Atomic investigation of the critical role of niobium in irradiation-induced hardening of molybdenum-niobium alloys

• In the Mo-Nb system, voids act as strong obstacles, exerting a significant pinning effect on the glide behavior of dislocation lines, while the composite defects formed by the void and niobium atoms around aggregated voids only slightly enhance this pinning effect. • In comparison to voids, the composite defects consisting of the loops with Nb atoms aggregated around them can exert a stronger hindering effect on gliding dislocations. • The pinning effect of Nb atoms on the movement of the adjacent Mo atoms can be attributed to the migration being energetically unfavorable due to the Nb atoms decorated with the loops. Building on this mechanism, the impediment exerted by Nb-decorated loops on the gliding dislocations is consequently strengthened to a significant degree. • The transformation of the interaction modes between Nb-decorated loops and the dislocations is essentially a result of the significant reduction in the gliding ability of the Nb-decorated loops caused by the aggregation of Nb atoms. The Nb-decorated loops can be transformed from mobile obstacles to strongly immobile ones once the Nb concentration in the decorated loops reaches a specific critical value. This critical value increases as the loop size grows. Molybdenum-niobium (Mo-Nb) alloys are considered the preferred materials for energy conversion components in space thermionic reactors because of their excellent creep resistance and high vacuum work function. However, the fundamental mechanism of irradiation hardening caused by solute Nb atoms remains unclear. Here, the interaction between edge dislocations and Nb-decorated irradiation-induced defects was studied using molecular dynamics (MD) simulations, and the results were compared to interactions with irradiation-induced defects and Nb-rich clusters. The simulation results showed that Nb segregation to loops greatly affects the interaction between these loops and edge dislocations. The loops can change from weak barriers to strong obstacles when decorated with Nb, especially when the loop orientation is parallel to the glide plane and inclined relative to the Burgers vector of the dislocation (<11 1 ¯ > loops). The pinning strength of Nb-decorated <11 1 ¯ > loops on gliding dislocations also depends on Nb concentration. Once the Nb concentration exceeds a certain critical level, the <11 1 ¯ > loops can no longer be absorbed or sheared by the dislocations. Additionally, the critical Nb concentration increases with <11 1 ¯ > loop size. This happens because the average binding strength between Nb atoms and loops decreases as <11 1 ¯ > loop size grows, requiring a higher Nb concentration to compensate. These findings offer new insights into the mechanisms behind irradiation hardening in the Mo-Nb system.