EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Oxide Dispersion Strengthened (ODS) ferritic steels are promising candidates as cladding material for 4th Generation Sodium-cooled Fast nuclear Reactors. Conventional reinforcement is achieved by introducing titanium and yttrium oxide Y2O3 during mechanical milling with matrix steel powder. The strengthening particles are dissolved or reduced as sub-nanometric clusters during this step, before precipitating as complex oxides during heating and hot extrusion consolidation process. These ferritic ODS grades exhibit exceptionnal high-temperature strength and thermal creep lifetime. Nevertheless, they are difficult to cold form as tubes and are characterized by a low Charpy impact upper-shelf energy (USE) and brutal creep rupture. These halftone properties are mainly attributed to the uncontrolled precipitation : some nano-precipitates, such as small Y-Ti-O oxides, appear to be highly beneficial to the mechanical behaviour of the material, while coarser ones segregated at grain boundary – e.g. TiC, Y-Al-O, Y2O3 - embrittle it.

In order to avoid these detrimental phenoma, an innovative alloy-design approach is developed at CEA. Nanostructured Y2Ti2O7 pyrochlore phase oxides are elaborated by mechanochemical synthesis, followed by annealing under oxygen atmosphere. Microstructure and time-driven oxygen stoichiometry of pyrochlore oxides are studied through X-ray Diffraction Rietveld refinements.

Added instead of yttria, stoichiometric Y2Ti2O7 prevents from the presence of coarsed Y2O3 particles and reactive titanium excess. Results show that the introduction of these new strengthening nanoparticles reduces the micrometric precipitates volume fraction, and improves the ductility of ODS ferritic steels. Mechanical tests reveal a significant improvement in USE, while maintaining a good tensile strength up to 650°C. A 60% cold rolling rate has been achieved without any intermediate thermal processing, demonstrating the cold-forming possibilities of these new ferritic ODS grades.