Effects Of The Content Of Al On Thermal Age–hardening Mechanisms Of 15Cr ODS Steels

Al–alloyed high–Cr oxide dispersion strengthened(Fe Cr Al–ODS)steel has been considered as one of the most promising candidate cladding materials for Gen–IV nuclear fission energy systems and the accident tolerant fuel(ATF)cladding of the light water reactors because of the favorable combination of excellent resistance to oxidation,corrosion,creep and irradiation with superior high temperature strength and very good compatibility with nuclear fuels.However,when the Cr content exceeds 12 wt.%,Fe Cr Al–ODS steel may suffer from thermal age–hardening and/or age–embrittlement during long–term service in the high tem–perature environment due to ?(Fe–enriched)–?'(Cr–enriched)phase separation and some specific precipitation behavior.Therefore,the effects of the content of Al on thermal ageing-hardening mechanisms of 15 Cr ODS steels has been studied in this work.The nanoscale precipitates of 15 Cr,15Cr–7Al ODS steels after isothermally ageing at 475 °C for 300 h and,moreover,15 Cr,15Cr–5Al,15Cr–7Al and 15Cr–9Al ODS steels after isothermally ageing at 475 °C for 9000 h were investigated by atom probe tomography(APT).Based on the dislocation barrier models,the increment in yield strength of the ODS steels was theoretically calculated according to the dispersion morphology of the ?' phase and ?' phase,which is consistent with the corresponding actual values,indicating that the amounts of age–hardening are well correlated with the evolution characteristics of nanoscale structural characterized by APT.The following conclusions can be drawn:(1)?–?' phase separation occurs in 15 Cr ODS steel but not in 15Cr–7Al ODS steels after isothermally ageing at 475 °C for 300 h,indicating that the ?–?' phase separation in 15 Cr ODS steel could be suppressed when the content of Al is equal to or higher than 7 wt.% in early–stage thermal ageing.According to theoretical calculations,age–hardening of 15 Cr ODS steel is due to the ?–?' phase separation,age–hardening occurred in 15Cr–7Al ODS steels should be attributed to the(Ti,Al)–enriched ?' phase precipitation.(2)?–?' phase separation occurs in 15 Cr and 15Cr-5Al ODS steels but not in15Cr-7Al and 15Cr-9Al ODS steels after isothermally ageing at 475 °C for 9000 h,indicating that the ?–?' phase separation in 15 Cr ODS steel could be completely suppressed when the content of Al is equal to or higher than 7 wt.% in long-term thermal ageing.Significant precipitation of(Ti,Al)–enriched ?' phase occurs in all the Al–alloyed 15 Cr ODS steels.(3)Complex–oxide–core & Cr–enriched–shell structured particles are formed in15 Cr and 15Cr–5Al ODS steels,in which ?–?' phase separation occurs,while there is no such core–shell structure in 15Cr-7Al and 15Cr-9Al ODS steels,indicating that the formation of core–shell structured oxide particle could be closely associated with the process of ?–?' phase separation.(4)According to theoretical calculations,age–hardening of 15 Cr ODS steel is due to the ?–?' phase separation.Among the four ODS steels,the most significant age–hardening occurs in 15Cr–5Al ODS steel attributed to both the ?–?' phase separation and ?' phase precipitation.The age–hardening in 15Cr–7Al and 15Cr–9Al ODS steels is only because of the ?' phase precipitation.The strengthening of the ?' phases become more significant with the increasing of Al content from 5 wt.% to 9 wt.%.(5)The age–hardening mechanisms of the 15 Cr ODS steels are proposed based on the dislocation barrier models where the amounts of age–hardening are well correlated with the evolution characteristics of nanoscale structural features characterized by APT,the strengthening mechanisms of the ?' phases and the ?' phases are the shearing mechanism and bypassing mechanism of Orowan models,respectively.