Abstract: In this study, a gradient ultra-nanograined (GUNG) Cu was prepared by surface rolling and shearing processing at liquid nitrogen temperature. Microstructural analysis reveals a significant presence of ultra-nanograins (∼5-20 nm) within the topmost surface layer (SL), transitioning to coarser grains beneath, culminating in a gradient structure over 600 µm deep. The GUNG Cu exhibits an exceptional strength-ductility synergy, achieving yield strengths of 250-330 MPa and uniform elongations of 17 %-30 %. The deformation mechanisms of GUNG Cu are elucidated through in-situ electron backscatter diffraction and microscopic digital image correlation, highlighting the interlayer-compatible deformation of GUNG Cu under tensile loading. It is noteworthy that the topmost ultra-nanograined SL (within depths of 0-2 µm) in GUNG Cu maintains high mechanical stability with minimal change in grain size during tensile plastic deformation, whereas the subsurface layer (at a depth of ∼15 µm) displays a deformation-driven grain coarsening behavior, facilitating deformation compatibility across individual layers. The enhanced strength-ductility synergy exhibited in GUNG Cu can be attributed to the interplay between interlayer compatible deformation and hetero-deformation induced (HDI) hardening, in which softer and harder layers interact with each other, thus promoting the strain hardening throughout the GUNG structure. The present findings provide a more profound understanding of deformation compatibility and HDI hardening mechanisms in gradient structures, demonstrating how tailored microstructural heterogeneity can potentially circumvent the traditional strength-ductility trade-off in nanostructured materials.

Key words: Gradient nanostructures, Deformation compatibility, Mechanically-driven grain coarsening, Hetero-deformation induced hardening, Strength-ductility synergy