Effect of S content in deposit on low-cycle fatigue properties of Ni film electroplated from sulfamate baths

Nickel was electrodeposited from sulfamate baths containing various amounts of thiourea. The effect of S in the deposit on the fatigue properties of electroplated Ni was studied by low-cycle fatigue test and by the observation of the fractured surface of the deposit. The results obtained were as follows: (1) In the deposit of low S content (1.3×10^-4 mass%), the low-cycle fatigue properties were able to be estimated by the tensile test on the basis of Manson's cogradient law. However, the result of tensile test was not applicable to the deposits of high S content (46×10^-4 mass%) because of their poor fatigue properties. (2) The striation was recognized on the fracture surface of the deposit containing 1.3×10^-4 mass% S, which suggested that transgranular fracture peculiar to the low-cycle fatigue occurred. On the contrary, the deposit of 46×10^-4 mass% S was consisted of polycrystal colonies with the polygonal shape (several millimeters in size). Since the strength in colony boundary was lower than that in colony itself, the fracture proceeded along the colony boundary. (3) During annealing, S in the deposits diffused into the boundary between grains recrystallized. Therefore, the fracture might occur along the grain boundary embrittled by S.