• Mechanism innovation: By adopting a two-step heat treatment method of “solution treatment + aging”, we can avoid the drawbacks of single heat treatment while giving full play to the advantages of single heat treatment. A microstructure was constructed with a uniform distribution and ideal size of strengthening phases such as precipitates and dislocations. This will help to further explore the performance potential of NiTi alloys using more sophisticated processes. • Process innovation: By studying the mechanical properties of LPBFed NiTi alloys over a wide temperature range (40℃), the mechanism of microstructure sensitivity to temperature was elucidated. This will help to further broaden the application scenarios of NiTi alloys. This study clarified the influence mechanisms of these two different heat treatment routes (aging treatment alone (Group A) and solution treatment followed by aging (Group SA)) on the microstructure and properties of LPBFed NiTi alloys, thereby helping to resolve this issue. The results show that the densities of Ti 4 Ni 2 O x precipitates and dislocations in Group SA samples are lower, However, they are retained in Group A samples. Microstructural features such as large particles of Ti 4 Ni 2 O x and high density of dislocations can be detrimental to the NiTi alloy matrix because they induce stress concentrations under applied stress. In addition, the distribution of Ni 4 Ti 3 precipitates and dislocations in Group SA samples is more uniform, which promotes more homogeneous stress in the matrix of NiTi alloy during compression. Moreover, as the critical stress of the A → M transition in the NiTi alloy increases with ambient temperature, the detrimental effects of Ti 4 Ni 2 O x and the chaotic high-density dislocations on the matrix become increasingly pronounced. Therefore, when the test temperature gradually increases, especially the test temperature window reaches 30 K, the superelasticity, eCE, and coefficient of performance ( COP ) values of Group SA samples are significantly better than those of Group A.
Chen et al. (Sun,) studied this question.