Age-hardening mechanisms of 15Cr ODS ferritic steels with 5, 7 and 9 wt.% Al at 475 °C for 9000 h

Abstract Thermal ageing behaviors of 15 wt.% Cr-oxide dispersion strengthened (ODS) steels with the contents of Al of 5, 7 and 9 wt.%, which were isothermally aged at 475 °C for 9000 h, have been investigated by Vickers hardness measurement, tensile test and atom probe tomography (APT) to correlate the age-hardening with the evolution of nanometer-scale structures for understanding the age-hardening mechanisms. The age-hardening of 15Cr ODS steel is attributed to α−α′ phase separation. The age-hardening of 15Cr–5Al ODS steel is considerably higher, relative to 15Cr, 15Cr–7Al and 15Cr–9Al ODS steels, which is due to the strengthening by both Cr-enriched α′ phases and (Ti, Al)-enriched β′ phases formed in 15Cr–5Al ODS steel. In 15Cr–7Al and 15Cr–9Al ODS steels, however, the age-hardening is only attributed to the β′ phase strengthening because α−α′ phase separation is completely suppressed in the ODS steels when the content of Al is ≥ 7 wt.%. In all the Al-added ODS steels, fairly large amounts of β′ phases with crystallography closely related to Heusler-type Fe2AlTi precipitated during the thermal ageing. The strengthening by the β′ phases becomes more significant with the increasing of Al content from 5 wt.% to 9 wt.%. Core/shell structured oxide particles were observed only when α−α′ phase separation occurred, indicating that the formation of such core/shell structure might be closely associated with the process of α−α′ phase separation. The age-hardening mechanisms of the ODS steels with different Al-contents were proposed based on the dislocation barrier models where the amounts of age-hardening were correlated with the nanostructures characterized by APT. The theoretically predicted values of the age-hardening of the FeCrAl-ODS steels agreed well with the values experimentally determined.