Fig. 1. Calculated helium concentration and selected positron stopping profiles. Also shown are implanted concentration profile of helium with depth (left axis) as calculated by the SRIM code. Stopping profiles of positrons corresponding to the energies used in this study are superimposed (right axis). Note that the maximum depth probed at 12 keV is somewhat less than 400 nm.

Fig. 2. Helium concentration profile averaged over distance (depth) by reflecting the shape of the stopping profiles of the positrons used in this study. The profile of the helium concentration (cHe) was calculated for each of the two experimental techniques used for microstructural characterization. The overlapping regions indicate the probing depth, to which both techniques are sensitive. For instance, in depth of 400 nm, local cross-sectional TEM analysis probes a region with actual concentration of helium of 3000 appm. Due to the broadening of the Makhovian profile, slow positron beam with a mean implantation depth of 400 nm provides information from a volume characterized by average helium concentration of 20,000 appm.

Fig. 3. Bright-field TEM micrograph of the as-received sample of the ODS Eurofer steel. Oxide nanoparticles of various sizes are indicated by arrows.

Fig. 4. Cross-sectional TEM image of the two studied materials. The Fe9Cr steel (a) and its ODS variant (b) including simulated SRIM implantation profiles (linear scale, normalized to the area under the curve) and magnified insets of the 470-600 nm region. The peak values calculated by SRIM correspond to 29.3 dpa and 46 at.% He.

Fig. 5. Average bubble density and diameter obtained from TEM and PALS analyses. Average number density (a) and mean bubble diameter (b) obtained from the TEM analysis of the two studied materials, plotted as a function of the helium concentration. The error bars are obtained as a standard deviation of TEM analysis performed by three independent analysts. (a) includes quantitative data obtained from PALS analysis. (b) shows an extrapolation of the bubble size towards zero-damage, considered as a parameter in the bubble swelling calculations using the PALS technique. The qualitative information could not be precisely obtained from the PALS analysis due to unknown He/vacancy ratio.

Fig. 6. TEM micrograph of ODS Eurofer implanted by helium at 500 °C to fluence 5 × 1020 m-2. Some of the cavities associated with oxide particles are highlighted by red circles.

Fig. 7. Positron mean lifetime and DBS S parameter as a function of helium concentration. A relative increase in the average positron lifetime (a) and Doppler broadening S parameter (b) as a function of cHe. These data are presented as a ratio of the results of the measurements on implanted and virgin samples.

Fig. 8. Swelling as a function of displacement damage in Fe9Cr steel and its ODS variant. Positron stopping profiles are considered only in the calculation of dpa based on PALS data. All TEM data, as well as the top X-axis, are determined for the specific discrete region of the implantation peak. The behavior of swelling $\text{ }\!\!\Delta\!\!\text{ }V/{{V}_{0}}$ caused by the implantation of helium can be well approximated by an exponential decay function (1) of helium concentration cHe(appm)

Fig. 9. Schematic illustration of various lattice and interfacial defects in the studied materials. (a), (b) and (c) represent microstructure of ferritic Fe9Cr alloy (blue circles), while (d), (e) and (f) represent the same alloy containing nanoscale oxide dispersoids represented as green circles. A higher density of open-volume defects in ODS Fe9Cr results in a higher rate of positron trapping (represented by 511 keV annihilation gamma quanta in (a) and (d)), along with a more effective sinking of radiation-induced defects and helium atoms. The nucleation and coalescence of helium bubbles occur at lower helium concentrations in Fe9Cr alloy (b, c), compared to its ODS variant (f). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).