Mg-content dominates irradiation hardening in Al alloys by controlling the defect rounding at the initial stage

Solute elements significantly control the mechanical properties of aluminum (Al) alloys, yet the correlation between irradiation-induced microstructures, mechanical properties, and the interaction mechanism of solute magnesium (Mg) atoms with defects remains unclear to date. The influence of solute Mg atoms on irradiation defects and hardening behavior in Al alloys was systematically investigated via in-situ ion irradiation, nanoindentation, and first-principles calculations. The results revealed a strong correlation between Mg content, microstructural evolution, and irradiation hardening. Increasing Mg content inhibits the growth and interaction of dislocation loops at the initial stage, postponing the formation of the network unit and thereby reducing their size. This leads to alloys with high Mg-content exhibiting a lower number density of dislocation network junctions and delayed irradiation hardening at the same dose. In contrast, dislocation loops in low Mg-content Al alloys experience less growth impediment and tend to evolve into elongated morphologies along preferential directions. These phenomena can be attributed to the formation of solute-defect pairs, where Mg atoms—due to their larger atomic size and weak bonding with Al atoms—generate local elastic fields that preferentially trap vacancies. Although higher Mg content enhances initial solute strengthening, excessive Mg may promote solute clustering—leading to more clusters formed after irradiation. Increasing Mg also strengthens the local barrier effect on point defects. These contribute to inhibiting defect rounding and result in a smaller average size and higher number density of dislocation loops at low doses. Further, the formation of solute-defect pairs influences the kinetics of network formation. These findings provide new insights into the composition-defect-property relationship in irradiated Al alloys and offer guidance for the design of Al alloys for nuclear applications.