Nano-oxides and microstructural evolutions of oxide dispersion strengthened steels throughout their fabrication process

Oxide Dispersion Strengthened (ODS) steels with a Fe-Cr matrix are of great interest in the development of generation IV fission nuclear power plants as a fuel cladding material. These materials, elaborated by powder metallurgy, include a high density of Y-Ti-O oxide precipitates of a few nm, providing the main contribution to the ODS steels strength. During the fabrication process, powders of Fe-Cr steel, Y₂O₃ and TiH₂ are milled together to obtain a super-saturated solution of Y, Ti and O, otherwise insoluble in Fe. The obtained powder is subsequently processed by Hot Extrusion or Hot Isostatic Pressing around 1100 C. This induces the precipitation of Y-Ti-O and leads to a fully dense steel with finely dispersed nano-oxides in high density, extremely stable even at very high temperature. Moreover, the complex microstructure induced by the milling stage, including high density of dislocations and very small grain size, also evolve during the heating, in an intricate ways with the evolution of the nano-oxides that act as strong pinning points for both grain boundaries and dislocations. This features leads to abnormal grain growth (significant growth of some grains, while others remain stable) and therefore a bimodal grain size after consolidation. The aim of this thesis is to characterize both kinetics and chemical evolution of the nano-oxides during the fabrication process of these steels, as well as the grain microstructure evolution, which are still misunderstood. Such experimental characterization would be invaluable to further control the fabrication process and improve available model of the ODS precipitation kinetics. For this purpose, cold pressed specimens from as-milled powder were prepared (in order to achieve dense specimens without precipitation) and observed by means of various in-situ or ex-situ techniques. In particular, in-situ Small Angle X-ray Scattering measurements were performed during the heating until 1100 C to measure the precipitation kinetics of several ODS grades. The chemistry and structure of nano-oxides were assessed thanks to anomalous SAXS at Y (performed in-situ) and Ti (ex-situ) edges, ex-situ small angle neutron scattering and atom probe tomography. Then, the grain microstructure was monitored by in-situ X-ray diffraction. These characterizations allow to describe the whole kinetics of purely ferritic ODS, ferritic/martensitic ODS and ODS strengthened with variation of the strengthening powders. In particular, this study highlights a non-homogeneous as-milled stage with clusters containing Y, O and Ti in lesser extent. These clusters evolve in chemistry and structure until reaching a stable structure and stoichiometry, revealed to be Y₂Ti₂O₇ pyrochlore and Y₂TiO₅ orthorhombic depending on the ODS steel grade. Moreover, a kinetic modification induced by the in temperature phase transformation of the matrix in the ferritic/martensitic ODS (increasing the growth and coarsening rate) was observed. Correlation between nano-oxide and the grain microstructure evolutions were performed and help in the comprehension of the abnormal grain growth temperature and final grain morphology, from one ODS grade to another. Finally, comparisons of the precipitation simulations with experimental data allow to get more insights on the nano-oxides properties. (author)