Twofold age-hardening mechanism of Al-added high-Cr ODS ferritic steels

Age-hardening of oxide dispersion strengthened (ODS) ferritic steels with various contents of Cr and Al was investigated by Vickers hardness (HV) measurement and tensile test after ageing at 475°C for up to 9000 h. The correlation between age-hardening and the evolution of nanometer-scale structure and chemistry studied by atom probe tomography (APT) reveals that the age-hardening could be interpreted in terms of twofold hardening mechanism involving α–α′ phase separation and/or the precipitation of (Al, Ti)-enriched β′ phases. The results of APT characterization indicate that the Cr concentration and volume fraction of α′ phase increase significantly while the chemical compositions and volume fraction of β′ phases don't changed considerably during ageing from 300 h to 9000 h. At the early stage of ageing, the age-hardening due to α–α′ phase separation is very limited because of the low Cr concentration of α′ phase, which results in very weak modulus strengthening. With the increasing of ageing time, the age-hardening is increased because of the increasing of Cr content in the α′ phase, resulting in considerably enhanced modulus mismatch strengthening. However, age-hardening due to β′ phase precipitation is significant once the β′ phases are formed and, thereafter, it doesn't change apparently. The metal/oxide interfaces in ODS steels provide nucleation sites for α′ and β′ phases. Based on the comprehensive data developed, the roles of Cr and Al in the age-hardening of ODS ferritic steels were elucidated.