Twin Boundary Orientation and Spacing-Depended Dislocation Patterns in Nanotwinned Aluminum: An Atomistic Study

The high strength and ductility of structural materials are crucial for preventing sudden failure during service. In recent years, extensive research has highlighted the critical role of nanotwins in enhancing the strength of metals. However, further improvement relies on a deeper understanding of the relationship between microstructure and mechanical properties. In particular, the anisotropic dislocation patterns in nanotwinned metals, in the absence of conventional grain boundaries, remain poorly understood. To address this issue, this study employed molecular dynamics simulations to investigate the uniaxial tensile behavior of nanotwinned single-crystal Al in three representative directions, either perpendicular or parallel to the twin boundaries. The results indicate that loading along <111> and <110> directions initially activates the {111}⟨112⟩ slip system, whereas loading along the <112> direction first activates the {100}⟨110⟩ slip system. For <111> loading, the initial dislocations observed are flower dislocations, whereas for <112> and <110> loading, they appear serrated. Furthermore, under <110> loading, when twin boundary spacing is less than 7.015 nm, a distinctive ring-shaped necklace dislocation composed of jog dislocations emerges; above this spacing, jog dislocations become less pronounced or even vanish. In addition, during deformation, twinning dislocations nucleate on stacking faults, which leads to the formation of shell-like secondary twin regions. This study unveils fundamental deformation mechanisms in nanotwinned metals under different loading directions and twin boundary spacings, advancing the understanding of dislocation evolution during plastic deformation.