Characterization of microstrutural evolution of ODS alloys after aging treatment at 450DC and 500DC

International audienceOxide Dispersion Strengthened (ODS) steels envisaged as cladding materials in Generation IV fission nuclear reactors due to their good radiation resistance and improved high-temperature mechanical properties through the nano-oxides addition compared to conventional ferritic/martensitic steels. Cr-rich ferritic ODS alloys are in particular excellent candidates due to their high corrosion resistance, besides their excellent mechanical properties and creep resistance. However, these alloys, whose service temperature is in the range of 400-700C, can, face severe embrittlement problems particularly for lower temperatures. This embrittlement is attributed to the decomposition of the matrix into the alpha(Fe-rich) and alpha' (Cr-rich) phases. The effect of phase separation causes an increase in hardness and yield strength and reduction of the tensile elongation. Despite the significant body of research on microstructure and mechanical properties of ODS, there is still a significant lack of knowledge on this phenomenon in these alloys. In this work, Atome Probe Tomography (APT), Energy-Filtered Transmission Electron Microscopy (EFTEM) and thermoelectric power (TEP) were used to study the phenomenon of alpha' separation after aging at 450C and 500C for 5000 and 10000h hours in different Fe-18Cr and Fe-14Cr reinforced and non-reinforced steels, in order to study the effect of the Cr content, the duration and temperature on the kinetics of alpha alpha'-phase precipitation and differences between grades were indeed noticed. Moreover, we also study the behaviour of grain boundary after these heat treatments; we examine the relationships between the misorientation, the segregation and the grain boundary migration