Atomistic simulations of tension–compression asymmetry in dislocation nucleation for copper grain boundaries

Atomistic simulations are used to investigate how grain boundary structure influences dislocation nucleation under uniaxial tension and compression for a specific class of symmetric tilt grain boundaries that contain the E structural unit. After obtaining the minimum energy grain boundary structure, molecular dynamics was employed based on an embedded-atom method potential for copper at 10 K. Results show several differences in dislocation nucleation with respect to uniaxial tension and compression. First, the average nucleation stress for all 〈1 1 0〉 symmetric tilt grain boundaries is over three times greater in compression than in tension for both the high strain rate and quasistatic simulations. Second, partial dislocations nucleate from the boundary on the {1 1 1} slip plane under uniaxial tension. However, partial and full dislocations nucleate from the boundary on the {1 0 0} and {1 1 1} slip planes under uniaxial compression. The full dislocation nucleation on the {1 0 0} plane for boundaries with misorientations near the coherent twin boundary is explained through the higher resolved shear stress on the {1 0 0} plane compared to the {1 1 1} plane. Last, individual dislocation nucleation mechanisms under uniaxial tension and compression are analyzed. For the vicinal twin boundary under tension, the grain boundary partial dislocation is emitted into the lattice on the same {1 1 1} plane that it dissociated onto. For compression of the vicinal twin, the 1/3〈1 1 1 〉 disconnection is removed through full dislocation emission on the {1 0 0} plane and partial dislocation emission parallel to the coherent twin boundary plane, restoring the boundary to the coherent twin. For the Σ 19 boundary, the nearly simultaneous emission of numerous partial dislocations from the boundary result in the formation of the hexagonal close-packed (HCP) phase.