Study On The Thermal Ageing Behavior Of Corrosion Resistant Oxide Dispersion Strengthened(ODS) Steels

Oxide dispersion strengthened(ODS)steels,which are manufactured by mechanical alloying(MA),have been identified as candidate cladding materials for generation IV nuclear reactors.However,ODS steels with Cr contents below 13 wt.%are not suitable for the application in the highly corrosive environment of some nuclear reactors(e.g.,supercritical water cooled reactors(SCWR),lead-bismuth eutectic cooled fast breeder reactors(LBR))due to the insufficient corrosion resistance.Hence,high Cr ODS steels with Cr concentrations in the range of 14-22 wt.%were recently developed.Unfortunately,ODS steels with Cr content higher than 12 wt.%may suffer from thermal ageing hardening or even embrittlement after isothermally aged at 300-500 ~oC for long time due to the decomposition of?-(Fe,Cr)ferrite phase into Fe-rich ferrite(?)and the Cr-enriched ferrite(??)phase that can seriously degrade the mechanical properties,e.g.,“475 ~oC embrittlement”.In this work,in order to study the effects of Al/Cr concentrations on the???+??phase separation behavior,the nanometer scale characterizations on the structure and chemistry of the 12Cr(wt.%),15Cr(wt.%)and18Cr(wt.%)ODS steels,which undergone thermal aging at 475°C for 9000 h,have been performed by energy-filtered transmission electron microscopy(EFTEM)and atom probe tomography(APT).The following conclusions can be drawn:(1)No??phase was detected and no???+??phase decomposition in 12Cr,12Cr-5Al ODS steels after thermal ageing.(2)Significant phase separation occurred and,consequently,??phases were formed in 15Cr-5Al ODS steels and,indicating that 4.57 wt.%Al is not high enough to suppress the???+??phase decomposition in FeCrAl-ODS steels with the content of Cr being 14.16 wt.%;at the same time,intermetallic compounds(??phase)precipitated of15Cr-5Al ODS steels after thermal aging.(3)No??phase was detected in 15Cr-9Al ODS steels at all,indicating that 8.4 wt.%Al is high enough to suppress the???+??phase decomposition in FeCrAl-ODS steels with the content of Cr being 14.25 wt.%;at the same time,intermetallic compounds(??phase)precipitated of 15Cr-9Al ODS steels after thermal aging.(4)Significant phase separation occurred and,consequently,??phases were formed in 18Cr-5Al,18Cr-7Al ODS steels,indicating that 4.46 wt.%and 6.28 wt.%Al is not high enough to suppress the???+??phase decomposition in FeCrAl-ODS steels with the content of Cr being 16.78 wt.%and 16.73 wt.%,respectively.(5)No??phase was detected in 18Cr-9Al ODS steels at all,indicating that 8.16 wt.%Al is high enough to suppress the???+??phase decomposition in FeCrAl-ODS steels with the content of Cr being 16.84 wt.%.