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

Synopsis : Electrodeposition of Ni-W alloys was conducted from an unagitated sulfate solution containing citric acid at pH 5 and 60 °C under coulostatic (3.44×10⁵-6.22×10⁵ C/m²) and galvanostatic (30-5000 A/m²) conditions. Before annealing, the lattice constant of Ni increased linearly with an increase in the W content up to 40.7 mass% in accordance with Vegard's law, showing the W supersaturated solid solution into Ni. At W contents of < 37.1 mass%, the deposits showed a morphology of field oriented texture, which a preferred orientation of specific plane occurs toward the electric field in deposition, and the edges of platelet crystals were exposed at surface. At W contents of > 40.7 mass% of solid solubility limit, the cross section of deposits showed a layered morphology, while the surface became smooth with small granular crystals. After annealing, Ni₄W precipitated in deposits of W contents of 32.6 and 37.1 mass%, while both Ni₄W and NiW precipitated entire surface finely in deposits of W contents of 40.7 to 45.3 mass%. Before annealing, the hardness of deposits increased with W content, and the increase was particularly large at W content of 40.7 mass%. The hardness was almost constant regardless of current density at W contents of > 40.7 mass%. The alloy composition to change the hardness of deposits significantly corresponded with that to change the structure of deposits. The hardness of deposits increased at all W contents by annealing, and the degree of increase was particularly large at W contents of > 40.7 mass%.