Atomic-scale insights into the strengthening effect of Cu segregation on Al Σ9 (221)[11̄0] grain boundary
Nanoscale segregation of alien solute atoms at grain boundaries (GBs) can enhance the stability and mechanical properties of the GB. Systematic molecular dynamics simulations were conducted to clarify the strengthening effect of Cu segregation on Al Σ9 (221)[11̄0] GB. The predicted negative segregation energy indicates a strong driving force for Cu segregation at Al GBs, which is expected to improve GB stability and strength. Detailed structural analysis during uniaxial tensile testing reveals that Cu segregation reduces the free volume of GB atoms and restricts 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 exceptional stability of E structures at the GB, which retain their kite shape against structural transition during straining. With Cu segregation, the pattern of dislocation nucleation from the GB shifts from a shuffling-assisted regime to a collective-migration regime, the latter necessitating higher critical stress. Furthermore, Cu doping elevates the GB shear strength by blocking shear-coupled GB migration under 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.