Microstructural evolution of Zr-2.5Nb alloy under helium ion irradiation at room temperature and 350°C: grain morphology and allotrope effects

To elucidate the roles of α-Zr grain morphology and α/β phase difference in the irradiation response of Zr-2.5Nb alloy, TEM thin‑foil samples were irradiated with 50 keV He + ions at fluences of 3 × 10 16 cm −2 (∼0.3 dpa), 5 × 10 16 cm −2 (∼0.5 dpa) and 1 × 10 17 cm −2 (∼1 dpa) at room temperature (RT) and 350°C. The microstructural evolutions in morphologically different α-Zr grains (elongated lath‑shaped L‑type vs. equiaxed E‑type) as well as in α‑Zr and β‑Zr grains were characterized by TEM. Post‑irradiation characterization shows that at 350°C, He bubbles in L‑type grains are consistently larger than in E‑type grains, the size difference increasing to ∼1 nm at ∼1 dpa; at RT the difference is negligible. Dislocation loops in α‑Zr are mainly ‑type and no precipitates form. Notably, under the irradiation condition of 350°C and ∼1 dpa, α‑Zr and β‑Zr exhibit marked differences: the size and density of He bubbles/dislocation loops are much larger in α‑Zr, which contains discernable loops, whereas β‑Zr shows mostly “black‑spot” defects. α‑Zr/α‑Zr grain boundaries have larger but less dense He bubbles than α‑Zr/β‑Zr boundaries. EDS analysis reveals that increasing fluence enhances elemental segregation (Zr depletion, Nb enrichment) in β‑Zr. These results demonstrate that β‑Zr possesses superior irradiation tolerance under identical conditions, providing direct evidence for irradiation‑induced Nb enrichment and offering guidance for phase/boundary engineering of Zr‑2.5Nb alloys in nuclear applications.