Effect of Fe addition on the microstructure, transformation behaviour and superelasticity of NiTi alloys fabricated by laser powder bed fusion

NiTi-Fe ternary shape memory alloys were produced by laser powder bed fusion (L-PBF) using the pre-alloyed NiTi and elemental Fe powders. A large process window exists to fabricate dense NiTiFe0.3 and NiTiFe0.5 as alloys, while severe cracks occur in the NiTiFe2 alloy. The cracking susceptibility is studied through thermodynamic analysis. Fe is mainly dissolved in the NiTi matrix, which is mainly due to the low Fe addition and rapid cooling associated with L-PBF process. The martensite transformation temperatures (MTTs) decrease monotonously with the increase of scanning speed, the decrease of laser power, or the increase of Fe addition, providing a feasible approach to tailoring the MTTs. Due to the decrease of the MTTs, the NiTiFe0.3 and NiTiFe0.5 alloys show good superelasticity, e.g. the NiTiFe0.3 alloy shows a superelastic recoverable strain of 5.2% (pre-strain of 6%). This work suggests that L-PBF is a potential method to in-situ synthesise ternary NiTiFe alloys.