Radiation effect on nano-indentation properties and deformation mechanisms of a Ni-based superalloy X-750

Nano-indentation analysis was employed to investigate the mechanical response of heavy ion irradiated X-750 Ni-based superalloy in both solution-treated (ST) and precipitation-hardened (PH) conditions. Helium pre-implantation was carried out at 300 °C up to 5000 appm followed by Ni⁺-ion irradiation up to 1 dpa at room temperature or 400 °C. Cross-sectional TEM examination was used to characterize the microstructural evolution of the irradiated material and correlate this with nano-scale mechanical test results. Nano-hardness measurements after irradiation at 400 °C showed a similar trend of irradiation-induced hardening, for both ST and PH materials. In contrast, radiation at room temperature resulted in different mechanical responses, with hardening in the ST condition compared to softening in PH. The hardening behaviour was attributed to the creation of irradiation-induced defects including, cavities, Frank loops and small defect clusters; whereas, the γ′-precipitate instability (disordering/dissolution) in PH material resulted in the observed softening. The individual and combined contribution of each type of defects under irradiation hardening were estimated by employing three different obstacle models; results were verified by nano-indentation data for both ST and PH materials. In addition, the softening of the irradiated PH material which results from disordering and dissolution was separately calculated, allowing an estimate of the total yield strength change. TEM analysis of post-indentation microstructure revealed that un-irradiated X-750 deformed by homogenous dislocation motion; however, localized deformation in the form of nano-twins was the dominant deformation mechanism in irradiated X-750.