## 5 Structural Properties of the Electrodeposited Nickel Parts
### Fatigue Failure and Other Properties  
Beyond its mechanical characteristics, electrodeposited nickel exhibits a range of material properties that influence its performance. The specific heat capacity of electrodeposited nickel is reported to be around 0.44 J/g\(\cdot\)K, and its coefficient of thermal expansion falls between 13.6 and 17.2 \(\times\) 10\({}^{-6}\) K\({}^{-1}\), both comparable to those of bulk nickel [48]. The thermal conductivity of the electrodeposited nickel tends to be slightly lower than that of pure nickel, due in part to grain boundary and defect scattering, while electrical resistivity is somewhat elevated--typically in the range of 6.5-8.0 \(\upmu\)J\(\cdot\)cm--because of the refined grain structure and minor impurities introduced by additives [67]. The ferromagnetic properties remain largely intact, with saturation magnetization near 55-60 emu/g, though fine-grained deposits may display increased coercivity [48]. Porosity and corrosion resistance of the electrodeposited nickel parts are highly dependent on the deposition quality; uniform, low-stress deposits achieved through optimal mechanical agitation and a stable DC power supply yield higher corrosion resistance and lower porosity [13]. The anode-to-cathode area ratio is also important, ensuring efficient nickel ion replenishment and preventing passivation, particularly in complex shapes where a higher ratio may be necessary [62]. Toward employing the electrodeposited components for structural applications, it is critical to understand and model the various failure mechanisms of these materials. Among various failure modes, fatigue failure is the most dominant in most of the engineering applications [68; 69]. As most of the applications of nickel electrodeposition so far have been limited to electroplating, where a layer of nickel is coated on a base metal, direct evaluations of the fatigue of electrodeposited nickel components have not been realized and require thorough experimentation and analysis. As fatigue failure is dominated by the microstructural features of the part and the internal stresses, as well as surface conditions [70; 71], extensive research should be carried out to understand, model and improve the fatigue behavior of these parts, accounting for factors specific for these parts such as their surface condition, internal stresses, and presence of defects. It should be noted that electrodeposited materials, without any post-processing, inherently contain internal and surface defects, which may limit their structural applications to short- and medium-life fatigue regimes. Furthermore, depending on the process parameters and the electrolyte, various phases can form inside the material, which can be the subject of future research. These new microstructural phases can affect the crack formation and growth behavior in structural components. Structural components fabricated via electrodeposition from a Watts-type bath without additives have tensile internal stresses ranging from 110 to 210 MPa, relatively independent of plating temperature or pH, and this internal stress rises steeply with increased chloride content and current density [15]. The presence of tensile internal stress accelerates the  
Figure 6: Effect of organic additives to Watts electrolytes on internal stress [59]. fatigue failure of the part by opening micro-cracks that form around the co-deposited sulfur and trapped hydrogen. A drop of -45% in fatigue endurance limit when highly tensile Watts nickel was compared with a compressive deposit on identical specimens [72]. The highly tensile internal stresses can be counteracted with additives such as saccharin, thereby increasing the fatigue life [15,72]. Since crack nucleation is a surface-dominated phenomenon, any micro-notches left by brightener-induced growth bands or by hydrogen evolution can dominate the fatigue life [70].