The study reports transmission electron microscope (TEM) observations of deformation twinning in plastically deformed nanocrystalline aluminum, which is not observed in coarse-grained pure aluminum. The presence of these twins is directly linked to the nanocrystalline structure. The authors propose a dislocation-based model to explain the preference for deformation twins and stacking faults in nanocrystalline materials. They find that as the grain size decreases to tens of nanometers, the deformation mechanism transitions from normal slip to partial dislocation-controlled deformation. This transition is supported by experimental evidence of deformation twins and stacking faults in nanocrystalline aluminum, which are not seen in coarse-grained materials. The study also discusses the implications of these findings for interpreting the unusual mechanical behavior of nanocrystalline materials.The study reports transmission electron microscope (TEM) observations of deformation twinning in plastically deformed nanocrystalline aluminum, which is not observed in coarse-grained pure aluminum. The presence of these twins is directly linked to the nanocrystalline structure. The authors propose a dislocation-based model to explain the preference for deformation twins and stacking faults in nanocrystalline materials. They find that as the grain size decreases to tens of nanometers, the deformation mechanism transitions from normal slip to partial dislocation-controlled deformation. This transition is supported by experimental evidence of deformation twins and stacking faults in nanocrystalline aluminum, which are not seen in coarse-grained materials. The study also discusses the implications of these findings for interpreting the unusual mechanical behavior of nanocrystalline materials.