Atomic size mismatch effects on irradiation-induced defect evolution in Nb-Zr-Ti-V multi-principal element alloys

Multi-principal element alloys (MPEAs) generally exhibit high radiation tolerance because the substantial lattice distortion associated with atomic size mismatch and chemical disorder may enhance the migration energy barriers. The atomic size mismatch effects are less understood because of the complex coupling among defect production, chemical redistribution, and structural instability under irradiation. Here, a Nb-Zr-Ti-V alloy with a body centered cubic structure is used as a model system to elucidate the role of atomic size mismatch in irradiation-induced defect evolution under helium ion irradiation. First-principles density functional calculations show that Frenkel pairs associated with smaller Nb and V atoms have significantly lower formation energies than those associated with oversized Zr, indicating that increasing Nb and V contents enhances early-stage vacancy availability under irradiation. This defect-formation tendency is consistent with the experimentally observed earlier appearance of detectable helium bubbles in Nb/V-rich alloys. Under irradiation, coupled vacancy and interstitial fluxes toward bubbles produce inverse Kirkendall-like radiation-induced segregation, with V enrichment near bubble interiors and pronounced Zr accumulation in the surrounding matrix. The Zr enrichment near bubbles triggers a Bain-path-related BCC→FCC (FCT) transformation and the formation of plate-like radiation-induced precipitates. These precipitates act as strong defect sinks, enhancing vacancy trapping and helium bubble growth and establishing a positive feedback loop between precipitation and bubble evolution. These results highlight atomic size mismatch as an important bubble size-related descriptor that couples with local chemical effects to influence irradiation tolerance in BCC MPEAs.