Speaker
Description
Refractory High Entropy Alloys (RHEAs) such as the BCC NbMoTaW system represent a promising class of structural materials for advanced nuclear reactors due to their exceptional physical and mechanical properties. In nuclear environments, structural materials face severe irradiation damage, which is driven by the accumulation of microscopic irradiation defects such as displacement-induced vacancies and transmutation-produced helium (He) atoms. Because He has an extremely low solubility in solids, it aggregates into bubbles, causing severe material degradation such as swelling, blistering, surface roughening, and embrittlement. Since He bubble formation is directly dictated by point defect energetics and kinetics, this study utilizes Density Functional Theory (DFT) to investigate the formation, and migration of monovacancies, as well as interstitial and substitutional He defects formation in the NbMoTaW matrix. The DFT calculations revealed that the formation energies for all investigated point defects exhibit wide distributions driven by the severe lattice distortions and large differences in the local chemical environments characteristic of HEAs. Furthermore, He atoms strongly prefer occupying substitutional positions over interstitial ones, with the octahedral position being energetically favored over the tetrahedral position among the interstitial sites. Crucially, the formation energy distributions of the point defects show significant overlap, indicating a strong tendency for defect recombination that potentially suppresses and reduces the overall formation of He bubbles. Additionally, the migration energies display a wide distribution, revealing that vacancies follow distinct, preferential pathways through the alloy matrix. Ultimately, these results give preliminary insights into the behaviour of point defects in NbMoTaW RHEA and their further impact on He bubbles formation and diffusion.