1.06.6.1 ISHI Studies and Thermal Stability of Nanofeatures in NFA MA957

In this section, we describe the status of developing a potentially transformational new class of materials, we call nanostructured ferritic alloys (NFA), with emphasis on He management for radiation tolerance as discussed in Section 1.06.3.2 , NFA manifest high tensile, creep and fatigue strengths, unique ther­mal stability, and remarkable irradiation tolerance. The outstanding characteristics of NFA result from the presence of an ultrahigh density of Y-Ti-O rich nanofeatures (NF). The multifunctional NF, which are remarkably thermally stable, impede dislocation climb and glide, enhance SIA-vacancy recombina­tion, and, perhaps most importantly, trap He in small, high-pressure gas bubbles. The bubbles reduce the amount of He reaching GBs, thus mitigating tough­ness loss at lower temperatures and potential degra­dation of creep rupture properties at higher temperature. He trapped in a high number density of small bubbles also mitigates many other manifesta­tions of irradiation effects, including void swelling.

As discussed in Section 1.06.2, ISHI in mixed spectrum fission reactor irradiations provides an attractive approach to assessing the effects of He-dpa synergisms. To reiterate, the basic idea is to use Ni (or B or Li)-bearing implanter layers to inject high-energy a-particles into an adjacent material that is simultaneously undergoing fast neutron-induced displacement damage. The a-particles can be pro­duced by two-step 58Ni(nth, g)59Ni(nth, a) thermal neu­tron (nth) reactions. A series of ISHI irradiation experiments have been carried out in HFIR. Micron — scale NiAl injector coatings were used to uniformly implant a-particles to a depth of «5-8 mm in TEM disks for a large matrix of alloys irradiated over a wide range of temperatures and dpa at controlled He/dpa ratios ranging from ^1 to 40appmdpa~ . Here, we compare the cavity structures in a 14Cr NFA, MA957, to those in an 8Cr FMS, F82H, following HFIR irradiation at 500 °C to 9 dpa and 380appm He. The experimental details are given

23,50,51

elsewhere.

Through-focus sequence TEM images were used to characterize the bubbles and voids, with care taken to avoid surface artifacts. Bubble-like features were generally not found in the unimplanted regions of either MA957 or F82H. As illustrated in a typical underfocused image in Figure 40(a), a high number density (Nb « 4.3 x 1023 m~3) of very small (average db « 1.2 nm) bubbles are observed in the NFA. The inserts in Figure 40(a) show examples of the decora­tion of larger features with cavities. Image overlap analysis suggests that most bubbles are associated with a similar number density («6.5 x 1023 m~3) of NFs.51,2 0 However, the degree of bubble-NF associ­ation has not yet been fully demonstrated and quan­tified. The boundary in MA957 in Figure 40(a) appears to be relatively cavity free, and there does not seem to be a large nearby NF-cavity denuded zone. Assuming equal partitioning of all the 380 appm He to 4.3 x 1023 m~3 bubbles («80 He atoms/bubble), g = 2Jm~2 is consistent with ty « 2g/kmkT« 0.6nm at 500 °C, where к is the real gas compressibility factor, which is in remarkable agreement with the measured average cavity size. Thus, we conclude that the He is primarily stored in near-equilibrium bubbles at a capillary pressure of 2g/r, « 6500 MPa in this case. A higher He content of2000 appm parti­tioned to the same number of bubbles («400 He atoms/bubble) increases гу to «1.1 nm, still far below the critical size for void formation, which is estimated to be well over 10 nm. The 9 dpa irradia­tion at 500 °C has no observable effect on the NFs.

As shown in Figure 40(b) and 40(c), a lower number density of (Nb « 5.3 x 1022 m~3) of somewhat larger (db « 2.1 nm) bubbles (the smaller population of cavities in this case) are observed in F82H, along with much larger faceted cavities; the larger cavities are likely voids. The smaller matrix bubbles in F82H are clearly formed on dislocations, as high­lighted by the black and white contrast insert in Figure 40(c). Figure 40(d) shows that the cavity size distribution is much narrower in the MA957, with a maximum diameter of less than 2.5 nm. In contrast, the largest diameters exceed 10 nm in F82H and it appears that a bimodal bubble-void

(d) ^bubble (nm)

Figure 40 (a) Underfocused TEM image of in situ He injected MA957; (b) and (c) underfocused TEM image of in situ He injected F82H; (d) cavity size distributions in MA957 versus F82H.

cavity size distribution is developing.51 Model-based extrapolation of these results suggests that significant swelling may develop at higher He and dpa.

The ISHI results suggest that NF are effective in trapping He in fine-scale bubbles at least up to 500 °C. The sink strength of 4.3 x 1023 m~3, 1.2 nm bubbles is Zb ~ 3.2 x 1015m~2, which is significantly higher than the typical total sink strength in FMS alloys (<«1015m~2). Presumably, Zb could be increased by an additional factor of at least two to three in alloys with larger numbers of NFs and bigger associated bubbles at higher He levels.

It is important to emphasize that both He bubbles and NFs are key to highly irradiation-tolerant alloys. The primary role of the NFs is to provide preferred and thermally stable sites for forming bubbles. In principle high densities of stable bubbles can seques­ter He up to high levels and serve as sinks that, in principle, mitigate all manifestations of displacement damage. Helium management schemes based on these principles are critical to developing fusion energy and may also play a role in fission applications intended to reach very high dpa levels. The effect of He bubbles on defect damage accumulation is being
investigated using the in situ implantation technique, including at higher He and dpa, for a wide range of irradiation temperatures and large alloy matrix.