SANS investigation of microstructure evolution in high chromium ODS steels after thermal ageing

In the present paper the small-angle neutron scattering (SANS) technique was used to investigate the phase separation in two oxide dispersion strengthened (ODS) ferritic steels, PM2000 and MA956, during isothermal ageing at 475 °C. The ferritic matrix of the investigated materials separates into Fe-rich α and Cr-rich α′ phases during ageing at this temperature. The samples were measured under magnetic field in order to separate the nuclear and magnetic scattering intensities. The ratio of these two contributions was used to gain information on the chemical composition of the scatterers. The SANS measurements revealed the formation and subsequent growth of Cr-rich precipitates on the nanometer scale. The size distributions and the volume fractions of the precipitates were evaluated from the scattering data. Microhardness was found to significantly increase as a function of ageing time for both materials. The hardening is related to the formation of the Cr-rich α′ phase.