The proposed study will investigate the microstructural evolution under irradiation by Kr++ heavy ions and He co-implantation of three candidate compositionally complex alloy (CCA) compositions from two alloy systems of interest for sodium fast reactor claddings and core applications. Conventional alloys optimized for claddings and ducts such as austenitic D9 and ferritic-martensitic G92 and 9-12Cr steels show dramatic degradation after hundreds of displacements per atom (dpa), far short of the needs of advanced reactors, triggering exploration of CCA. Preliminary studies have shown that these alloys exhibit excellent strength, temperature resistance, and tolerance to radiation damage, promoting their candidacy for cladding and core applications. Especially interesting are recent findings that complex Ni-based alloys show a reduction in defect formation and void swelling compared to their single-element constituents and of their NiFeCr and NiFeMn conventional counterparts. Improvements in material properties are attributed to the compositional complexity related to the number and choice of constituent elements. Since Co-free FCC CCA have been found to show similar irradiation hardening and microstructural evolution to 316SS, the benefit of compositional complexity may be of similar magnitude to dilute alloying element additions, and thus demands fundamental mechanistic understanding. This study has identified two compositions in the FCC CrFeMnNi family, Cr18Fe27Mn27Ni28 and Cr15Fe35Mn15Ni35. The former has mechanical properties comparable to 316SS and is predicted by CALPHAD to phase separate at ~760 ̊C, while the latter is predicted to be a single FCC phase down to ~575 ̊C. Although both were found to phase separate after ageing at 700 ̊C, sluggish diffusion slows phase separation in all these CCA for the duration of an IVEM irradiation. To advance fundamental understanding of the radiation resistance of compositionally complex base matrices, in situ IVEM studies are necessary for the dynamic observation of defect formation and evolution under irradiation. This technique provides the high temporal and spatial resolution needed for characterizing loop and void formation, growth, migration, and stability. These CCA have already been studied by the PI at IVEM, and results indicate that defect cluster formation under single-beam irradiation is reduced at 50K in CCA compared to less compositionally complex materials. At high temperature, interstitial loop growth kinetics were slowed in Cr15Fe35Mn15Ni35. This study builds upon that work with co-implantation experiments at elevated temperatures to observe defect clustering and void growth behavior in a He-producing environment. Arc-melted or vacuum-induction melted samples are homogenized and formed into disks for electro-polishing before 1MeV Kr++ and 12 keV He+ irradiation at 773 and 873K to a dose of 10 dpa and 0.5% He/dpa. A total of 10 days on the IVEM are requested over the next year. The goal of this effort is three-fold: to compare the radiation tolerance of these alloys to simple metals and model alloys; to inform future CCA design and improvements by mapping the physical response of the alloys under these conditions; and finally, to characterize the effect of compositional complexity on the mobility of point defects and larger defect structures.

The proposed effort aims to improve structural alloys, enabling development of next-generation advanced fission reactors. A primary limiting factor in deploying sodium fast reactors (SFR) is developing materials with optimal mechanical and corrosion properties which can tolerate radiation damage at high levels of dpa. Overcoming the limitations of conventional alloying design requires shifting to innovative and high-throughput alloying design. While conventional alloys consist of one or two primary elements, compositionally complex alloys (CCA) consist of multiple principle metallic elements mixed in near-equimolar proportions in a single-phase solid solution. CCA exploit compositional complexity for improved mechanical properties and have shown a high tolerance to microstructural and micromechanical changes under irradiation, including reduced defect formation and growth, fewer surviving defects, less radiation induced segregation, and a reduction in void swelling. While preliminary results attribute enhanced radiation tolerance to a compositionally complex base matrix, the formation of voids has not been captured dynamically in these materials, nor has a rigorous understanding of the mechanisms behind this tolerance been attained. Since FCC CCA have shown similar irradiation hardening and microstructural evolution to 316SS, CCA may show the most promise as a replacement base matrix preceding more advanced alloy design. To select the most effective CCA composition for in-core applications, it is first necessary to understand the microstructural evolution under irradiation in helium-producing environments that may enhance performance over single-element base matrices. Two compositions in the well-studied CoCrFeMnNi FCC system, Cr18Fe27Mn27Ni28 and Cr15Fe35Mn15Ni35 have been selected for the proposed experiment. The first has been previously studied and has mechanical properties which are comparable to that of 316SS, but with the promise of mitigated void swelling; and the second is predicted by CALPHAD to be a single FCC phase at 600 ̊C. In a companion single-beam irradiation IVEM experiment, Cr15Fe35Mn15Ni35 showed higher interstitial loop nucleation and slower loop growth than Cr18Fe27Mn27Ni28. Mobility of point defects may vary strongly in CCA even across small compositional changes. Inclusion of two CCA compositions and less compositionally complex reference materials will evaluate the role of both compositional complexity and smaller compositional changes on void formation in the presence of helium. Arc-melted samples will be heat treated for recrystallization and formed into 3mm disks for electro-polishing. Electro-polished 3mm disks of model alloy E90, pure Ni, and NiFe binary reference materials will be used as reference materials. The study is designed to dynamically observe the evolving microstructure under irradiation using the IVEM and evaluate the mechanisms that govern swelling behavior. This effort will improve the foundational understanding of damage accumulation in these alloys which can be used: to create predictive models for the development of improved alloys; for application into improved simulations of cladding alloys in reactor environments, such as the NEAMS MOOSE-BISON-MARMOT (MBM) fuel performance code structure; and to provide guidance for alloy design for advanced in-core fast reactor environments. This effort fulfills a necessary leg of the scientific approach: focusing on experiments needed for understanding material response of advanced alloys under irradiation to inform and design models and simulations.