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

Abstract

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 outgazing and hot consolidation. The temperature increase during these last two stages triggers the precipitation of a fine dispersion of nano-oxides. The outgazing stage is generally conducted at temperature between 300 and 800 °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 properties by comparison between SAXS and Atom Probe Tomography (APT) during a thermal treatment at 700 °C for 5 h, simulating the outgazing stage. We show that despite that the nano-oxides slightly grow during the 700 °C heating, the final properties of the nano-oxides in term of size and number density are extremely close to those where no 700 °C isothermal was applied. Moreover, the growth rate of the nano-oxides is surprisingly higher at 700 °C than 1100 °C. APT and SAXS comparison shows that the nano-oxides do not stabilize in terms of stoichiometry and structure in the studied conditions, and that a small quantity of Al remain in the precipitates during this 700 °C isothermal treatment. Both of these factors could influence the nano-oxides kinetics at low temperature. • The outgazing shows no influence on the final nano-oxides size and number density. • No stabilization of the nano-oxides is observed at 700 °C after 5 h isothermal. • The nano-oxides growth rate is surprisingly higher at 700 °C than at 1100 °C. • Nano-oxides contains Al on the 700 °C isothermal, which is removed after heating at 1100°C. • TiC or Ti(C,N) is likely to form after the outgazing, between 700 and 1100 °C.