Resumen de Effects of ion irradiation on oxide dispersion strengthened steels for fusion reactors
Nuclear fusion is one of the most sustainable solutions to the current global energy and climate crisis. Structural materials part of future nuclear fusion reactors will have to withstand the high doses consequence of the 14 MeV neutron irradiation, operating temperatures that could be as high as 700 °C, and large amounts of He and H produced by transmutation reactions. One of the main structural candidate materials are oxide dispersed strengthened reduced activation ferritic steels (ODS RAFS). A fine, homogeneous, dispersion of thermally stable nanoparticles such as Y2O3 would improve the mechanical performance of these steels, enhancing creep strength by impeding dislocation motion and increasing their upper operating temperatures. Moreover, they could also improve radiation damage resistance. However, more investigations are needed to clarify the effect of irradiation on these materials.
The main objective of this thesis is to investigate the microstructural stability of different ODS RAFS with nominal composition Fe–14Cr–(2W–(0.3-0.4)Ti)–0.3Y2O3 (wt%) (referred to as ODSM, ODS1, ODS2 and ODS3) and model Fe-14Cr (wt%) alloys (referred to as M1 and M2) subjected to ion irradiation to simulate the effect of a fusion environment.
Single irradiations with 1 MeV Fe+, to a damage of 15 dpa were performed to -80 °C and RT, and single 1MeV Fe+ irradiation to damage of 22dpa was performed at RT. Single 50 keV He+ irradiation was performed to 8000 appm at RT, 4700 appm at 400 °C and 8000 appm at 450 °C. Two sequential dual irradiations were perfomed with alternating ion order. SEQ He/Fe irradiation was performed with 50 keV He ions and 1 MeV Fe ions at RT to 8000 appm and 15 dpa, respectively. SEQ Fe/He irradiation was performed with 1 MeV Fe ions and 45keV He ions at RT to 22 dpa and 8000 appm, respectively. Dual simultaneous irradiation SIM Fe/He was performed with 3.6 MeV Fe3+ and 800 keV He+ at 400 °C to 30 dpa and 4700 appm respectively. Simultaneous triple irradiation was performed with 14 MeV Fe5+, 1.6 MeV He+, and 500 keV H+ at 600 °C to 30 dpa, 600 appm and 1500 appm, respectively. Ion irradiations have been carried out at the JANNUS-Saclay, CMAM and CIEMAT facilities.
The stability of the nanoparticles present in the ODS steels has been investigated by Transmission electron microscopy (TEM), Scanning electron microscopy (SEM) and Atom Probe Tomography (APT). The irradiation induced damage has been characterized in all materials, mainly focusing in the study of open volume defects and irradiation induced bubbles, by TEM, Positron Annihilation Spectroscopy (PAS) and/or nanoindentation. Transmission electron microscopy has been carried out at the Department of Materials at the University of Oxford (UK) and at the Centro Nacional de Microscopía Electrónica (Madrid, Spain). Atom probe tomography and nanoindentation measurements have also been accomplished at the Department of Materials (University of Oxford). Slow positron annihilation measurements at NEPOMUC and ELBE facilities.
The microstructure of the unirradiated alloys has been characterized. The M1 model alloy has an equiaxed grain structure with mean size being 100 ± 60 µm. Precipitates observed by EDS are found to be Cr oxides in the size range 1-8 μm and few small impurities rich in Ca and Ni with sizes between 10 and 50 nm. No dislocations were observed by TEM. Similarly, grains of the M2 model alloy are also equiaxed in structure with larger sizes, being the mean size 170 ± 120 µm. This alloy contained no visible precipitates nor dislocations. The ODSM alloy has a homogeneous grain structure with grain sizes < 3 µm and Y rich nanoparticles dispersed throughout the matrix. TEM investigations found their mean size and mean number density to be 8 ± 6 nm and (2.1 ± 0.4) × 1022 m-3 respectively. The mean size and number densities of the nanoclusters found by APT are 2.6 ± 0.9 nm and (1.43 ± 0.14) × 1024 m-3 respectively. The ODS1 steel has a bimodal grain structure with nanometre-sized grains (200-800 nm) and recovered grains with sizes up to 15 µm. Nanoparticles are Y-Ti rich and their mean size and mean number density were measured to be 8 ± 5 nm and (2.4 ± 0.5) × 1022 m-3 respectively (TEM) and 2.9 ± 1.1 nm and (0.7 ± 0.2) × 1024 m-3 respectively (APT). The ODS2 steel was mainly investigated by HAADF-STEM. The mean size of the Y-Ti rich nanoparticles and their mean number density were measured to be 4 ± 2 nm and (6.3 ± 1.1) × 1022 m-3. The ODS3 steel was also investigated and the mean size and number density of Y-Ti rich nanoparticles was found to be 8 ± 6 nm and (1.4 ± 0.3) × 1023 m-3, respectively.
