Nanoscale segregation of alien solute atoms at the grain boundary (GB) can enhance the stability and mechanical properties of the GB. Systematic molecular dynamic simulations were conducted to clarify the strengthening effect of Cu segregation on Al Σ9 (221)[ \(1\bar{1}0\) ] GB. The as-predicted negative segregation energy suggests the strong inclination of Cu segregation at Al GBs. Such segregation is expected to improve GB stability and strength. Detailed structural analysis during the uniaxial tensile test indicates that Cu segregation can reduce the free volume of GB atoms and restrict GB atomic displacement, thereby retarding dislocation nucleation and increasing the tensile strength of the GB. The suppressed atomic migrations by Cu doping also give rise to the exceptional stability of E structures at GB, which can retain their kite shape against structural transition during straining. With Cu segregation, the pattern of dislocation nucleation from GB was shifted from “shuffling-assisted regime” to the “collective-migration regime”, wherein the latter necessitates higher critical stress. Further, Cu-doping was also shown to elevate the GB shear strength via blocking the shear-coupled GB migration when subjected to shear deformation. The enhanced GB resistance against shear straining is attributed to the stabilized E structures with Cu segregation featuring reduced atomic free volume. This study provides atomic-scale insights into the stabilizing and strengthening effect of Cu segregation on Al GBs.
