2018 | ANNUAL REPORT 63 dislocation loops in the Fe-3Cr to Fe-18Cr alloys irradiated at 593 K to about 1.8 dpa [15] to estimate the obstacle strength factors (αj) for loops, solute clusters and α′ precipi- tates.This study involved analyzing the combined microstructural and ∆σy measurements (based onVickers microhardness, ∆Hv) by least-square fitting the dispersed barrier strength factors αj and an independently derived strength superposition model. As shown in Figures 7b and c, the optimized hardening predictions are in good agreement with experiment, within expected uncertainties. Sequen- tial 168 h isochronal anneals of the Fe-6Cr alloy from 573 to 673 K, in 25 K increments, coarsened the loop size distribution by a factor of three, and decreased the number density by a factor of 8.TEM also showed that isothermal 300 h anneals at 773 and 873 K, coarsened and dissolved the precipitates, respectively, qualitatively consistent with APT observations [9]. Coarsening was shown to be consis- tent with bulk diffusion controlled kinetics (aka Ostwald Ripening). Nanostructured Ferritic Alloys (NFA) Nanostructured ferritic alloys (NFA) are Fe-12Cr to Fe-18Cr ferritic stainless steels, which are dispersion strengthened by an ultrahigh density ofY-Ti-O nano-oxide precipitates. These materials have both remarkable high-temperature strength and irradia- tion tolerance.As shown in Figure 8, advancedTEM (through focal series, exit wave and STEM diffraction) techniques demonstrate that large- and medium-sized oxide nanoparticles produce all the expected extra spots in the fast Fourier transformation aver- aged power spectra for the fcc pyro- chloreY2Ti2O7 complex oxide [16]. Figure 8c shows the corresponding spot pattern for the matrix without the oxide particles. Figure 8d and e show that the spot patterns for larger selected areas containing populations of much smaller oxide nanoparticles are the same in both as-fabricated and UCSB-2 library irradiated MA957. While the extra spots are blurred or streaked, and all are not visible, these patterns are also consistent with the presence ofY2Ti2O7. Figure 8e shows similar spot patterns in another unir- radiated NFA. Future Activities Figure 7. a) the correlation between high load and two measures of NI hardness using a Berkovich indenter; b) a bar graph showing the predicted and measured ∆σy for the optimized dispersed barrier model for the fitted indicated strengthening factors for loops, solute clusters and α’ precipitates, along with their individual contributions; c) a scatter plot of the predicted versus measured ∆σy. a) c) b)