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

Abstract

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 NiTiFe 0.3 and NiTiFe 0.5 alloys, while severe cracks occur in the NiTiFe 2 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 NiTiFe 0.3 and NiTiFe 0.5 alloys show good superelasticity, e.g. the NiTiFe 0.3 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.