Dose-dependent evolution of dislocation structures and bubble formation in He irradiated Fe-Cr epitaxial film

Abstract

Irradiation-induced damage in fusion and fission environments drives complex microstructural changes in structural materials, critically influencing their performance under extreme conditions. Our approach to investigate irradiation-induced microstructural evolution employs a well-defined material structure that allows for a precise assessment of He-induced bubbles and defects. In this study, a 100 nm Fe-8Cr epitaxial film was synthesized on MgO (001) substrate using molecular beam epitaxy, resulting in a grain boundary-free microstructure. The Fe-8Cr alloy film was subsequently irradiated at room temperature with 30 keV He⁺ at fluences of 1.7 × 10¹⁶ and 1.7 × 10¹⁷ ions/cm², corresponding to peak-damages of 0.5 and 5 displacements per atom (dpa), respectively. Cross-sectional transmission electron microscopy revealed swelling of 2.7% for 0.5 dpa and 8.1% for 5 dpa. The defect morphology evolved from isolated dislocation loops primarily oriented along <111> at low fluence to complex dislocation structures at high fluence. Notably, smaller bubbles with low number density were observed at lower fluence, whereas larger bubbles with higher number density developed at higher fluence, coinciding with the formation of an extensive dislocation network. These results provide fundamental insights into the dose-dependent microstructural evolution of Fe-8Cr alloys under irradiation, offering a foundation for understanding defect interactions in model ferritic systems.