Influence of Ti addition on MX precipitation and creep-fatigue properties of RAFM steel for nuclear fusion reactor

Abstract

As part of the research framework to improve the mechanical properties of the reduced activation ferritic/martensitic (RAFM) steels suitable for higher temperatures, the effects of Ti addition on microstructures and low cycle fatigue (LCF) properties have been investigated. The addition of 0.015 wt% Ti to conventional base RAFM steel (9.3Cr‒0.93W‒0.22V‒0.094Ta‒0.10C‒Fe) increased the volume fraction of MX particles (2.1–4.6%). Atom probe tomography results showed that Ti atoms were rarely partitioned to (V,Ta)-rich MX precipitates with the size of 4‒13 nm, and instead most Ti atoms were involved in the formation of TiC precipitates with the size of 2‒8 nm. The nano-sized TiC particles were homogeneously distributed with the highest population. The LCF property of the Ta/Ti-added RAFM steel was inferior to the base RAFM steel. The cyclic stress reduction rate (softening rate) was higher in the Ta/Ti-added RAFM steel due to a higher density of dislocations at the initial state. The 10 min hold time at the peak tensile strain was imposed on the LCF testing to add creep damage to the cumulative fatigue damage. The creep-fatigue life of the Ta/Ti-added RAFM steel was about 2 times longer than the base RAFM steel. These results indicated that Ti addition-induced nano-sized MX particles are highly resistant to creep damage rather than fatigue damage. The higher fraction of nano-sized TiC and (V,Ta)-rich MX particles was responsible for the substantial retention of fine laths with a high density of dislocations during creep. The cyclic softening mechanism was discussed in terms of microstructure evolution during LCF and creep-fatigue deformation to explain the contradictory creep and fatigue properties. © 2022 Elsevier B.V.