Abstract:A bulk nanocrystalline (nc) pure copper with a high purity and a high density was synthesized by means of electrodeposition. An extreme extensibility (elongation exceeds 5 000%) without strain hardening effect was observed when the nc Cu specimen was rolled at room temperature. Microstructure analysis suggests that the superplastic extensibility of the nc Cu originates from a deformation mechanism dominated by grain boundary activities rather than lattice dislocation, which is also supported by the tensile creep studies at room temperature. This behavior demonstrates new possibilities for scientific and technological advancements of nc materials.