Divorced eutectic microstructural NiTiNb shape memory alloy with high recovery rate and recovery stress prepared by laser powder bed fusion

NiTiNb is a wide-hysteresis shape memory alloy (SMA) with great application potential in many industries, such as the aerospace and biomedical fields. However, a series of hot working processes, from casting to subsequent hot working processes, such as forging, heat treatment, and machining, are required owing to the limitations of long cycles, high energy consumption, and limited product structure. Meanwhile, during traditional equilibrium solidification, many eutectic structures composed of matrix and β-Nb phase are formed. The plastic deformation of the large β-Nb phase in the eutectic region may seriously deteriorate the shape memory performance. However, the eutectic structure is inherited, and its influence on shape memory properties cannot be eliminated by subsequent traditional hot workings. In this study, considering non-equilibrium solidification in 3D printing, laser powder bed fusion (LPBF) was attempted to rapidly fabricate NiTiNb alloys. Excellent shape memory performance with a divorced eutectic structure could be tailored using suitable LPBF parameters. It was observed that, in terms of the recovery rate, recovery stress, and thermomechanical cycle stability, the LPBF-NiTiNb alloys were superior to the traditional cast, forged, and extruded NiTiNb alloys. The recovery rate of the LPBF-NiTiNb alloy after 6% pre-strain at low temperatures reached approximately 98.8%, which was 21.5% higher than that of the as-cast alloy. The maximum recovery stress of LPBF-NiTiNb alloy reached 420 MPa, which was 55.9% and 15.4% higher than traditional forged and extruded alloys, respectively. The improved shape memory performance was essentially caused by the solidification difference between the non-equilibrium rapidly solidifying additive alloys and equilibrium solidifying as-cast alloys. To reduce costs and shorten fabrication processes using additive manufacturing, this finding provides a new perspective for tailoring the microstructure and improving the shape memory performance of NiTiNb alloys.