Synthesis and mechanical properties of nanocrystalline nickel electroforming layer in pulse deposition

Electroforming is the precision manufacturing process in which a metallic part is produced in an electrolytic bath by the electrodeposition of metal onto a cathode mandrel. Improvement of the electrodeposit properties has been paid to considerable attention during the last decade. In this paper, a new method was developed to synthesize bulk nanocrystalline(nc) nickel. The high frequency pulse current and high deposit current density were employed to increase the deposit cathodic overpotential and the nucleation rate. With increasing peak current density, the deposit grain sizes was decreased markedly, e.g. average grain sizes ranging from 200 nm to 50 nm were determined when the peak current density varied from 60A dm⁻² to 300A dm⁻² at the on-time of 10 μs and the off-time of 90 μs. The effect of the grain sizes on the mechanical properties was discussed for room and a medium temperature. It was found that the properties of the electrodeposited nickel such as micro-hardness, yield stress (at room temperature and 473 K) were improved substantially with the decrease in the grain size. A deviation of yield stress from Hall-Petch relationship for nc nickel was observed when the grain sizes were decreased to less than 70 nm at room temperature. A low yield stress value of nc nickel was obtained at a temperature of 673 K as a result of its thermodynamic instability.