Role of non-equilibrium solidification effects in tuning martensitic transformation of laser powder bed fusion-fabricated NiTi shape memory alloys

The martensitic transformation behavior of NiTi shape memory alloys fabricated using laser powder bed fusion (LPBF) is sensitive to processing parameters. This sensitivity is closely tied to non-equilibrium microstructure formed during the LPBF processing. In the present work, the microstructure of LPBF-fabricated NiTi intermetallic compounds was characterized using electron backscatter diffraction (EBSD), high-resolution transmission electron microscopy (HRTEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Three types of non-equilibrium microstructures were observed in LPBF-fabricated NiTi specimens: fine grains, high-density dislocations introduced by post-solidification thermal cycling and point defects generated by rapid solidification. Among these, the point defects were identified as the primary factor governing martensitic transformation in NiTi. These findings show that by tuning the LPBF processing parameters and heat-treatment conditions, the evolution of non-equilibrium microstructure can be controlled, enabling the tailoring of the shape memory properties of NiTi components.