The stability of nanoparticles was investigated for the ODSM and ODS1 alloys irradiated with Fe+ at -80 °C. In the irradiated ODSM alloy, the mean size of Y rich nanoparticles is lower than the corresponding mean size in the non-irradiated sample as seen by TEM. Moreover, from the distribution of the sizes and distances between these nanoclusters investigated by APT, it can be concluded that the smallest nanoclusters are ballistically dissolved due to the low temperature irradiation. Results for the irradiated ODS1 steel show that, unlikely to the case of the model ODS alloy, it appears that the larger Y-Ti rich nanoclusters are being partially dissolved and there is a nucleation of new, smaller nanoclusters due to irradiation.
PAS and nanoindentation techniques have been applied to characterize the defects induced by Fe+ irradiation at -80 °C on the M2 model alloy and the ODSM and ODS1 steels. For all the samples, the S-parameter value was higher post-irradiation, indicating higher concentration and/or larger vacancy clusters forming under the irradiation. The DB measurements showed that the S-parameter for the model alloy after irradiation is higher than in the case of both ODS samples, highlighting the impact that nanofeatures, such as Y rich nanoparticles, alongside other sources of positron traps (grain boundaries, dislocations, precipitates, etc.) have on reducing the irradiation damage. Results of the CDB showed that, unlikely to the case of the unirradiated ODSM alloy, the role of the Y rich nanoparticles in the annihilation of positrons in the irradiated ODSM sample is not so predominant. This agrees with what is seen with APT, i.e. larger Y rich nano-oxides coarsen by the total dissolution of smaller ones, thus lowering the amount of Y rich positron traps in the sample. Nanoindentation results did not show much difference in hardness values before and after Fe+ irradiation at -80 °C.
The open-volume defects induced by Fe+ irradiations at RT have been characterized by PAS. Samples investigated were two model alloys irradiated up to different damages (M1 to 15 dpa and M2 to 22 dpa), and one ODS steel (ODS1 to 15 dpa). In all cases, it appears that the number of vacancies/their size have increased due to Fe+ irradiations at RT. Both model alloys M1 and M2 have a similar S-parameter in the region of maximum damage due to irradiation, which could point to the saturation effect of the positron traps. The increase of S-parameters for both irradiated model alloys is 18 %, while for previous -80 °C Fe+ irradiation that difference was 21 %, both measured at the maximum, so it appears that the temperature of these irradiations did not play a significant role and both -80 °C and RT irradiations can be considered low temperature. The ODS1 steel shows a higher resistance to radiation damage as it was also the case after -80 °C Fe+ irradiation. CDB was done for M2 and ODS1 samples. In the case of the M2 model alloy, the chemical environment is not changing between the deformation-induced and irradiation-induced positron traps, while the shape of the CDB profile reflects annihilation of positrons in the Fe-adjacent vacancies. The CDB profile of the irradiated ODS1 steel suggests the existence of traps associated with the Y signal that do not appear in the ODSM irradiated with Fe+ at -80 °C. These results support the APT investigations performed on the ODSM and ODS1 alloys irradiated at -80 °C, which show a better stability of the Y-Ti rich nanoclusters present in the ODS1 steel.
Room temperature He+ irradiations were performed for the model M1 alloy and ODS1 steel and have been investigated by PAS techniques. The effect of He+ irradiation is weaker compared to the Fe+, as predicted by the SRIM calculations. The change is more obvious in the model alloy M1 where the difference in S-parameters before and after irradiation is 9.5 %, compared to 3.3 % in the case of the ODS1 steel. CDB profiles point to some irradiation induced changes. In the case of the M1 alloy, the high momentum portion of the CDB profile has varied, thus it appears that the chemical environment of open-volume traps has changed due to the He+ irradiation, possibly due to creation He-vacancy complexes. In the case of the irradiated ODS1 sample, the CDB profile undergoes a change with respect to its unirradiated profile and becomes identical to the profile of the irradiated M1 sample suggesting that in the ODS1 after irradiation positron traps are not associated with nanoparticles but are in the ferritic matrix.
