Early-stage thermal ageing behavior of 12Cr, 12Cr-7Al and 18Cr-9Al ODS steels

The early-stage thermal age-hardening and the resultant nanoscale structure of 12Cr, 12Cr–7Al and 18Cr–9Al ODS steels have been studied by Vickers hardness measurements and atom probe tomography (APT), respectively, after isothermally ageing at 475 °C for 300 h. The thermal age-hardening of 12Cr, 12Cr–7Al and 18Cr–9Al ODS steels was measured to be 4 HV, 45 HV and 34 HV, respectively. The results of APT analyses revealed that no α–α′ phase separation occurred in all the three ODS steels, while a significant precipitation of (Ti, Al)-enriched β′ phase with crystallography closely related to Heusler-type Fe2AlTi took place in both 12Cr–7Al and 18Cr–9Al ODS steels. The chemical compositions of the β′ phase precipitates in 12Cr–7Al and 18Cr–9Al ODS steels were measured to be Fe_2.3AlTi_0.6Cr_0.3 and Fe_2.3AlTi_0.5Cr_0.4, respectively. In 18Cr–9Al ODS steel, no α–α′ phase separation was detected, indicating 9 wt.% Al addition into 18Cr ODS steel could intensely suppress the phase decomposition. The occurrence of β′ phase in 12Cr–7Al and 18Cr–9Al ODS steels indicates that β′ phase should be more thermodynamically stable because of its lower formation energy. The early precipitation of β′ phase may result from the higher diffusion coefficients of Al and Ti in the bcc steel matrix, relative to Cr. It was concluded that the significant early-stage thermal age-hardening in 12Cr–7Al and 18Cr–9Al ODS steels should be due to the β′ phase precipitation. The measured thermal age-hardenings agree well with the values estimated based on dislocation barrier model of Orowan mechanism.