## 3 Results and Discussion
### Tensile Test  
Mini tensile samples were extracted along the horizontal (parallel to the substrate surface) and vertical direction (parallel to the BD). For the horizontal one, the entire sample is mainly included in the region of 8-12 mm above the interface. The vertical one spans from 13.5 to 3.5 mm above the bottom. Then, the gauge length of the vertical sample locates in the range of 7-10 mm. To avoid sample misalignment, mini tensile samples need a preload at the beginning of the test, which can also be found in previous works [38]. All tensile tests were carried out at a constant strain rate of 0.003 mm/mm/s until the sample fracture finally occurred. The stress and strain data points were collected and plotted as stress-strain curves, shown in Figure 10.  
Figure 10: Stress–strain curves: (**a**) Horizontal tensile sample; (**b**) vertical tensile sample.  
Figure 9: Vickers hardness distribution along the BD.  
The stress-strain curves of both horizontal and vertical samples are recorded and plotted in Figure 10, where the horizontal and vertical samples correspond to Figure 10a,b, respectively. The ultimate tensile strength (UTS) values of the horizontal sample and the vertical sample are 622 MPa and 447 MPa, respectively. The two curves are different in feature mainly due to their different compositions of martensite and austenite. The concave part of the vertical sample at the tensile strain range between 0.01 and 0.05 indicates the martensite reorienting stage [39], while the horizontal sample includes less martensite; this plateau region is not found obviously.  
The SEM images of the fracture surface of the fractured samples are listed in Figure 11, where Figure 11a,b indicates the horizontal sample and Figure 11c,d represents the vertical sample. The fracture surface in Figure 11a,b,d shows the mix mode of the fracture with both dimple-like features and microcracks within the fracture surface image. The ductile feature is due to the ductile matrix, while the defects and secondary phases can cause microcracks under tensile loading and finally result in brittle fracture. Figure 11c shows brittle features. The martensite-rich vertical sample shows a lower UTS and mostly brittle fracture at the fracture surface. The strain value at the fracture point of the vertical sample is also lower than the horizontal sample. On the fracture surface of the vertical sample, more pure brittle fracture regions can be found, such as that shown in Figure 11c. The vertical sample displays more brittle features and a lower UTS result, as it includes more Ti\({}_{2}\)Ni secondary phases along the direction of BD.  
Researchers in [40; 41] fabricated DEDed TiNi binary alloy parts through elemental powder blend, and the UTS values of 250 MPa and 320 MPa were obtained, respectively.  
Figure 11: Fracture surface of the fractured tensile sample under SEM: (**a,b**) Horizontal sample; (**c,d**) vertical sample.  
The fracture shows brittle features in [40]. While in [42], TiNi was built by DED with pre-alloyed TiNi powder, and the UTS exceeds 700 MPa. Some other available reported UTS values of TiNi using selective laser melting (SLM) and wire arc additive manufacturing (WAAM) include 601 MPa (SLM [43]), 690 \(\pm\) 15 MPa (SLM [44]), and 571.4 \(\pm\) 18.6 (WAAM [45]). These reported data are tabulated in Table 2 for comparison. Therefore, this work obtains a much higher UTS value among DED works using an elemental powder blend, and it is comparable with the TiNi binary alloys fabricated with AM processes other than DED. In addition, it can still be improved when comparing with DED works with pre-alloyed powders.