Effects Of Ti,Zr And/or Ce On Nano-Oxides In FeCrAl Oxide Dispersion Strengthened (ODS) Steels

FeCrAl oxide dispersion strengthened(ODS)steel is one of the most promising candidate cladding materials for super-critical water cooled reactors(SCWRs)and leadcooled fast reactors(LFRs)due to its superior creep strength and excellent resistance to corrosion,oxidation and irradiation.Although Al addition is beneficial to the improvement of corrosion and oxidation resistance,it degrades high–temperature strength due to the formation of large size Y–Al complex oxides with sparse dispersion.Fortunately,the dispersion morphology of nanoparticles in FeCrAl ODS steel could be considerably improved and,consequently,comprehensive performance be significantly enhanced,by addition of the minor reactive elements.However,the effects of Ti,Zr and/or Ce on the morphology and phase of oxides,and structure of metal/oxide interfaces,and the corresponding formation mechanisms have not been completely clarified up to now.In this work,the phase and morphology of nano-oxides,and the structure of metal/oxide interfaces in FeCrAl ODS steels,including Fe–15Cr–2W–4Al–0.51Zr–0.35Y2O3(wt.%)and Fe–15Cr–2Al–0.1Ti–0.37Ce–0.35Y2O3(wt.%),were investigated by using scanning transmission electron microscopy(STEM)and high–resolution transmission electron microscopy(HRTEM).The main concluding remarks are as follows:(1)By single addition of Zr and/or simultaneous addition of Zr and Ti,the phases of nanoparticles and their corresponding proportions in Fe–15Cr–2W–4Al–0.35Y2O3 ODS steel were changed,leading to the occurrence of Y–Zr complex oxides with number fractions of 73.8% and 87.4%,respectively,and with other types of complex oxides minor.(2)By single addition of Zr and/or simultaneous addition of Zr and Ti,the formation of Y–Al complex oxides is intensively suppressed,contributing to the occurrence of fine Y–Zr complex oxides,reducing mean size and increasing number density of nanoparticles.Especially,the simultaneous addition of Zr and Ti is more effective in improving the dispersion morphology of nanoparticles in FeCrAl ODS steels,relative to the single addition of Zr.The reason is that Ti addition can promote the formation of finer Y–Ti complex oxides,further reducing the average size of nanoparticles and increasing number density.(3)By simultaneous addition of Ce and Ti,the phases and their corresponding proportions of nanoparticles in Fe–15Cr–2Al–0.35Y2O3 ODS steel were changed,leading to the occurrence of Y2Ce2O7,Y2 Ti O5,and Ce2Ti2O7 complex oxides with number fractions of 52.7%,15.7%,and 8.6%,respectively,and with other types of complex oxides minor.(4)By simultaneous addition of Ce and Ti,the formation of Y–Al complex oxides is intensively suppressed,contributing to the occurrence of Y–Ce complex oxides.However,the dispersion morphology of nanoparticles is not improved by simultaneous addition of Ce and Ti,but reducing mechanical properties,owing to the non-uniform distribution,large size,and low number density of Y–Ce complex oxides.