LAUSR

Conventional polycrystalline metals become stronger with decreasing grain size, yet softening starts to take over at nanometer regime, giving rise to the strongest size at which the predominate strengthening mechanism switches to softening. We show that this critical size for the onset of softening shifts from ~12 nm for homogeneous nanograined copper to ~7 nm for its gradient nanograined counterpart, and this strongest size decreases with increasing the grain size gradient. The decrease in the strongest size is prompted by mitigation of grain boundary-mediated softening processes accompanying by enhanced intragranular plastic deformations. We found the nanograins as small as 6 nm, mainly involving intergranular sliding in homogeneous structures, reveal anomalous plastic deformation in gradient systems, which is mediated by partial dislocations nucleation and motion. The results on extended dislocation slip and gradient stress and plasticity, stemming from the structure heterogeneity, shed light on an emerging class of heterogeneous nanostructured materials of improved strength-ductility synergy.