The nano-oxides under irradiation in the oxide dispersion strengthened steels

International audienceOxide Dispersion Strengthened (ODS) ferritic steels are considered as promising candidates for nuclear application as cladding tubes for GEN IV reactors. In such reactors, the irradiation damage can reach more than 150 dpa at temperatures ranging between 400DC and 650DC. Thus, the stability of nano-particles has to be guaranteed in order to ensure the materials' excellent properties. Further, since the nano-particles act as sinks for point defects, the stability of their interface under irradiation is also of great interest. Transmission Electron microscopy exams in a Fe-14Cr-1Ti-0.3Y2O3 ODS steel show that particles with size below 3 nm are (Y, Ti, Cr, O) compounds exhibiting an unknown f.c.c structure and are embedded with full coherent configuration. After annealing at 1300DC, these particles relax toward equilibrium by adopting a Y2Ti2O7 crystal structure and undergo shape transition to accommodate the strain energy owing to the difference of lattice parameter between nano-oxide and matrix. The formation of a Cr shell around the relaxed particle is also observed, hypothesis on its origin will be discussed. Ion bombardment were conducted to assess the stability of these relaxed particles under irradiation. Using Fe ions, ODS steels were irradiated at 500DC up to 150 dpa. Under these conditions, the system evolution consists in a decrease of the nano-particle density and an increase of the nano-particle size. This phenomenon is explained using the radiation-induced Ostwald ripening theory. Further, after irradiation, the flat interfaces of the nano-oxides appear as destabilized since they display morphological changes and diffuseness. From an applied point of view, the modification of precipitate shapes under irradiation may alter their growth and even conduct to a coarsening saturation while the interface instability is also susceptible to influence the interaction between nano-particles and point defects. In the end, comparison with the results obtained after neutron irradiation will be proposed