Thermal Aging Behavior Of Fe-Cr-Al Oxide Dispersion Strengthened (ODS) Steels Used For Advanced Nuclear Reactors

Fe-Cr-Al oxide dispersion strengthened(ODS)steel is one of the most promising candidate cladding materials of advanced nuclear reactors due to the excellent compatibility with high temperature steam,superior creep resistance and outstanding irradiation tolerance.However,it may suffer from thermal aging hardening&embrittlement after long term service in the high temperature environments due to the Fe–Cr phase separation of the ferrite phase into the two phases,i.e.,Fe–rich ferrite(α)and Cr–rich ferrite(α’).This may be a key issue for the application of ODS ferritic steels as the structural materials of advanced nuclear reactors.The nanometer-scale structure and chemistry of two kinds of 12Cr ODS steels isothermally aged at 475~oC for300h and five kinds of 15Cr ODS steels isothermally aged at 475~oC for 9000h were investigated by transmission electron microscopy(TEM),energy-filtered transmission electron microscopy(EFTEM)and atom probe tomography(APT)techniques to understand the Fe-Cr phase separation and solid-state phase transformation.Precipitates in ODS steels after early-stage and long-term thermal aging were examined to explore the underlying reason of hardening.New guidlines of alloy design are provided based on the thermal aging behavior and the corresponding mechanism and,thus,novel types of ODS steels could be developed in the future for the application in advanced nuclear reactors.The following conclusions can be drawn:(1)There is no Fe-Cr phase separation in 12Cr5Al and 12Cr9Al ODS steels after thermal aging at 475~oC for 300h.However,Ti-enriched and Al-enriched precipitates occurred in these alloys.The hardening phenomena of 12Cr5Al and 12Cr9Al ODS steels in early-stage aging is due to the occurrence of the Ti-enriched and Al-enriched precipitates.(2)Significant Fe-Cr phase separation occurred in 15Cr and 15Cr5Al ODS steels after thermal aging at 475~oC for 9000h and,consequently,a large number density of Cr-enrichedα’phases came to be formed.The addition of 4.57 wt.%Al is not high enough to suppress the Fe–Cr phase separation behavior of FeCrAl ODS steel when the content of Cr is 14.16 wt.%.Fe-Cr phase separation is the reason for the hardening phenomena of 15Cr and 15Cr5Al ODS steels after long-term thermal aging.(3)There is no Fe-Cr phase separation behavior in 15Cr7Al ODS steels after thermal aging at 475~oC for 9000h.The addition of 6.44 wt.%Al is high enough to suppress the Fe–Cr phase separation behavior of FeCrAl ODS steel when the content of Cr is 14.18 wt.%.A large number of Ti-enriched and Al-enriched precipitates came to be formed in this alloy after thermal aging,which can explain the hardening phenomena of 15Cr7Al ODS steel after long-term thermal aging.(4)Cr–rich precipitates,which presumably are notα’phases in nature,were detected in 15Cr–7Al–0.4Zr–0.07Ex.O and 15Cr–7Al–0.4Zr–0.21Ex.O ODS steels after thermal aging at 475~oC for 9000h.Some composite particles composed of Cr–rich precipitates and oxide particles came to be formed after thermal aging in these alloys,which can explain the hardening phenomena of 15Cr–7Al–0.4Zr–0.07Ex.O and 15Cr–7Al–0.4Zr–0.21Ex.O ODS steels after long-term thermal aging.(5)A large number of core-shell structures came to be formed in the ODS steels suffered from Fe–Cr phase separation,i.e.,15Cr and 15Cr5Al ODS steels.However,there was no core-shell structure in the ODS steels without Fe–Cr phase separation,i.e.,15Cr7Al,15Cr–7Al–0.4Zr–0.07Ex.O and 15Cr–7Al–0.4Zr–0.21Ex.O ODS steels.The formation of the core-shell structures should be closely related to the diffusion behavior of Cr during the process of Fe–Cr phase separation.