## 3 Results and Discussion
### Microstructure Analysis of NiTiNb Prepared by LSF
#### 3.1.2 Microstructure Features  
It can be seen from Figure 5 that there are B2 NiTi phase, Nb phase, and Ni\({}_{4}\)Ti\({}_{3}\) phase in the samples prepared by LSF. The detection temperature is room temperature. Comparing the transformation temperatures (Table 2), it is found that the martensitic transformation start temperature (\(M_{\text{s}}\)) of the sample is 30 \({}^{\circ}\)C or above. The state of the sample should be a martensitic phase at room temperature, but B19\({}^{\prime}\) phase is detected only in the L6 sample with \(P=2500\) W, \(v=1000\) mm/min, and \(E_{\text{V}}=33.33\) J/mm\({}^{3}\), The B2 parent phase still accounts for the majority, indicating that the high-temperature austenite phase is retained due to the rapid cooling rate during the preparation of LSF. It can be inferred that the samples prepared by LSF have uneven phase composition and element distribution in different building directions. It can be seen from Figure 6 that because of the large energy input of sample L1, the molten pool is enlarged, the powder is exposed to high temperatures for a long time, and the grains are relatively coarse (Figure 6a). The dendrite structure grows in a preferred orientation, and the direction closest to the temperature gradient direction tends to occupy the most advantageous position in the dendrite growth. The dendrite microstructure with a large difference between the growth orientation and the temperature gradient direction disappears, and then presents directional dendrite growth along the building direction, as the energy input increases, the microstructure along the building direction evolves from short and thick columnar crystals to slender columnar crystals. The spheroidal particles in Figure 6f may be the etch pit caused by the corrosion process. \begin{table}
\begin{tabular}{c c c c} \hline
**Location** & **Ni (at\%)** & **Ti (at\%)** & **Nb (at\%)** \\ \hline
1 & 49.67 & 41.93 & 8.39 \\
2 & 44.99 & 46.32 & 8.69 \\
7 & 45.12 & 44.82 & 10.06 \\ \hline \end{tabular}
\end{table} 
Table 2: Atomic ratio of Ni: Ti: Nb. Figure 5: The XRD diagram of NiTiNb prepared by LSF: (**a**) v = 900 mm/min, (**b**) v = 1000 mm/min. Figure 7 is a high magnification microstructure of the L1 and L6 samples. It can be found that there are many fine precipitated phases inside the grains of the NiTiNb alloy prepared by LSF. The L1 sample with a large energy input presents a cell morphology, and the L6 sample with a small energy input presents an obvious dendritic morphology, the eutectic region is obvious, and the grain size is smaller. During the process, layers have experienced constant remelting, and the grains in the remelted area are refined, and equiaxed grains are found in both samples. Figure 6: The OM image of the NiTiNb parallel to the building direction prepared by LSF: (**a**) Sample L1, (**b**) Sample L2, (**c**) Sample L3, (**d**) Sample L4, (**e**) Sample L5, (**f**) Sample L6. Figure 7: High magnification OM microstructure of NiTiNb prepared by LSF: (**a**) Sample L1, (**b**) Sample L1, (**c**) Sample L6, (**d**) Sample L6. Figure 8 shows the XRD curves of sample L7 position 1, sample L7 position 2, sample L7 position 4, sample L8 position 7 at room temperature. All four positions have been heat-treated (see Figure 4 for the position). Ni\({}_{4}\)Ti\({}_{3}\) precipitation and B2 austenite phase are present in all positions, but the Nb phase is not detected in all positions, which indicates that the composition of the sample is unevenly distributed in space.