Fig. 3: Tensile properties and thermal stability of nc-Cu composites.

a Representative tensile engineering stress-strain curves for pure nc-Cu and nc-Cu composites (0.4 vol.% and 0.8 vol.% C, respectively). All tensile tests were performed at a strain rate of 5 × 10⁻⁴ s⁻¹ and room temperature. The strain at which necking occurs is marked with an open triangle for pure nc-Cu. The inset shows the simultaneous improvement of yield strength and uniform elongation of nc-Cu composites over pure nc-Cu. b Experimentally measured strain hardening rate dσ/dε (with σ and ε being the true stress and true strain, respectively) for nc-Cu composites and nc-Cu. c Strain hardening exponent (n = d(lnσ)/d(lnɛ)) as a function of strain. The reference nt-Cu plot was calculated from the literature²⁶. d Yield strength versus uniform elongation of nc-Cu composites as compared with those of other Cu-based materials, including heterogeneous Cu, nanostructured Cu alloys and complexion-engineered Cu. Superior properties are observed for nc-Cu composites. Sources of the references are cited in the Methods. e Uniform elongation versus grain size of nc-Cu composites as compared with those of pure nc-Cu. f Thermal stability of nc-Cu composites: cumulative area fraction of grain size of the nc-Cu composite (0.8 vol.% C) annealed at various temperatures (873 K and 973 K) for 1 h, in comparison with that of the reference nc-Cu annealed at 673 K for 1 h. Inset shows pronounced grain growth in nc-Cu, as indicated by the ion-channelling cross-sectional image. In contrast, no grain growth was observed in nc-Cu composites after annealing at 973 K for 1 h.