The impact of outgassing on the nano-oxides kinetics of ferritic oxide dispersion strengthened steel

Oxide Dispersion Strengthened (ODS) steels are studied for their application in fission and fusion nuclear power plants. These steels are processed by powder metallurgy involving a high energy milling of Fe-Cr with oxides powders, followed by outgassing and hot consolidation. The temperature increase during these last two stages triggers the precipitation of a fine dispersion of nano-oxides. The outgassing stage is generally conducted at temperature between 300 and 800 degrees C for several hours, which can already trigger the formation of nano-oxides, and thus may have an influence on the final nano-oxide characteristics. In this work we assess the nano-oxides size by Small Angle X-Ray Scattering (SAXS) and get insight on the nano-oxides chemical and structural prop-erties by comparison between SAXS and Atom Probe Tomography (APT) during a thermal treatment at 700 degrees C for 5 h, simulating the outgassing stage. We show that despite that the nano-oxides slightly grow during the 700 degrees C heating, the final properties of the nano-oxides in term of size and number density are extremely close to those where no 700 degrees C isothermal was applied. Moreover, the growth rate of the nano-oxides is surprisingly higher at 700 degrees C than 1100 degrees C. APT and SAXS comparison shows that the nano-oxides do not stabilize in terms of stoi-chiometry and structure in the studied conditions, and that a small quantity of Al remain in the precipitates during this 700 degrees C isothermal treatment. Both of these factors could influence the nano-oxides kinetics at low temperature.