## 3 Results and Discussion
Below the titanium dioxide layer, there is a nickel-rich layer (the chemical composition close to Ni\({}_{3}\)Ti) followed by the NiTi core. It is worth noting that TiO\({}_{2}\) is non-stoichiometric in this case, consistent with the findings in [39]. The Al\({}_{2}\)O\({}_{3}\) layer is absent in the image. Figure 7c shows the chemical element contents along the path denoted by the orange line in Figure 7b. The only chemical elements found in the sample using the line scan were Ni, Ti, O\({}_{2}\), and W. Several other surface examinations did not reveal any presence of Al\({}_{2}\)O\({}_{3}\), most likely due to delamination after many loading \begin{table}
\begin{tabular}{l l l l l} \hline \hline \multirow{2}{*}{**NiTi Sample ID**} & \multirow{2}{*}{**Load Applied During ALD (kg)**} & \multicolumn{2}{c}{**Stress**} & \multirow{2}{*}{**Nf\_ald to Applied Load (MPa)**} & \multirow{2}{*}{**Nf\_ald**} & \multirow{2}{*}{**Nf\_ald\_mean**} & \multirow{2}{*}{**SD\_ald**} \\  & & & & & \\ \hline ALD\_wire\_1 & 1.7 & 1359 & 5302 & & \\ \hline ALD\_wire\_2 & 2.2 & 1759 & 320,024 & & \\ \hline ALD\_wire\_3 & 2.2 & 1759 & 30,417 & & \\ \hline ALD\_wire\_4 & 1.7 & 1359 & 752 & & \\ \hline ALD\_wire\_5 & 2.8 & 2238 & 1,112,882 * & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: List of fatigue-tested NiTi wire samples with ALD coating. Nf denotes the number of mechanical cycles (ranging from 0 to 700 MPa, with the minimum stress between 0 and 50 MPa) at room temperature until fracture occurred. cycles. The failure to detect any traces of Al\({}_{2}\)O\({}_{3}\) in the coated sample after the fatigue test raises questions regarding the proposed mechanism for the improved fatigue resistance of the coated samples, specifically the formation of compressive stress around the Al\({}_{2}\)O\({}_{3}\) layer deposited within microcracks. Another potential mechanism will be proposed in the following subsection. Figure 5: The stress—strain diagrams until the fracture at room temperature for the individual noncoated samples (**a**–**f**). Figure 8 presents a comparison of the tensile loading curves up to fracture for both the as-purchased wires (four samples) and the ALD-coated wire under a load of 2.8 kg. No significant difference is observed between the stress-strain curve of the ALD-coated sample and those of the as-purchased wires. The incidental heat treatment during the ALD process is not substantial enough to induce a noticeable change in the ultimate elongation or ultimate tensile strength. Figure 6: The stress—strain diagrams until the fracture at room temperature for the individual ALD-coated samples (**a**–**e**) and the electrical resistance—number of loading cycles diagram (**f**) for NiTi wire sample ALD_wire_3. Alternative Mechanism for Improved Fatigue Resistance  
During the ALD process, the loaded-wire samples, which contain a nonzero fraction of austenite and martensite, are exposed to an elevated temperature. This fulfills the condi  
Figure 8: The comparison of the tensile loading curves of the as-purchased and ALD-coated wires (under load of 2.8 kg) loaded until the fracture. Figure 7: HAADF (high-angle annular dark field) image of a thin lamella cut parallel to the wire axis and the radial direction (**a**,**b**). The atomic content (at.%) of selected chemical elements (**c**) along the orange line shown in panel (**b**). tions for low-temperature shape setting (LTSS) [40]. LTSS is associated with phenomena observed when a NiTi element containing oriented martensite is heated under external constraints that inhibit both strain recovery and transformation to austenite. Consequently, the reverse transformation occurs alongside dislocation slip [41]. As for our cold-drawn samples, the local plastic deformation due to LTSS that occurs in regions with the highest stress concentrations during the ALD treatment contributes to the alleviation of stress around microcracks [21].