-1 for high He/dpa ratio values, possibly relating to the dissolution of large B-carbides due to transmutation reactions.