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NbMo crystallizes in the orthorhombic Cmmm space group. The structure is two-dimensional and consists of two NbMo sheets oriented in the (0, 1, 0) direction. Nb is bonded in a 4-coordinate geometry to four equivalent Mo atoms. All Nb–Mo bond lengths are 2.79 Å. Mo is bonded in a 4-coordinate geometry to four equivalent Nb atoms.
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 μm in 9Cr-Ta versus 4.3 ± 0.5 μm 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 0.45T m ), irradiation-induced hardening at 400 °C was observed for both alloys. Irradiation at 490 °C causes the two alloys to exhibit different tensile behavior: 9Cr-Ta softens by 208 MPa in yield stress, whereas 9Cr-NbMo maintains strength. Additionally, 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.
NbMoNbCrMo crystallizes in the orthorhombic Fmm2 space group. The structure is one-dimensional and consists of four NbCrMo ribbons oriented in the (1, 0, 0) direction and four NbMo ribbons oriented in the (1, 0, 0) direction. In each NbCrMo ribbon, Nb is bonded in a 8-coordinate geometry to two equivalent Cr atoms. Both Nb–Cr bond lengths are 2.66 Å. Mo is bonded in a 2-coordinate geometry to two equivalent Cr atoms. Both Mo–Cr bond lengths are 2.56 Å. Cr is bonded in a 2-coordinate geometry to two equivalent Nb and two equivalent Mo atoms. In each NbMo ribbon, Nb is bonded in a 10-coordinate geometry to two equivalent Mo atoms. Both Nb–Mo bond lengths are 2.82 Å. Mo is bonded in a 8-coordinate geometry to two equivalent Nb atoms.