## 4 Conclusions  
In this work, a TiNiCu ternary shape memory alloy single-wall structure was fabricated by the DED process through an elemental powder mixture. As AM is a layer-based manufacturing technique and the elemental powder mixture is a novel concept in AM, the single-wall structure was equally separated into five location points along the BD to perform comprehensive material characterization on the chemical composition, phase, mechanical properties, and functional properties with regard to various locations of interest. The main conclusions are summarized below:  
For the overall element composition along the BD, at the upper locations, the compositions of the three elements are much closer to the as-mixed composition. The small area EDS analysis results show that all matrix phases are TiNi phases, where a small portion of Ni atoms are replaced by Cu. At the lower locations, the main secondary phase is Ti\({}_{2}\)Ni. The Ti\({}_{2}\)Ni composition decreases when the location goes up due to the decrease in the Ti element composition. At the top of the as-deposited single-wall structure, a small amount of the Ni-rich secondary phase exists where the Ti composition decreases to slightly below 50 at.%.  
The XRD patterns indicate the increase in austenite TiNi and the decrease in martensite TiNi from the bottom to the top, which is related to the changing of the compositions of Ti and (Ni + Cu). The variations in the martensitic TiNi and austenitic TiNi compositions agree well with the atomic compositions of Ti and (Ni + Cu). The average Vickers hardness at the martensite-rich area (200-250 HV0.2) is lower than the austenite-rich area (300-450 HV0.2), which also matches the XRD results well.  
The UTS of the as-deposited TiNiCu tensile sample reaches 622 MPa along the horizontal direction in the area with less of the martensite phase. The vertical sample that includes martensite-rich areas within the gauge length gets a UTS of 447 MPa, and the strain at the fracture is also lower than the horizontal sample. The UTS values of both are comparable with other AM-based TiNi binary alloys.  
Strong martensite-austenite transformation signals are witnessed from the DSC results in sections S12, S23, and S34 within the 0-120 \({}^{\circ}\)C temperature range, which reflects a good phase transformation response at room temperature. The three sections' thermal hysteresis values are much smaller than AM-based TiNi binary alloys. These results agree well with reducing thermal hysteresis by adding Cu in the Ti-Ni binary system.  
Conceptualization, Y.C.; methodology, Y.C., X.Z.; formal analysis, Y.C., X.Z., J.W.N.; investigation, Y.C., M.M.P.; writing--original draft preparation, Y.C., X.Z., M.M.P.; writing--review and editing, Y.C., X.Z., M.M.P., J.W.N., F.L.; supervision, J.W.N., F.L.; project administration, F.L.; funding acquisition, F.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by NSF, grant number CMMI 1625736.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material Type** & **Processing** & **Hysteresis (\({}^{\circ}\)C)** & **Ref.** \\ \hline \multirow{4}{*}{TiNiCu} & DED & 11–16 & This work \\  & Plasma skull push–pull & 21.1 & [30] \\  & Vacuum arc melting & 23.8–30.6 & [46] \\  & Ingot metallurgy & 5–19 & [47] \\  & Magnetron sputtering & 9 & [49] \\ \hline \multirow{4}{*}{TiNi} & DED & 35.3–45.9 & [50] \\  & SLM & 30 & [51] \\ \cline{1-1}  & SLM & 28 & [44] \\ \cline{1-1}  & Powder metallurgy & 30–36 & [52] \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison of thermal hysteresis of TiNiCu and TiNi.