Effect of Ion Implantation on Mechanical Properties and Kinetic Deformation Mechanisms in Inconel X-750

Loabat Shojaei-Kavan, The University of Western Ontario

Abstract

The recent mechanical tests on the ex-service Inconel X-750 spacers have indicated significant embrittlement and reduced load carrying capacity compared to as installed condition. This is an immense safety concern to the nuclear industry, as in-service spacer examination and their replacement within fuel channels are very costly and impractical. The primary degradation mechanism is complex, and thus provides the focus of the current investigation. Despite few previous reports about hardness of non-irradiated and irradiated Inconel X-750, thermally-activated time-dependent deformation conditions have not been studied. This dissertation has attempted to overcome this scarcity of data by providing a series of fundamental investigations involving the use of novel pyramidal nanoscale indentation test techniques to measure the time-dependent plastic deformation of He⁺ at 300 ºC and Ni⁺ at 25 ºC at different doses separately. Established theories were applied to grain by grain nano indentation testing to assess the hardness of the X-750 alloy under different irradiation doses and temperatures data. Both He⁺ and Ni⁺ ion implantation at lower doses, resulted in softening of the X-750 alloy, however at higher irradiation doses indentation hardness was increased. Constant-load pyramidal indentation tests performed at 25 ºC showed that the average indentation stress, and strain rate increases with decreasing indentation depth and increasing levels of irradiation. The apparent activation energy of the obstacles that limit the rate of dislocation glide during indentation, increased with increased He⁺ implantation dose whereas, it was decreased by increasing Ni⁺ irradiation as irradiation defects increased. On the other hand, apparent activation volume and activation area (Δa) decreased by an increased indentation stress for both He⁺ and Ni⁺ irradiated samples. This indicates that the obstacles controlling deformation process are rate sensitive “thermal” obstacles. These approaches are unique to the literature since they demonstrate the individual effect of irradiation damage on mechanical behavior and operative kinetic plastic deformation on irradiated Inconel X-750.