## 3 Results and Discussion
### Phase Transformation of NiTiNb Prepared by LSF
NiTi alloy powder melts and solidifies rapidly under the action of a high energy density laser as the preparation process proceeds, the previously solidified material undergoes a cyclic heating and cooling process. Therefore, the complex thermal history during the processing will lead to complex microstructures evolution [45], and the phase transformation behavior of NiTi alloy is very sensitive to microstructure changes [1], so the change \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**LocationTTs (\({}^{\circ}\)C)** & \(R_{\rm s}\) & \(R_{\rm f}\) & \(M_{\rm s}\) & \(M_{\rm f}\) & \(A_{\rm s}\) & \(A_{\rm f}\) & \(R_{\rm P}\) & \(M_{\rm P}\) & \(A_{\rm P}\) & \(A_{\rm P}-M_{\rm P}\) \\ \hline
2 & 7.4 & \(-16.8\) & \(-23.1\) & \(-67.3\) & \(-24.1\) & \(16.9\) & \(-6.3\) & \(-37.4\) & \(3.7\) & \(41.1\) \\
3 & — & — & \(-3.8\) & \(-70.4\) & \(-28.1\) & \(23.5\) & — & \(-36.4\) & \(-2.4\) & \(34\) \\
4 & — & — & — & — & — & — & — & — & — & — & — \\
5 & — & — & \(-25.1\) & \(-67.8\) & \(-31.5\) & \(13.7\) & — & \(-38.8\) & \(-3.6\) & \(35.2\) \\
6 & — & — & — & — & — & — & — & — & — & — & — \\
7 & 9.6 & \(-17.5\) & \(-33.2\) & \(-69.4\) & \(-18.1\) & \(16.1\) & \(2.5\) & \(-52.1\) & \(-4.1\) & \(48\) \\
8 & — & — & \(-16.9\) & \(-70.9\) & \(-24.8\) & \(21.3\) & — & \(-31.5\) & \(-0.9\) & \(30.6\) \\
9 & 18.6 & \(-4\) & \(-17.5\) & \(-61.6\) & \(-22.3\) & \(27\) & \(5.5\) & \(-38\) & \(-2.7\) & \(35.3\) \\
10 & — & — & \(-53.8\) & \(-88.3\) & \(-58.6\) & \(-22.3\) & — & \(-65.8\) & \(-33.4\) & \(32.4\) \\
11 & — & — & \(-53.1\) & \(-98.6\) & \(-61.1\) & \(-6.9\) & — & \(-80.8\) & \(-48.3\) & \(32.5\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Phase transformation temperature of sample L7 and sample L8. of preparation process parameters will affect the microstructure of NiTi alloy and thus significantly affect the phase transformation. In previous studies, the input energy density is often considered to be the key factor affecting the phase transformation. Studies have found that the martensite transformation temperature generally increases with the increase of the input energy density [11; 31; 34; 35; 56; 57; 58; 59]. Even under the same SLM energy density, the NiTi alloys prepared with different combinations of process parameters have a phase transformation temperature change of 205-277 K [38; 39]. In the research of this article, it was also discovered that the phase transformation temperatures of NiTi prepared by different process parameters under the same energy density are not the same. Therefore, the input energy density is not the only factor that determines the quality of NiTi forming [46]. In this study, the phase transformation behavior of samples prepared with different process parameters is different and the phase transformation behavior of the same sample at different positions is also different [40]. After the samples were annealed at 850 \({}^{\circ}\)C for 3 h, not only did the phase transformation temperatures increase, but the phase transformation behavior also changes from one-step transformation to two-step transformation. Aging leads to precipitation of Ni\({}_{4}\)Ti\({}_{3}\) from the Ni-rich matrix, which reduces the Ni content in the NiTi matrix [55], thus increasing the phase transformation temperature [60]. In addition to affecting the phase transformation temperature, reducing the Ni content can also increase the thermal hysteresis width and phase transformation heat [5]. There is also another explanation that the change of Ni/Ti ratio caused by the loss of Ni is the main reason for the change of phase transformation temperature [31; 39], and the decrease of Ni/Ti will cause the martensite transformation temperature to increase [5; 45]. This is one of the reasons why the phase transformation temperature and the phase transformation hysteresis increase after the sample undergoes heat treatment. [PERSON] et al.