Towards a reliable nanohardness-dose correlation of ion-irradiated materials from nanoindentation tests: A case study in proton-irradiated vanadium

Nanoindentation has been commonly used for evaluating the hardening effects of ion-irradiated materials. Nonetheless, establishing a reliable correlation between the hardness and irradiation dose is never trivial, due to not only the intrinsic analytical challenges of this technique, such as size effects, pile-up effects, etc., but also the fact that the irradiation dose is usually uneven inside the stress volume under the indenter, especially near the depth of dose peak. In the present work, the hardening in pure V irradiated with 1 MeV proton at various fluences is investigated by using nanoindentation tests, combined with the characterization of both irradiation defects and dislocations of the indented material. Under the cross-sectional indentation, we demonstrate that the nanohardness-dose correlation can be unified from the samples irradiated to different fluences and at different depths on each sample, as long as the lateral spanning of indenter is carefully considered. Crystal-plasticity finite-element-modeling simulation results can well describe the measured hardening-dose correlation, as well as the observed features on the change in strained fields and pile-up effects after irradiations. Moreover, the measured hardness is further corrected for the dose-dependent pile-up based on the surface profiling, and the indentation size effects based on surface indentation tests for deeper indentation depth, to reach a reliable connection between the hardening effects and the irradiation dose. Furthermore, microstructural characterization of the indented materials shows the pinning of dislocation by irradiation defects and the sweeping of those defects during dislocation migration. Molecular dynamics results suggest that the drag of loops by edge dislocations might cause the annihilation or aggregation of small loops, which could be responsible for the lower density but the larger size of irradiation loops in the strained region.