The objective of the proposed research is to quantify the microstructure of Alloy 625 developing after neutron irradiation. Commercial Ni-based alloy, Alloy 625, is used as control rod components in light water reactor. The degradation of this Ni-based alloys under irradiation is linked to the possible precipitation of embrittling Pt2Mo-type ordered phase, dislocation loop formation, void swelling, and the dissolution of strengthening γ” precipitates. Neutron-irradiated data on this alloy is limited, which hinders the understanding of its irradiation response. As a starting point, we selected the three irradiation conditions (0.2, 1.1, and 2.15 dpa at 90 °C) currently available on Alloy 625 in the Nuclear Fuels and Materials Library, which will serve as guideline to understand the microstructural development of Alloy 625 under neutron irradiation and inform future irradiation. The results of this proposed research would be used to understand: possible phases formed and microstructures developed in Alloy 625 at low temperatures and differences and comparison between existing proton/ion-irradiated and neutron-irradiated microstructures (work already performed) to understand possible dose rate effects. Surface preparation will be performed at the INL Materials and Fuels Complex (MFC). Transmission electron microscopy (TEM) and atom probe tomography (APT) specimens will be prepared using standard focused ion beam (FIB) lift-out process and further cleaned using Precision Ion Polishing System (PIPS) to eliminate FIB damage. TEM and APT instruments will be used at Center for Advanced Energy Studies (CAES) for characterization in terms of the formation of voids, dislocation loops, any nano-sized solute clusters, segregation, and precipitates. The experiment results will complement the limited existing neutron-irradiated microstructures of Ni-based alloys, benchmark further comparison or emulation using other proton/ion irradiation conditions, and help clarify the possible phases form at low temperatures and their stability under irradiation. The experiments proposed here are expected to be completed within six months after the proposal is awarded, and the total instrument time needed is anticipated to be within two weeks.