Kinetic Paths of Radiation-Induced G-Phase Precipitation in Fe-MnNiSi Ferritic Model Alloys

Abstract

A high-purity Fe-0.7Mn-1.8Ni-0.8Si model reactor pressure vessel alloywas irradiated with 22.5 MeV Fe9+ ions at 400◦C, producing a radiation dose of 1–2 dpa at a depth of 1μm, following by annealing at the same temperature for up to 34 weeks.  Irradiation led to the formation of fourMn-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 L21symme-try, 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 Ni16(MnxFe1−x)6Si7, with increased Fe substitution in the Mn sub-lattice under irradiation. G-phase precipitates exhibited thermodynamic stability during post-irradiation annealing, while sandwich-like phases dissolved,indicating metastability. Heterogeneous precipitation of the G-phase on dis-location 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 certainly formed 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. The incubation time for homogeneous G-phase nucleation suggests a late blooming phase. While SIAs primarily influence phase selection, vacancies in excess lowers the solubility limit. Additionally, growing G-phase precipitates act as biased point defect sinks, promoting vacancy elimination, which may explain the absence of observed vacancy clusters in this alloy.

Keywords: Point defects Precipitation, Transmission electron microscopy (TEM), X-Ray Diffraction (XRD), Atom probe tomography (APT)