Mechanisms of radiation-induced G-phase precipitation in Fe-MnNiSi ferritic model alloys at 400 °C

A high-purity $\text{Fe-}0.7\text{Mn-}1.8\text{Ni-}0.8\text{Si}$ model reactor pressure vessel alloy was irradiated with 22.5 MeV Fe^9＋ ions at 400 °C, producing a radiation dose of 1–2 dpa at a depth of $1\mu m$, following by annealing at the same temperature for up to 34 weeks. Irradiation led to the formation of four Mn-Ni-Si-enriched features: spherical precipitates in the bulk and on dislocation lines, sandwich-like precipitates, and toroidal segregation on dislocation loops. High-resolution transmission electron microscopy and synchrotron X-ray diffraction identified bulk precipitates as G-phase, exhibiting L2${}_1$ symmetry, a cube-on-cube orientation with the matrix, and semi-coherent interfaces. Atom probe tomography showed that the bulk G-phase precipitates have the formula ${\text{Ni}}_{16}{\left({\text{Mn}}_x{\text{Fe}}_{1-x}\right)}_6{\text{Si}}_7$, with increased Fe substitution in the Mn sublattice under irradiation. G-phase precipitates exhibited thermodynamic stability during post-irradiation annealing, while sandwich-like phases dissolved, indicating metastability. Irradiation reduces the matrix solubility limit, which is evidenced by the reduced volume fraction of G-phase precipitates observed in post-irradiation annealed samples. Heterogeneous precipitation of the G-phase on dislocation lines likely occurs through radiation-induced segregation (RIS) of Ni, Si, and Mn. Bulk sandwich-like precipitates with a dense Ni-Si-enriched phase in the center are strongly evidenced to form through a two-step mechanism: the SIAs clustering into a dense NiSi phase, followed by the kinetics of vacancy-solute clustering, leading to the precipitation of the less dense G-phase.