This study reports transmission electron microscope (TEM) observations of deformation twinning in plastically deformed nanocrystalline aluminum. The presence of deformation twins is directly related to the nanocrystalline structure, as they are not observed in coarse-grained aluminum. The researchers propose a dislocation-based model to explain the preference for deformation twins and stacking faults in nanocrystalline materials. These results indicate a transition from normal slip to partial dislocation-controlled deformation mechanisms when grain size decreases to tens of nanometers, with implications for understanding the mechanical behavior of nanocrystalline materials. Nanocrystalline aluminum films with thicknesses of ~200 nm and ~400 nm were prepared by physical vapor deposition. These films were deformed by microindentation and manual grinding to introduce large plastic strains and facilitate TEM observations. The deformed films showed deformation twins, identified by mirror symmetry between the twin and matrix in atomic-resolution images. The twin boundaries were found to be parallel to the {111} planes and several atomic planes thick, caused by tilted twinning dislocations. The study also compared the critical shear stresses required to nucleate perfect dislocations and Shockley partial dislocations. The results showed that twinning becomes a preferred deformation mode in aluminum with grain sizes around 10 nm, consistent with HRTEM observations. The model also explains the preferential generation of partial dislocations, leading to stacking faults and deformation twins in nanocrystalline grains. The findings provide insights into the mechanical behavior of nanocrystalline materials and offer an alternative explanation for grain size strengthening and strain hardening.This study reports transmission electron microscope (TEM) observations of deformation twinning in plastically deformed nanocrystalline aluminum. The presence of deformation twins is directly related to the nanocrystalline structure, as they are not observed in coarse-grained aluminum. The researchers propose a dislocation-based model to explain the preference for deformation twins and stacking faults in nanocrystalline materials. These results indicate a transition from normal slip to partial dislocation-controlled deformation mechanisms when grain size decreases to tens of nanometers, with implications for understanding the mechanical behavior of nanocrystalline materials. Nanocrystalline aluminum films with thicknesses of ~200 nm and ~400 nm were prepared by physical vapor deposition. These films were deformed by microindentation and manual grinding to introduce large plastic strains and facilitate TEM observations. The deformed films showed deformation twins, identified by mirror symmetry between the twin and matrix in atomic-resolution images. The twin boundaries were found to be parallel to the {111} planes and several atomic planes thick, caused by tilted twinning dislocations. The study also compared the critical shear stresses required to nucleate perfect dislocations and Shockley partial dislocations. The results showed that twinning becomes a preferred deformation mode in aluminum with grain sizes around 10 nm, consistent with HRTEM observations. The model also explains the preferential generation of partial dislocations, leading to stacking faults and deformation twins in nanocrystalline grains. The findings provide insights into the mechanical behavior of nanocrystalline materials and offer an alternative explanation for grain size strengthening and strain hardening.