Abstract: Designing alloys capable of withstanding irradiation is a crucial aspect of developing materials for nuclear reactors and aerospace applications. Local chemical order (LCO) has recently been recognized as a new microstructural parameter to leverage, and its effect on the mechanical properties of body-centered cubic (BCC) multi-principal element alloys (MPEAs) has attracted much attention. However, the impact of LCO on the dynamic evolution of irradiation-induced defects in BCC MPEAs remains much less explored. In this study, we engineered varying degrees of LCO and local lattice distortion in NbZrTi BCC MPEAs by alloying them with different concentrations of interstitial oxygen solutes, and analyzed their effects on the evolution of radiation-induced defects during He irradiation at 673 K to 873 K, with a fluence of 5 × 10¹⁶ ions/cm² and a peak dose of approximately 1 DPA. Using first-principles calculations and atomic-scale analysis of microstructures and chemical elements, we discovered that interstitial oxygen atoms enhance LCO and increase local lattice distortion. These heterogeneities increase the formation energy, and localize the diffusion, of vacancies, hence effectively reducing the transport of aggregating helium that causes bubble swelling. The initiation and growth of dislocation loops and precipitates are depressed as well. The manipulation of irradiation defects in BCC MPEAs, through orchestrating interstitial oxygen solutes and the LCO they provoke, adds a practical strategy for designing advanced alloys for nuclear applications.

Key words: Multi-principal alloys, Local chemical order, Radiation defects, Interstitial oxygen atoms, Transmission electron microscopy