Unraveling the enhanced stability and strength of Al Σ9 (221)[ $$ 1\bar{1}0 $$ ] symmetric tilt grain boundary with Mg segregation

The marked Mg segregation at grain boundaries (GBs) in nanocrystalline Al alloys usually contributes additional GB segregation strengthening. To gain a further understanding of this phenomenon, molecular dynamics simulations were conducted to reveal the Mg segregation behavior at Al Σ9 (221)[ $$ 1\bar{1}0 $$ ] symmetric tilt grain boundary (STGB) and its effects on the GB stability and strength. Results reveal that Mg dopants have a large driving force to segregate at Al GBs. Such Mg segregation not only enhances the strength of Σ9 (221) STGB but also improves GB stability. It is found that the Mg segregation turns to enlarge and narrow the strain intervals of stable and thickening stages of Σ9 (221) STGB during tensile test, indicative of the Mg-induced stabilizing effect on the GB structural integrity. Calculations further elucidate that the segregated Mg dopants increase the critical stress for dislocation nucleation, which accounts for the remarkably increased tensile strength of Σ9 (221) STGB with Mg segregation. Such retarded dislocation nucleation is ascribed to the decrement in boundary free volume by Mg segregation. This work will provide important atomic-scale insights into the extra GB strengthening in Al alloys deriving from Mg segregation.