## 3 Results and Discussion
### Phase Transformation of NiTiNb Prepared by LSF  
The phase transformation of NiTiNb during heating and cooling is as follows: During cooling, when the temperature drops to the martensite transformation start temperature (\(M_{\rm s}\)), austenite begins to transform into martensite, and this is called martensite transformation when the temperature continues to decrease to the martensite transformation finish temperature (\(M_{\rm f}\)), the martensite transformation ends, and it is exothermic during cooling. During heating, when the temperature rises to the reverse martensite transformation start temperature (\(A_{\rm s}\)), martensite begins to transform into austenite, and this is called reverse martensite transformation (austenite transformation). Continue to increase the temperature until the reverse martensite transformation finish temperature (\(A_{\rm f}\)), the reverse martensite phase transformation ends, and it is endothermic during heating. \(M_{\rm p}\) and \(A_{\rm p}\) correspond to the peak temperature of the forward and reverse transformation, respectively, indicating that about 50% of the transformation has occurred at this temperature. There are two transformation behaviors in NiTi, one is that B2 austenite directly transforms into B19\({}^{\prime}\) martensite, called one-step transformation B2-B19\({}^{\prime}\), the other is that B2 austenite transforms into R phase first, and then into B19\({}^{\prime}\) martensite, called two-step transformation B2-R-B19\({}^{\prime}\). Figure 11 shows the DSC diagram of sample L1 and sample L6. The transformation temperatures (TTs) of samples are summarized in Table 3. Aging will produce a variety of precipitated phases. The precipitation of nickel-rich particles such as Ni\({}_{4}\)Ti\({}_{3}\) in NiTi alloy will cause the change of Ni/Ti, thereby making the phase transformation temperature higher than room temperature. The diffusion of the Nb element will replace the Ni site in the matrix phase, causing a slight change in the atomic ratio of Ni/Ti in the matrix, which will also cause a change in the martensite transformation temperature. Although the process parameters of L1 and L6 are different, their phase transformation temperature values are very close except for the \(M_{\rm p}\) and \(A_{\rm p}\) values, which means that even if the energy density and the combination of process parameters are different, the phase transformation temperature may still be similar, so consider it may be that the heat treatment at 950 \({}^{\circ}\)C for 15 h had reduced the NiTiNb phase transformation temperature difference. This phenomenon also shows that after long-time heat treatment, the sample with small energy input needs to reduce or increase the temperature to reach 50% phase transformation. The phase transformation hysteresis temperature (\(A_{\rm s}-M_{\rm s}\)) of sample L1 and sample L6 is only 3 \({}^{\circ}\)C, which does not exert the effect of the addition of Nb element on the wide hysteresis effect of NiTiNb alloy. Under the heat treatment at 950 \({}^{\circ}\)C for 15 h, only L1 and L6 of the L1-L6 samples prepared by LSF show phase transformation behavior, but the L6 sample has poor forming accuracy (see Figure 3), therefore, we take the L1 process parameter combination as a reference, and considering that energy density is mostly considered to be an important factor affecting the structure and performance of the prepared samples, we choose energy density \(E_{\rm V}=66.67\) J/mm\({}^{3}\), then reselect the process parameter combinations to explore. The long-term high-temperature heat treatment did not improve the phase transformation properties of L2-L5, Therefore, we appropriately lower the heat treatment temperature, reduce the annealing time, and choose 850 \({}^{\circ}\)C 3 h as the follow-up heat-treatment process. The phase transformation curves of sample L7 and sample L8 are shown in Figure 12. The phase transformation temperatures at different locations are summarized in Table 4.