The SPNI have produced a very large database on changes in strength and toughness properties up to «20 dpa, 1800 appm He, and 450 °C. The focus here is on two topics: (1) helium-induced hardening and (2) helium embrittlement.

1.06.4.2.1 Helium effects on tensile properties and He-induced hardening effects

The tensile properties of various FMS and AuSS after SPNI have been extensively investigated in the last decade.17,216,217,220-231 The tests were conducted at either ambient or the irradiation temperature.

The SPNI data are first compared with those for neutron irradiations of FMS, which show significant irradiation-induced hardening at <325 °C, decreas­ing hardening between 325 and 400 °C, and little or no hardening at greater than 400 °C.20 In the neutron case, the hardening measured as changes in the yield stress (Asy), initially increases with the square root of dpa but approaches a saturation level at higher doses. The saturation hardening depends on the irradia­tion temperature, and is «480 MPa at 10dpa and 200-300 °C. The uniform elongation (eu) following neutron irradiation decreases to less than 1% within several dpa. The postnecking strains are less affected, and the corresponding total elongation (et) decreases to between 3 and 10%. Figure 30 compares the SPNI yield strength increase data (Asy) with neutron data trends. Up to «10-13 dpa, the SPNI Asy are gener­ally similar to, or slightly lower than, the neutron data trends. However, the SPNI Asy do not saturate up to the maximum dose of 20 dpa, where the hard­ening reaches remarkable levels in excess of 700 MPa. The higher increment of SPNI hardening is even more pronounced at 350 °C and extends to well above 400 °C.17,231 The additional hardening above 10 dpa is primarily attributed to helium bubbles and, perhaps, with an additional contribution from the

higher loop density. Again note that, in some cases, the refined microstructures may be due to variable temperature history effects noted previously.

Figure 31 shows that the corresponding et of FMS after SPNI (symbols)221,222 is generally similar to, or only slightly less than, for neutron irradiations at 325 °C (solid lines)232 up to about 10-12 dpa for irradiations between 80 and 350 °C and room tem­perature tests (note that the modest differences in total elongation may be at least partly due to differ­ences in the size and geometry of the tensile speci­mens). However, at higher doses between 10 and 18 dpa and «750-1300 appm He, the et of FMS fol­lowing SPNI approaches 0 and, in some cases, the tensile specimens break during elastic loading at frac­ture stresses less than the yield stress, ay The fracture surfaces of the high-dose SPN-irradiated samples show a mixed brittle IG and transgranular cleavage fracture,227 similar to that observed in T91 and EM10 FMS after implantation at 250 °C with 2500 appm He producing 0.4 dpa.28

In contrast to the «20 dpa and 1800 appm He data reported here, high levels of hardening that can be attributed to He have generally not been observed previously, either in high-energy implanta­tion studies, at less than «500 appm He,233,234 or in low-temperature SPNI.225 Indeed, excess hardening was not observed in the LANSCE irradiations at <100 °C at He levels up to 2000 appm. Helium implantation at 200 °C235,236 and 250 °C28 indicated that significant hardening due to He occurred only at high He concentration levels above «5000 appm. All these results suggest that at less than 400 °C and «600 appm He, irradiation hardening is dominated by defect clusters and loops. Coupled with the small size of He-vacancy clusters (< 1 nm), a partial expla­nation may be found in recent molecular dynamics (MD) simulations237,238 showing that He bubbles can cause significant hardening but that their contribu­tions are reduced if they are overpressurized.

The data in Figure 32 more directly show the hardening contributions of He in STIP irradiated alloys that were annealed at 600 ° C for 1 h to remove the defect clusters and loops, while leaving the more stable bubbles unaffected.239 The residual bubble hardening is significant and increases with the square root of dpa and He. Note that the latter pro­vides a crude measure of the volume fraction of bubbles. The data in Figure 32 were combined with TEM measurements of 4> and Nb and were used to evaluate the dislocation obstacle strength (a) of the «1 nm bubbles, based on the relation Acy « 3ADPH

Figure 31 Dose dependence of total elongation of FMS irradiated in STIP at <350 °C and tested at RT. Trend lines for fission reactor irradiations are shown for comparison. Reproduced from Dai, Y.; et al. J. Nucl. Mater. (2011), 415, 306.

sy « 3ADPH « 3aGb^Nbdb, yielding an estimated a « 0.1. Note that this value of a is much lower than estimates based on MD simulations discussed in Section 1.06.5. This difference may be because strength superposition effects for combining bubble obstacles with preexisting strengthening features in the FMS were not accounted for in this evaluation. Strength superposition effects may also help rationalize the smal­ler hardening from bubbles below about 500appm He. Combining estimates of a > 0.1 with the TEM data
discussed in the previous section suggests that signif­icant hardening by bubbles (and voids) will extend to temperatures up to 500 °C at high He levels.