Low-cycle fatigue fracture behavior of a Mg alloy (AZ61) after alkaline Cu, alkaline followed by acidic Cu, Ni/Cu, and Cr-C/Cu electroplating

Micrometer-thick alkaline Cu-, alkaline followed by acidic Cu-, Ni/Cu- and Cr-C/Cu-coated AZ61 fatigue specimens were prepared, and their low-cycle fatigue behavior was studied. The experimental results show that the number of stress cycles to fatigue failure gradually decreased when the AZ61 specimen was electroplated with alkaline Cu or with alkaline followed by acidic Cu undercoats. Electroplating a protective Cr-C coating on Cu-coated AZ61 increases AZ61's low-cycle fatigue resistance, although some cracks perpendicular to the applied cyclic stress developed in the as-plated Cr-C coating. The Cr-C deposition, which has a high hardness, increases the number of stress cycles for crack initiation. Conversely, electroplating a crack-free Ni deposit on the Cu-coated AZ61 decreases its low-cycle fatigue resistance. This decrease is attributed to inducing a relatively high residual tension stress after Ni electroplating. A large fracture plateau was observed from the coated AZ61 specimens with a relatively low fatigue resistance. In Cu-coated AZ61 specimens, cracking initiated in the interface between Cu and AZ61 and then propagated into the AZ61 substrate due to inducing a residual tension stress.