The objective of this research program is to investigate the neutron irradiation effects on the mechanical properties of Fe-Cr-C ternary model alloys. The selected model alloys are Fe-9Cr-0.1C and Fe-12Cr-0.2C, the composition of which are similar to commercial alloys T91 and HT9, respectively. Nanoindentation will be carried out to characterize the mechanical properties such as hardness and Young’s modulus of the irradiated specimens. In total eight samples (four Fe-9Cr-0.1C and four Fe-12Cr-0.2C) will be examined. The nominal irradiation temperature for all samples are 300C and the doses are 0.01 dpa, 0.1 dpa, 0.5 dpa, and 1.0 dpa. The samples will be prepared using flash polishing.

It is well known that for ferritic/martensitic (F/M) steels T91 and HT9, the most detrimental effects induced by irradiation are irradiation hardening and embrittlement, especially at lower temperatures such as 300C. The underlying mechanisms have not been well understood, due to the complicated microstructure and interactions under energetic neutron bombardment. Various irradiation-induced defects such as Cr-rich alpha prime, G phase, dislocation loops and networks contribute to the degradation in mechanical performance. This program seeks to advance the understanding of degradation mechanisms by quantify the irradiation effects in simple Fe-Cr-C model alloy systems so that effects due to other substitutional alloying elements are separated out. The experimental data from this program will be compared with (1) nanoindentation data of neutron-irradiated Fe-Cr binary model alloys studied in a previous research project, and (2) nanoindentation data of neutron-irradiated commercial alloys T91 and HT9 that are being investigated in another on-going research program supported by the Department of Energy.

This research project on Fe-Cr-C ternary model alloys will contribute to the state-of-the-knowledge in three ways: (1) provide unique experimental data for advancing the understanding of neutron irradiation effects in Fe-Cr-C model alloys, (2) provide a basis for comparison with neutron irradiation results of Fe-Cr binary alloys, and (3) advance the understanding of radiation-induced degradation mechanisms in commercial alloys such as T91 and HT9.

The neutron irradiation experiments have been completed and the samples are available for post-irradiation examination (PIE). All the PIE experiments are expected to be completed within the six-month period. Only nanoindenter is needed and the instrument time needed is estimated to be 8 days.

T91 and HT9 are two representative ferritic/martensitic (F/M) alloys that have promising structural applications in advanced fission reactors and future fusion reactors. However, due to the complexity of the alloy systems, the irradiation performance and underlying degradation mechanisms are still not well understood.

This research effort was constructed to carry out nanoindentation investigations on neutron irradiation effects of two Fe-Cr-C ternary model alloys, Fe-9Cr-0.1C and Fe-12Cr-0.2C. These two model alloys are selected because their chemical compositions are close to T91 and HT9, and effects due to other substitutional elements such as nickel, manganese, and molybdenum are avoided.

Systematic nanoindentation data of Fe-Cr-C specimens that were irradiated at 300C at various dose levels will provide in-depth and helpful information for solving two critical problems: (1) quantifying irradiation hardening in Fe-Cr-C ternary systems and correlating with Fe-Cr binary model alloys that were irradiated under the same conditions (same reactor positions, same doses, and same irradiation temperatures); (2) advancing the understanding of neutron irradiation hardening of commercial alloys T91 and HT9 investigated in an on-going DOE-supported research program. This proposed research will contribute to filling the knowledge gap and establishing the relationship of binary Fe-Cr, ternary Fe-Cr-C and complex alloy systems such as T91 and HT9. The results of this research will contribute to the deployment of next-generation advanced reactors by advancing the understanding of irradiation hardening of Fe-Cr, Fe-Cr-C, and commercial T91 and HT9 alloys.

In summary, this project is highly related to past PIE programs on Fe-Cr model alloys and current DOE-supported research programs for developing radiation-resistant alloys for advanced reactors. This program will provide a valuable reference for the development of nuclear structural materials and contribute to the DOE Nuclear Energy Roadmap in the search for materials that can meet the challenges in materials R&D for advanced reactor systems that contribute to the Administration’s energy security and climate change goals.