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

Abstract

Ni-W alloys were electrodeposited onto steel sheets with an initial Ni plating. The electrodeposition was conducted in unagitated sulfate solution containing citric acid at pH 5 and 60°C under coulostatic (9.0 × 105 C·m-2) and galvanostatic (100-3 000 A·m-2) conditions. The effect of annealing duration on the structure and hardness of the deposited Ni-W alloys was investigated. Compared with the non-annealed case, annealing for several seconds significantly increased the crystallite size of the Ni in the deposits. Moreover, the crystallite size gradually increased with annealing duration. When the holding time at 700°C was increased to 100 s and 1 000 s, the Ni4 W and NiW precipitates increased and coarsened in the upper layers of the deposits. Annealing increased the hardness of all deposits, especially at high current densities (i.e., at high W content in the deposits). In deposits with 45 mass% W content, the hardness was maximized at a holding time of 10 s at 700°C, and gradually decreased as the holding time increased. Under the optimal precipitation conditions, the Ni4W and NiW precipitates were fine and uniformly distributed throughout the deposits.