## 3 Results and Discussion
### Microstructure Analysis of NiTiNb Prepared by LSF
#### 3.1.2 Microstructure Features
The microstructure and phase transformation temperature of the sample parallel to the building direction are not the same, the range of reverse martensitic transformation start temperature (martensite begins to transform into austenite, \(A_{8}\)) of sample 7 is \(-\)31.5 \({}^{\circ}\)C to \(-\)24.1 \({}^{\circ}\)C, the range of reverse martensitic transformation finish temperature (martensite completely transforms into austenite, \(A_{I}\)) is 13.7 \({}^{\circ}\)C to 23.5 \({}^{\circ}\)C, the test temperature of XRD is room temperature, which is between \(A_{8}\) and \(A_{I}\), and so we may consider with regard to the martensite detected in position 1 that the reverse martensite transformation is not completed and the martensite remains. It is considered that the martensite detected at position 7 may be the martensite phase caused by the repeated rapid solidification and stress caused by remelting during the preparation process and it is retained in the subsequent heat treatment. The OM diagrams of sample L7 and sample L8 are shown in Figure 9. Compared with the microstructure parallel to the forging direction in the hot forged structure, which shows large and highly elongated grains [9; 10], the NiTiNb microstructure prepared by LSF is closer to the cast microstructure. From the low magnification OM image, it can be observed that there are epitaxially grown columnar grains at position 1 parallel to the building direction [49] (Figure 9a). In the high-magnification OM image of this position, it is found that the crystal grain direction has changed during the growth of columnar crystals, and the sample microstructure no longer shows regular epitaxial growth, but grows along an irregular direction. Because the solid-liquid interface gradient will gradually change from the vertical laser scanning direction at the bottom of the molten pool to the scanning direction when the molten pool is solidified, the growth direction of the microstructure from the bottom to the top will change, and the adjacent layer is not prone to continuous epitaxial growth. During the preparation process, the temporal and spatial fluctuations of temperature cause the temperature gradient to deviate, and the microstructure will change in space [50]. Comparing the high-magnification OM microstructure of position 2 and position 4, it is found that in position 2 close to the substrate of sample L7 (Figure 9f),  
Figure 8: XRD diagram of L7 and L8 samples with heat treatment. the grain size is larger, and in position 4 close to the top of the sample L7 (Figure 9g), the grain size becomes smaller, and the proportion of equiaxed grains increases. The grain size increases with the increase of laser power, but no obvious change is observed with the increase of laser scanning speed [51]. Figure 9f,h shows that as the laser power increases, the crystal grain size increases, and the proportion of equiaxed crystals increases. The martensite transformation temperature of the sample with reduced grain size will decrease [52], and the experimental results confirm the martensite transformation start temperature of position 7 is lower than that of position 2. Figure 10 is the element distribution diagram of positions 1, 2 in sample L7 and positions 7 in sample L8. The structure of LSF-NiTiNb is similar to that prepared by traditional methods [53]. It is composed of NiTi matrix and \(\beta\)-Nb, eutectic \(\beta\)-Nb phase is obvious, meanwhile, large numbers of \(\beta\)-Nb precipitates densely distribute in the B2-NiTi  
Figure 9: OM diagram of sample L7 and sample L8 (the left column is the low magnification microstructure, and the right column is the high magnification microstructure): (**a**) sample L7 position 1, (**b**) L7 position 2, (**c**) sample L7 position 4, (**d**) sample L8 position 7, (**e**) sample L7 position 1, (**f**) sample L7 position 2, (**g**) sample L7 position 4, (**h**) sample L8 position 7.  
matrix between eutectic regions.