High strength and high ductility of electrodeposited nanocrystalline Ni with a broad grain size distribution

A nanocrystalline nickel with a broad grain size distribution was prepared by direct current electrodeposition, which exhibited high ultimate strength of 1440–1916 MPa and good ductility of 5.6–11.3% under tensile test at a wide strain rate range (1.35E−6 to 1.35 s−1) and room temperature. The good ductility of nc Ni can be attributed to the increased strain hardening ability induced by the inhomogeneous microstrucutre. Two distinguished strain rate sensitivity (0.016 and 0.045) and flow stress activation volume (26b3 and 11b3) were obtained at normal and high strain rate and low strain rate, respectively. The intrinsic microstructure and extrinsic deformation conditions (low strain rate) lead to the transition in the deformation mechanism from the dislocation mediated deformation to both grain boundary and dislocation deformation with decreasing strain rate. The high near-uniform elongation at low strain rate can be explained by its corresponding high m value. The variation of fracture surface morphology with strain rates also implies the underlying deformation mechanism transition.