First-principles interpretation toward the effects of Cu, Mg co-segregation on the strength of Al Σ5 (210) grain boundary

The co-segregation of Cu and Mg solutes at grain boundaries (GBs) in Al alloys is widely observed in experiments, but this segregation pattern is much less understood. Herein, the Cu, Mg co-segregation behaviors and their effects on Al Sigma 5 (210) GB properties were systematically studied via first-principles calculations. The segregation order of Cu and Mg atoms was firstly determined wherein the segregation tendency of Cu is comparatively stronger than Mg. Calculations demonstrate that Cu and Mg atoms exhibit attractive interaction, which would promote synergistic co-segregation of both dopants at the GB. It is revealed that Cu, Mg co-segregation not only improves GB thermodynamic stability, but also enhances the GB fracture strength. Detailed analysis of charge density, bonding characteristics, etc., suggests that the newly formed Al-Cu bonds upon Cu segregation are the primary contribution to the increased GB cohesion. The further Mg doping at the Al GB with Cu segregation results in charge depletion and meanwhile decreases Al-Cu bonding length, the net results of which increase GB cohesion, suggesting a synergistically enhanced GB strength with Mg and Cu co-segregation. This work is supposed to provide fundamental insights into the effect of Cu, Mg co-segregation on the strength of Al GBs.