Mechanism of shape memory effect alteration in Ni _50.4 Ti _49.6 via laser powder bed fusion from the perspective of martensitic twin

The phase transformation from martensitic twins to austenite phase in NiTi shape memory alloys (SMAs) determines their shape memory effect (SME). However, systematic studies are indeed needed to reveal which martensitic twin type is effective in enhancing the SME for NiTi SMAs due to the diversity of martensitic twins. In this work, based on laser powder bed fusion (LPBF) NiTi SMAs, an excellent strength-ductility combination of 900.5 MPa and 13.9% was obtained by utilising a low laser power (70 W) and scanning speed (200 mm/s), which represents one of the highest values attainable among current LPBF NiTi SMAs. Furthermore, based on transmission kikuchi diffraction, it is found that high-density [Formula: see text] I-type martensitic twins enhanced the SME of LPBF NiTi SMAs significantly, and the maximum SME recovery ratio is 95.8% after 10 cycles at controlled strain (6%) cyclic tensile loading-unloading. Conversely, the formation of [Formula: see text] I-type martensitic twins in the matrix weakened the SME of LPBF NiTi SMAs remarkably and lost SME after 10 cycles at controlled strain (6%) cyclic tensile loading-unloading. This research offers some valuable insights into the relationship between martensitic twin and SME for LPBF NiTi SMAs and paves the new way for modulation of microstructure to enhance mechanical properties and SME for NiTi SMAs via LPBF.