Abstract: This study investigates the abnormal nucleation and growth of helium (He) bubbles in NbZrTi-based refractory high entropy alloys (RHEAs). NbZrTi, NbZrTiHf, and NbZrTiHfV alloys were subjected to 400 keV He ions irradiation with a fluence of 1 × 10 17 cm - 2 at temperatures ranging from 623 to 723 K. Spherical aberration-corrected transmission electron microscopy was employed to analyze the distribution of He bubbles in each irradiated alloy, as well as the distribution of elements and atomic structure modifications around the bubbles. In NbZrTi, semi-coherent Zr-rich precipitates were observed near He bubbles, whereas NbZrTiHf and NbZrTiHfV exhibited semi-coherent hexagonal close-packed precipitates enriched in Zr and Hf. Additionally, NbZrTiHfV displayed coherent body-centered cubic precipitates enriched in V throughout the irradiated region. The results indicate that semi-coherent precipitates, owing to their higher interfacial energy, significantly affect He bubble nucleation and growth, leading to a non-uniform distribution of large He bubbles. Notably, all three alloys exhibited abnormally large He bubbles compared to pure Nb and face-centered cubic HEAs under identical irradiation conditions. This can be attributed to the lower vacancy formation and migration energies in NbZrTi-based RHEAs, which increase vacancy concentration and mobility during irradiation, thereby promoting the diffusion and growth of He atoms. These findings provide insights into the irradiation behavior of NbZrTi-based RHEAs, contributing to the design of advanced materials for nuclear applications.

Key words: Refractory high entropy alloy, Radiation-induced precipitates, He bubble, Transmission electron microscopy, Local lattice distortion