The Impact of Outgassing on the Nano-Oxides Kinetics of Ferritic Oxide Dispersion Strengthened Steel

Oxide Dispersion Strengthened (ODS) steels are candidates materials to be used in nuclear fusion and 4th generation fission power plants. They are processed by powder metallurgy, following a Mechanical Alloying (MA) where Fe-Cr powder is mixed with 0.3 %wt Y2O3 and 0.3 %wt TiH2 powders resulting in an as-MA powder where Y, Ti and O are dissolved into the Fe-Cr matrix. The as-MA powder is then consolidated at 1100 degrees C by Hot Extrusion (HE) or Hot Isostatic Pressing (HIP), where the Y-Ti-O nano-oxide precipitation occurs. In this study, two grades of ODS steels are studied: a Fe-14Cr ferritic ODS, with higher corrosion resistance but non-isotropic mechanical properties when processed by HE; and a Fe-9Cr Ferritic / Martensitic (F/M) ODS, which exhibits a matrix alpha -> gamma phase transformation around 950 degrees C resulting in a more isotropic microstructure (and then mechanical properties) after HE but a lower corrosion resistance. This work demonstrates the influence of the phase transformation of the F/M ODS steels on the precipitation kinetics, showing an increase of the nano-oxides growth rate after the alpha -> gamma matrix transformation, resulting in a nano-oxide size two times bigger than ferritic ODS steels, thanks to in-situ Small Angle X-ray Scattering (SAXS) measurements up to 1100 degrees C. Very small clusters (similar to 1 nm size) containing at least Y and O had been observed at the as-MA stage of both ODS grades. The comparison between SAXS and Atom Probe Tomography (APT) data allows to determine that the nano-oxides stoichiometry and structure stabilize at roughly similar to 1000 degrees C, for both alloys.