## 3 Results and Discussion
### Phase Transformation of NiTiNb Prepared by LSF
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**SampleTTs (\({}^{\circ}\)C)** & \(M_{\rm s}\) & \(M_{\rm p}\) & \(M_{\rm f}\) & \(A_{\rm s}\) & \(A_{\rm p}\) & \(A_{\rm f}\) & \(A_{\rm p}-M_{\rm p}\) & \(E_{\rm V}\) (J/mm\({}^{3}\)) \\ \hline L1 & 32 & 10 & \(-\)2 & 35 & 51 & 71 & 41 & 66.67 \\ L6 & 35 & 19 & 0 & 38 & 58 & 70 & 39 & 33.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Phase transformation temperatures of L1 and L6 prepared by LSF. Figure 11: DSC diagram of NiTiNb prepared by LSF: (**a**) v = 900 mm/min, (**b**) v = 1000 mm/min. Figure 12: _Cont_. Figure 12: DSC diagram of NiTiNb prepared by LSF: (**a**) heat-treated part of sample L7, (**b**) non-heat treated part of sample L7, (**c**) heat-treated part of sample L8, (**d**) non-heat treated part of sample L8. When the sample has not been heat-treated, the phase transformation temperature of the L7 sample with high laser power is higher than that of the L8 sample with low laser power, which may be related to the volatilization of Ni caused by high laser power, because the volatilization of Ni will increase the phase transformation temperature. Firstly, comparing Figure 12a and 12b, it is found that heat treatment will change the phase transformation behavior of the sample: from one-step B2-B19\({}^{\prime}\) to two-step B2-R-B19\({}^{\prime}\), and increase the characteristic temperature and phase transformation hysteresis. The characteristic temperatures related to martensitic transformation increase in a small range, the characteristic temperatures related to inverse martensitic transformation increase in a large range, and the transformation hysteresis (\(A_{\rm P}-M_{\rm P}\)) increase by about 5.9 \({}^{\circ}\)C. Secondly, it can be seen from Figure 9a that the phase transformation behavior of the same sample after heat treatment is different at different building heights. The position of the sample close to the substrate is a two-step transformation; the middle position of the sample is a one-step transformation, and no phase transformation is detected near the top of the sample, which indicates that there is uneven microstructure in the building direction. However, no matter whether the sample is heat-treated or not, no phase transformation is detected near the top of the sample. It is speculated that the mixed powder near the top of the sample does not undergo a sufficient remelting process, and the uniformity of microstructure is damaged. The specific reasons need to be further explored. It is found that heat treatment changes the phase transformation behavior of the sample by comparing Figure 12c,d. Like sample L7, the phase transformation behavior of sample L8 also changes from one-step to two-step after heat treatment. The characteristic temperatures of phase transformation increase obviously, and the phase transformation hysteresis (\(A_{\rm P}-M_{\rm P}\)) increases by about 2.8 \({}^{\circ}\)C. It can be seen from Figure 12c that the transformation behavior of the same sample after heat treatment is also different in the area perpendicular to the building height. R transformation occurs at both ends of the sample, and a small shoulder appears in the reverse martensitic transformation (austenite transformation), which may be related to the internal stress and high temperature during processing, similar to aging under pressure [55]. This indicates that the microstructure perpendicular to the building direction is also uneven. Different combinations of LSF processing parameters will produce different microstructures, which will affect the phase transformation temperature. Heat treatment will increase the phase transformation temperature and change the phase transformation behavior. However, due to the non-uniformity of the microstructure of LSF prepared by mixed powder, the change degree at different positions of the sample is not completely consistent.