Five samples were irradiated at intermediate temperatures with He+ ions: two M1 model alloys and three ODS steels (ODS2 and ODS3). S-parameters of the M1 sample irradiated with He+ at 400 °C and 450 °C are very similar in the zone of maximum damage, despite the dose in the case of He+ irradiation at 450 °C being double than in the case of 400 °C irradiation. It appears that there is a saturation of positron traps or there is some annealing happening at 450 °C. These profiles are also similar to the S-parameter profile after RT He+ irradiation, showing almost no effect of the temperature of irradiation. This is not the case for the ODS1 sample irradiated with He+ at 450 °C, where there is a clear increase of the S-parameter in the zone of maximum damage. S-parameter profiles of both the ODS2 and ODS3 samples are very similar before and after He+ irradiation at 400 °C. This indicates a transition in the irradiation induced defects in ODS steels when increasing dose and temperature. CDB profiles were obtained for M1, ODS2 and ODS3 samples irradiated at 400 °C. All three investigated samples after the irradiation seem to have a similar CDB profile in the area of high momentum. This could mean that the type of positron traps is independent of the nanoparticles and associated to the ferritic matrix, as it was the case for RT He+ irradiation.
TEM investigations of irradiation induced bubbles were done for M1, ODS2 and ODS3 samples irradiated with He+ at 400 °C. In all three samples, small bubbles (< 2.5 nm) were observed. Bubble sizes are similar for the two ODS steels and smaller than the ones found for the model alloy, being the visible bubble number densities higher for the two ODS steels. Although the majority of bubbles are observed in the ferritic matrix, ODS steels seem to be effective in reducing the coarsening of bubbles created under these irradiation conditions.
The stability of nanoparticles in the ODS1 steel was investigated after the SEQ He/Fe irradiation at RT. While the chemical composition of the particles appears unaltered, results point to some coarsening of nanoparticles while their number density remains similar suggesting partial dissolution of the smaller ones.
The DB PAS investigations performed on the M1 model alloy after the SEQ He/Fe irradiation at RT suggest association of He with the newly formed vacancy-type defects introduced by the subsequent Fe+ irradiation either by coarsening of the He-vacancy complexes or migration of pre-implanted He atoms.
DB PAS investigations performed on the model alloys also show that the different sequence of ions in the sequential dual irradiations has a large effect on the defect structures developed. It appears that while for pre-irradiation with He+, some He ions may have remained as interstitials or trapped by impurities, in the case of post-irradiation, all He ions are trapped by the open volume defects previously introduced by the Fe+ irradiation.
In the case of the ODS1 steel after the SEQ He/Fe irradiation at RT, the He trapping effect in the newly formed open-volume defects is still present, although not as visible compared to model alloys probably due to the initial complex defect structure in this material.
TEM investigations of irradiation induced bubbles after RT SEQ He/Fe irradiation were performed on the M1 and ODS1 alloys. Although for these irradiation conditions visible bubble sizes are similar for the ODS1 steel compared to the M1 model alloy (< 2 nm), the lower visible number density measured for the ODS1 steel as compared to the M1 alloy would point to bubbles being below the detection limit in the ODS steel, in agreement with the lower irradiation temperature (RT) as compared to the single He+ irradiation (400 °C).
The DB PAS investigations accomplished on the M1 model alloy after the SIM He/Fe irradiation at 400 °C suggest a possible entrapment of all injected He ions into the open-volume defects simultaneously created by the Fe3+ irradiation. This may also be a consequence of the enhanced mobility of vacancies and interstitials due to the intermediate temperature of the irradiation, promoting their recombination. The ODS2 and ODS3 steels behave similarly to the model alloy after this irradiation. The CDB profiles for these three samples show that there is no clear relation between positron traps and He.
After simultaneous triple ion irradiation, coarsening of smaller nanoparticles and a decrease in nanoparticle number densities was observed by TEM in both ODSM and ODS1 steels, while the chemical composition remained the same. This suggests that larger nanoparticles grow at the expense of smaller ones.
In the ODSM alloy, APT investigations conclude that there is a slight coarsening of nanoclusters, in agreement with TEM results, with distances between them increasing post irradiation and their number density lowering. It appears that the smallest nanoclusters (< 2 nm) are being completely dissolved under the present irradiation conditions.
Unlike the model ODS alloy, nanoclusters in the ODS1 steel seem to be more stable under the high temperature triple-ion irradiation. The dissolution of nanoclusters in the case of this sample was not complete, as evidenced by distance and number density that have remained similar after the irradiation, which is attributed to the healing properties of Ti as an alloying element .
For both ODS alloys after simultaneous triple ion irradiation nanoindentation experiments show no irradiation induced hardening, presenting very subtle differences with respect to the unirradiated alloys that would point towards some recovery in the irradiated ODS1 sample.
Both alloys have developed small (< 4 nm) irradiation induced bubbles consistent with previous studies. Bubble sizes appear to be slightly smaller for the ODS1 steel, in agreement with a higher refinement induced by the Y-Ti rich nanoclusters.
