Effects of corrosion-inhibiting surface treatments on irradiated microstructure development in Ni-base alloy 718 (original) (raw)

2018, Journal of Nuclear Materials

The objective of this study is to understand the influence of surface treatments, namely laser shock peening (LSP) and ultrasonic nanocrystal surface modification (UNSM), on the irradiation tolerance of a Ni-base alloy. Surface treatments such as LSP and UNSM have been demonstrated to mitigate the potential for primary water stress corrosion cracking (PWSCC). As such, there is emerging interest in the development and implementation of LSP and UNSM for nuclear reactor components. LSP process utilizes a laser-generated plasma, while UNSM utilizes mechanical contact at ultrasonic vibration speeds, to create near-surface compressive residual stresses, high dislocation densities, twins and subgrains, and nanostructuring of the workpiece. These resultant microstructural changes can substantially affect the creation and evolution of irradiation damage. Herein, we study precipitation hardened Ni-base Alloy 718+, which is utilized in reactor components exposed to PWSCC-inciting environments. Specimens are treated with LSP or UNSM, then irradiated with 2.0 MeV protons to 7 displacements per atom (dpa) at 500°C. The dislocation line density is roughly an order of magnitude larger in the unirradiated LSP and UNSM specimens than in the baseline (untreated) specimen. Irradiation-induced dislocation loop nucleation results in an increase in the areal density of dislocation-type defects. Irradiation also induces disordering of the γ' precipitates, although this disordering appears more extensive in the baseline than the LSP and UNSM specimens. These results considered in the context of overall sink strength. Finally, irradiation-induced softening is observed in all specimens through nanoindentation, and can be ascribed to the overall change in sink strength, which results from the competition between γ' disordering and dislocation loop nucleation.

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