The microstructure effects on irradiation response of ferritic - martensitic steels

Microstructural optimization to achieve greater mechanical strength has been one of the focuses in ferritic-martensitic steels development. However, these optimized microstructures' effects on the radiation response are not well known. In this work, two ferritic-martensitic steels (9Cr-NbMo and 9Cr-Ta) underwent neutron irradiation in the High Flux Isotope Reactor, and their room-temperature post-irradiation tensile properties and microstructure evolutions were investigated and compared. These two steels exhibit similar pre-irradiation tensile behavior, and their yield strengths are higher than that of other ferritic-martensitic steels by about 200-250 MPa. Microstructural characterization on pre-irradiated materials reveals a smaller grain size in 9Cr-Ta (2.8 +/- 0.3 mu m in 9Cr-Ta versus 4.3 +/- 0.5 mu m in 9Cr-NbMo) but higher dislocation density and precipitate density in 9Cr-NbMo. As is common for ferritic-martensitic steels at low irradiation temperatures (less than about 0.45Tm), irradiation-induced hardening at 400 degrees C was observed for both alloys. Irradiation at 490 degrees C causes the two alloys to exhibit different tensile behavior: 9Cr-Ta softens by 208 MPa in yield stress, whereas 9Cr-NbMo maintains strength. Microstructural characterizations were performed, including precipitate growth, dislocation, and defect formation. Using the barrier hardening model for microstructure-property correlation, the softening in irradiated 9Cr-Ta is primarily attributed to the significant dislocation recovery, while the strength lost from the slight dislocation recovery in 9Cr-NbMo was compensated by the additional strength from the irradiation-induced cavities. The microstructure effect (primarily precipitate, dislocation and boundary) on the radiation response is discussed herein.