The most recent source ofsignificant information on He effects on bulk microstructures and properties is from SPN irradiations of both AuSS and FMS to maximum doses of «20dpa and «1800 appm He at temperatures up to «450 °C.

1.06.4.1 Microstructural Changes

The microstructures of FMS alloys irradiated in STIP at temperatures below about 400 °C are domi­nated by defect clusters and bubbles, characterized by their respective number densities (Nd/b) and dia­meters (dd/b). Table 2 summarizes some recent TEM observations on FMS and AuSS after neutron and SPN irradiations. Figure 24 shows the development of defect structures in FMS F82H and T91 irra­diated in STIP-I209 at irradiation temperatures up to «360 °C. Note that the temperatures, dpa, and He levels (dose) are correlated with one another due to their mutual dependence on the proton flux. The microstructures in both FMS alloys are similar and are composed of small (1-3 nm) defects (likely small SIA cluster-dislocation loops, Figures 24(a)-24(d)), along with a lower number of larger dislocation loops (>3nm). Figure 25(a) plots the loop data for F82H as a function of irradiation temperature for the STIP-I and — II irradiations, along with some neutron data, including from an ISHI experiment. The STIP-I irradiation conditions were «10-12 dpa with He levels that increase with temperature from 560 to 1115 appm. The STIP-II data were at «10-19 dpa with between 750 and 1790 appm He. Note that the STIP-I irradiation initially ran at a much lower irradiation temperature while the STIP-II irradiation was more isothermal. Ignoring these confounding factors, the overall trends show that the dd increases with temperature while Nd decreases, after apparently peaking at «250 °C. The neutron data point at 10 dpa and 310 °C, with a low He content, following irradiation in the Peten High

Flux Reactor (HFR), is similar to the STIP-II data at higher He levels. The neutron data 400 °C from an ISHI study with 90 appm He at 3.8 dpa215 appears to be more consistent with the overall data trends than the corresponding STIP-II data point; however, this may be because of the lower dose in this latter case (Figure 25(b)).

As shown in Figure 26, the He bubble populations in the two FMS irradiated in STIP-I to 10 dpa at 295 °C are also similar. Due to the resolution limits, conventional TEM cannot image bubbles smaller than about 1 nm. Thus, bubbles are visible only at He concentrations and temperatures above «500 appm and «170 °C, respectively, in the STIP database.66 Figure 25(b) shows that 4> increases and Nb decreases with increasing temperature in the STIP — I and — II irradiations. The neutron data at 500 °C, from an ISHI study with 380 appm He at 9 dpa, is consistent with the STIP data trends.215 The smallest bubbles found in the SPNI studies, with an average diameter of «0.7 nm, were observed in F82H from the STIP-I irradiation to 9.9 dpa/560 appm He at 175 °C. Bubbles were not observed at lower tempera­tures and He levels.17,66,209,216,217 In this case, the He is presumably located in a very high concentration of subvisible He-V clusters, which may be overpressur­ized. At higher temperatures of 350 and 400 °C in the STIP-II irradiation, the apparent sizes of the bubbles are much larger than those for STIP-I.

Figure 27 shows the cavity structure in two F82H samples irradiated in STIP-II (left) and STIP-III
(right) to similar dose and He concentration at nomi­nal temperatures of 400 ± 50 and 440 ± 50 °C. Some of the cavities in the 400 °C STIP-II irradiation have transitioned from bubbles to larger voids, forming a bimodal size distribution, while there is a larger Nb of smaller bubbles in the 440 °C STIP-III case, with a monotonic size distribution. These differences are believed to be due to the fact that the STIP-III irradiation also ran at lower temperatures of about 200 °C during the initial phase of the irradiation up to «0.3 dpa and «30 appm. It is believed that the high Nb («5.1 x 1023 m~3) nucleated during this transient and then grew at higher temperatures and He levels. Thus, due to the large Nb, voids did not form in this case. In contrast, voids formed in the STIP-II irradi­ation, since it was more isothermal, resulting in a lower Nb («2.4 x 1023m~3) and closer to the peak swelling temperature for FMS of 400 °С.1 Large voids that are associated with precipitates are shown in Figure 28(a) for a STIP-II irradiation at 400 ± 50 °С. The corresponding microstructure for irradiations at lower irradiation temperatures of «350 ± 50 °С is again composed of a high density of small bubbles. Thus, the increase in the average sizes of the cavities in Figure 25 at 350 and 400 °С in STIP-II is due to the bimodal mix of bubbles and voids.

The strings of bubbles seen in Figure 28(b) also indicate a strong association between bubbles and dis­locations. Further, neither bubble denuded zones near GBs nor a significant number of grain boundary

bubbles have been found in STIP samples investigated to date. This suggests that grain boundary bubbles may be too small to image.

SPN irradiations also produce defect clusters, faulted Frank loops, and bubbles in AuSS (316L

60

50

40

10

Figure 25 Irradiation temperature dependences of (a) loop size and density and (b) bubble size and density of F82H specimens irradiated in STIP with SPN, in HFR with fission neutrons, and in HIFR with in situ He implantation (ISHI).

Figure 26 He bubble structures of (a) F82H irradiated to 9.7 dpa/670appm He and (b)T91 irradiated to 10.1 dpa/725appm He at 295°C. The scale shown in (b) is the same for (a). Reproduced from Jia, X.; Dai, Y. J. Nucl. Mater. 2003, 318, 207.

average sizes between 2.5 and 3.9 nm. In contrast, neutron irradiations of AuSS produce a lower density of larger loops. For example, mixed spectrum reactor irradiations at 400 °C, which produce smaller but significant amounts of He compared with the SPNI case, result in «30 times fewer and «6 times larger loops. The differences are even larger for fast reactor irradiations with much lower He levels. Thus, it appears that high He results in significant refinement of the loop structures in AuSS.

Figure 29 shows small 1-2 nm bubbles in 316LN AuSS for both 285 °C irradiations to 9.3 dpa and 705appm He at 285 °C and 19.4 dpa and 1800 appm He at 425 °C. Unlike the case of FMS irradiated at «400 °C, no large voids were observed in this case. The refinement of the bubble structures in SPNI with high He levels is even more profound. The mixed spectrum reactor irradiations spectrally tailored to produce «11 appm He/dpa produced 60 times fewer cavities with a bimodal distribution of bubbles and voids compared with the SPNI case with «7 times more He. The average cavity diameter is about 2.4 times larger in the mixed spectrum
neutron case. These observations are highly consis­tent with the concepts described in Section 1.06.3 with regard to higher He and Nb suppressing void formation. Again, however, the likely effects of the temperature history confound quantitative interpre­tations of both the loop and cavity microstructures in the AuSS as well as FMS.