Disordering of helium gas bubble superlattices in molybdenum under ion irradiation and thermal annealing

Abstract Self-organization of gas bubbles causes the formation of an ordered array of nanoscale gas bubbles (a gas bubble superlattice), a highly efficient mechanism for gas storage under irradiation. The stability of helium (He) gas bubble superlattices in molybdenum (Mo) under krypton (Kr) ion irradiation and thermal annealing has been investigated. It was found that He gas bubble superlattices gradually become disordered under Kr ion irradiation at 300 °C, and the order-disorder transition process completes at 2.5 dpa. Both transmission electron microscopy (TEM) and synchrotron-based small-angle X-ray scattering (SAXS) reveal that the order-disorder transition of He gas bubble superlattices is associated with a slight increase in average bubble size. Phase-field modeling indicates that inhomogeneous growth and coarsening of bubbles/voids cause the disordering of imperfect superlattices under irradiation and implies that highly ordered superlattices could potentially exhibit much stronger resistance to irradiation damage. Under thermal annealing, He gas bubble superlattices in Mo become unstable and disordered at 1000 °C, with bubble size increasing from ∼1.1 to ∼1.6 nm. The finding in this research provides insights into the disordering mechanisms of gas bubble superlattices, as well as guidance for designing stable defect superlattices in harsh environments.