• • Cu segregation at Al Σ9 (221)[11̄0] GB yields a negative segregation energy, indicating strong thermodynamic driving force for segregation; this improves GB stability and strength, with tensile strength enhancement attributed to reduced free volume and restricted atomic displacement.
• • Dislocation nucleation shifts from a shuffling-assisted regime to a collective-migration regime upon Cu doping, requiring higher critical stress; this delays dislocation nucleation and increases tensile strength of the GB.
• • Cu segregation stabilizes E structures at the GB, preserving their kite shape against structural transition during straining, which contributes to exceptional stability and reduced atomic free volume.
• • Under shear deformation, Cu doping elevates GB shear strength by blocking shear-coupled GB migration; the enhanced resistance is linked to stabilized E structures with reduced atomic free volume.