2018 | ANNUAL REPORT 59 associated with the smallest loops.At intermediate size, the Cr segregates to the outside of the loops while Si segregates to both the inside and outside. Both solutes segregate to the outside of the loops at larger sizes.The loops were analyzed in terms of their number density, size, habit plane and spatial distribution. Larger numbers of visible loops are observed near grain boundaries.The data from the 593 K irradiation also show segregation (enrichment) of Cr, Si (and P) at grain boundaries (Figure 3c).The C concen- tration is also slightly increased by irradiation, likely due to co-segregation effects with the higher Cr. Dependence of segregation on the type of grain boundary and solute enrichment at the individual dislocations were observed in low angle tilt boundaries, as demon- strated in Figure 3d. APT studies on the Fe-15Cr alloy presented in Figure 4a show a high density of very small Cr-Si-Ni-P clusters, again likely associated with small loops [8].These solute clusters are believed to play a critical role in irradiation hardening of Fe-9Cr tempered martensitic steels, especially at higher dpa. In addition to the expected solute clusters and α’ precipi- tates, grain-boundary Cr-carbide and nitride precipitates were also observed in Fe-15Cr alloy, which altered the local alloy chemistry. In general, less segregation of Cr at grain boundaries was observed in the 15Cr alloy, although this might be affected by nitride and carbide precipitates. Si, Ni and P generally segregate to all types of interfaces. Examination of α’ precipitation over a range of irradiation temperatures and dpa, as well as following post- irradiation annealing (PIA) at both 773 and 873 K [9] has been used to establish the full Cr solvus line. Figure 4c shows that the nominal α’ Cr content measured by APT is generally less than predicted by the Cr-rich thermodynamic phase boundary and is a function of the α’ precipitate size. However, a proxigram analysis (interface-to-center concentration profile) presented in Figure 4b showed that the Cr composition at the α’ core approaches equilibrium values in sufficiently large precipitates, especially at higher temperatures.This demonstrates, once again, that such matrix element enrichment in precipi- tates is an APT artifact. Note, the α’ interface composition may be affected by chemistry-dependent interface energies, as well as by cascade ballistic mixing at lower temperatures. Figure 4d shows that PIA results in α’ coars- ening at 773 K, and full α’ dissolution at 873 K, again consistent with the phase diagram. These results show the effect of neutron-irradiation-enhanced diffu- sion on accelerating thermally driven α’ precipitation in Fe-Cr alloys with ≥9% Cr. However, α’ precipitation is modified in similar alloys under ion irradiation at high dpa rates. Figure 5 (top) shows α’ formation in an Fe-18Cr alloy following both ion and neutron irradiations at around 593 K and dose rates from ≈10-7 to 10-4 dpa/s [10].The steady-state Cr content of the α’ depends on the ion irradiation dose and dose rate, confirming the contribution of ballistic mixing to diluting non- equilibrium α′ precipitates at high dpa rates. These APT data were used to guide the development and calibration of a model on the effects of the irradiating-particle type and dpa rate on α’ formation [11].The model was based on the Cahn-Hilliard phase- field theory, which includes dpa- rate-dependent radiation-enhanced diffusion and cascade ballistic mixing to simulate α’ evolution under neutrons, heavy ions, and electron irradiation at 573 K.As shown in Figure 5 (bottom), the model predicts that higher dpa rates increase the number density of precipitates while decreasing their radius, volume frac- tion, and Cr content. Furthermore, the model predicts that α’ formation is completely suppressed above about 10-5 dpa/s. In contrast to the ion and neutron irradiations, with high ballistic mixing rates associated with displacement cascades, electron irra- diation with weak mixing had little effect on α’ formation up to 10-3 dpa/s. These results show that both cascade mixing and radiation-enhanced diffusion must be accounted for, along with many other confounding factors, when attempting to emulate neutron- irradiation effects using accelerated cascade-inducing ion irradiations. Neutron Irradiation Effects onThe Constitutive Properties of Fe-Cr Ferritic Martensitic Steels and Fe-Cr Model Alloys: Property-Property- Microstructure Relations True stress-true strain constitutive relations, σ(ε), are the most impor- tant mechanical property used in engineering analysis and design, as a required input to finite-element calculations.The UCSB-1 irradiation included sub-sized tensile specimens for a variety of ferritic-martensitic and several other steels.The test results for subset of six of these alloys, five 9Cr to12Cr tempered martensitic