Oxide Dispersion Strengthened (ODS) steels are potential candidate materials for application as structural components of fission and fusion reactors, known for their high thermal stability, high resistance to creep and to radiation-induced damage. These attractive properties result from the presence of the fine and highly thermally stable yttrium‑oxygen (Y-O) based nanoparticles, which exert a strong Zener pinning force to hinder the grain boundary movement, and are able to pin dislocations and trap radiation induced defects. In the present work, the effect of annealing at 1400 K on the microstructure and oxide nanoparticles in a 0.3% Y₂O₃ ODS Eurofer steel was assessed. The material was characterized with Scanning Electron Microscopy, Transmission Electron Microscopy and Atom Probe Tomography in a reference condition and after annealing at 1400 K, followed by cooling at different rates. The results showed that the average diameter of the oxide nanoparticles increases from 3.7 ± 0.01 nm to 5.3 ± 0.04 nm, after annealing at 1400 K for 1 h. The particles present a well-known core/shell structure, with a core rich in Y, O and V and a shell rich in Cr. The effect of the increase in oxide nanoparticle size on the microstructure is discussed in terms of the Zener pinning force.

Oxide dispersion strengthened (ODS) steels are leading candidates for structural materials in nuclear fission and fusion power plants. Understanding the nature of nano-oxide particles in ODS steels is vital for a better control of the microstructure and mechanical properties to further their applications. In this study, electron microscopy and atom probe tomography (APT) have been used to investigate the nanocluster features in ODS Eurofer steel. With the addition of V and Ta in ODS Eurofer, the nanoclusters exhibit a higher number density with a decreased average diameter, indicating that V and Ta are beneficial for the formation of small clusters. Irrespective of the composition of the base material, the smaller particles have a variable stoichiometry while the larger particles are likely to have Y₂O₃ stoichiometry. The nanoclusters were found to have a core/shell structure, where Y, O and Ta are enriched in the core and Cr and V are predominant in the shell. The formation of the complex structure is possibly the result of a competing effect between Ta, Y, V and Cr binding with O. It is deduced that Ta tends to combine with O in the core (Y₂O₃) of the clusters due to a higher affinity, and pushes V and Cr to the surrounding shell during the formation of nanoclusters.