Twin boundary sliding in single crystalline Cu and Al nanowires

Twin boundary sliding (TBS) is a deformation mode that is typically observed in nanocrystalline metals and usually observed during the final deformation stage when deformation via twinning is limited. Unlike the well-established dislocation slip and necking deformation process that occurs in large crystals, recent theories that explain TBS are often controversial, and much remains unsettled. Herein, we develop a theory that enables the prediction of deformation pathways by quantitatively analyzing the relative tendency for the formation of partial dislocations based on the dislocation and fault energy theories. The developed theory considers the effects of static features, such as the stacking fault energy (SFE) as well as the crystal size and orientation, and dynamic structural states characterized by the various fault energies of a material. The theory is initially validated using a Cu crystal exhibiting low SFE for various orientations and sizes. In addition, micro-mechanical tensile tests and molecular dynamics simulations are conducted on Al nanowires to determine whether the proposed theory can be generically extended to crystals with high SFE. The developed theory produces self-consistent results even for metals exhibiting different SFE values. The observations can be used as a guideline to design nanoscale structures for load-carrying applications.