Effect of annealing on the structure and hardness of electrodeposited Ni-W alloys

The electrodeposition of Ni-W alloys was conducted from an unagitated sulfate solution containing citric acid at pH 5 and 60°C under coulostatic (3.44-6.22×10⁵ C/m²) and galvanostatic (30-5 000 A/m²) conditions. Before annealing, the lattice constant of Ni increased linearly with an increase in W content up to 40.7 mass% in accordance with Vegard's law. This shows that the W formed a supersaturated solid solution in Ni. At W contents of <37.1 mass%, the deposit morphologies showed a field-oriented texture, with a preferred orientation of the specific plane towards the electric field during deposition, and platelet crystal edges exposed at the surface. At W contents of >40.7 mass% of the solid solubility limit, the cross section of the deposits showed a layered morphology, with a smooth surface and small granular crystals. After annealing, Ni₄W precipitated with deposits of W contents of 32.6 and 37.1 mass%. Fine precipitates of Ni₄W and NiW formed over the entire surface with W contents of 40.7-45.3 mass%. Before annealing, the hardness of the deposits increased with W content, and the increase was particularly large at a W content of 40.7 mass%. The hardness was almost constant regardless of the current density for W contents of >40.7 mass%. The alloy composition required to change the hardness of the deposits corresponded with that required to change the deposit structures. The hardness of the deposits increased for all W contents by annealing, with the increase being particularly large for W contents of >40.7 mass%.