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

Electrodeposition of Ni-W alloys was conducted from an unagitated sulfate solution containing citric acid at pH 5 and 60 °C under coulostatic (9.0×10⁵) and galvanostatic (100-3000 A·m^-2) conditions onto steel sheets with an initial Ni plating to investigate the effect of annealing duration on the structure and hardness of deposited Ni-W alloys. The crystallite size of Ni in deposits with several seconds annealing increased significantly compared to without annealing, and gradually increased with annealing duration. With increasing the holding time at 700 °C to 100 and 1000 s, the precipitates of Ni4W and NiW were increased and became coarse at upper layers of deposits. The hardness of all the deposits increased with annealing, and the degree of increase became significantly large with increasing W content in deposit by increasing current density. The hardness of deposit with W content 45 mass% was maximum at holding time of 10 s at 700 °C, and gradually decreased with increasing annealing duration. The hardness of deposits increased when the precipitates of Ni4W and NiW were uniformly distributed in a fine size, which showing the optimum precipitation state at holding time of 10 s at 700 °C.