PNNL

Abstract

Inconel 718 (IN718) is a candidate structural alloy for advanced nuclear energy systems, where it is exposed to high irradiation doses and elevated temperatures. Here, we investigate the temperature-dependent microstructural and mechanical evolution of IN718 subjected to 5 MeV C²? ion irradiation at 30 °C, 500 °C, and 850 °C to doses up to 100 dpa. Positron annihilation lifetime spectroscopy (PALS) reveals negligible changes in mean positron lifetime across all irradiation conditions, indicating limited evolution in open-volume defect populations despite high displacement damage. Consistently, X-ray diffraction (XRD) confirms retention of the face-centered cubic (FCC) ? matrix under all conditions, while a progressive lattice contraction at elevated temperatures indicates solute depletion from the matrix . Transmission electron microscopy reveals a temperature-dependent microstructural evolution: at 30 °C, damage is depth-sensitive, characterized by Frank loops at 2 µm and a distinct carbon-rich zone at 2.6 µm; at 500 °C, Cr–C-rich periodic defect arrays appear in the Bragg peak region alongside d-phase precipitates emerge within the matrix; finally, at 850 °C, displacement damage triggers the ballistic dissolution of metastable ?’ precipitates within a 1.1 µm wide zone at the Bragg peak region, and are replaced by thermodynamically stable ? phases. These transformations represent a shift from the alloy’s intended metastable strengthening toward a radiation-stabilized microstructure, providing a framework for predicting long-term stability in high-temperature irradiation environments.. Microhardness measurements reveal pronounced irradiation-induced hardening across all conditions, with a non-monotonic temperature dependence and a peak hardness of 271.4 ± 25.9 HV at 850 °C and 100 dpa. Indentation depths (~2.6 µm) were constrained within the irradiated layer to ensure measurement fidelity. These results demonstrate that, while the FCC matrix remains structurally intact, high-temperature irradiation fundamentally destabilizes the ?'/?? strengthening architecture in IN718, promoting solute redistribution and carbide-mediated defect trapping. This work establishes distinct temperature-dependent regimes of radiation damage and phase stability, with important implications for the deployment of precipitation-strengthened Ni-based alloys in extreme nuclear environments.