Impact of micro-alloying in ion-irradiated nickel: From the inhibition of point-defect cluster diffusion by thermal segregation to the change of dislocation loop nature

Advanced austenitic materials are foreseen as cladding for next-generation fast reactors. They can suffer from void swelling, which is related to radiation-induced dislocation that may be strongly affected by chemical composition. Yet, the underlying mechanism leading to this latter effect is still unclear. Here, we investigate pure Ni, Ni-0.4wt.%Cr and Ni-0.4/0.8/1.2wt.%Ti as face-centered-cubic model materials, to gain an insight into the fundamental mechanism of solute effects on radiation damage in the early stage of irradiation. The microstructural evolution is studied using transmission electron microscopy in-situ self-ion irradiation at 450 and 510°C for doses up to 0.18 dpa, and supported by modelling using kinetic rate theory.Results reveal that micro-alloying has a spectacular impact on the dislocation loop nature, Burgers vector, mobility, number density, size, growth rate, and the critical sample thickness for their formation. A tiny amount of Ti effectively reduces the loop mobility and growth rate, and stabilizes self-interstitial loops against vacancy loops, depending on Ti content and temperature. Based on kinetic rate theory including production bias, this is explained by the suppression of interstitial loss to the surfaces. In the alloys, the equal distribution of Burgers vector and the relative strong increase of loop density is ascribed to the lower loop mobility and stacking fault energy, while both experiment and modeling show that irradiation leads to a Ti depletion at defects. Based on DFT data and present experimental evidences, we attribute these effects to the thermal segregation of oversized Ti atoms at strained lattice sites surrounding the defects.