In-situ alloying of NiTiNb ternary shape memory alloys via laser powder bed fusion using pre-alloyed NiTi and elemental Nb powders: Microstructure, phase transformation behavior and functional properties

Abstract

Laser powder bed fusion (L-PBF) provides not only the advantage to make geometrically complex structures, but also the potential for in-situ alloying. In this work, (NiTi)100–xNbx (x = 1, 3, 5, 9 at%) ternary shape memory alloys were in-situ synthesized through L-PBF using a mixture of pre-alloyed near-equiatomic NiTi and elemental Nb powders. Low-Nb (1, 3 at%) alloys exhibit coarse columnar grains and high cracking tendency, while high-Nb (5, 9 at%) alloys show fine microstructure and low cracking susceptibility. Increasing Nb content leads to a transition from planar to cellular/dendritic growth, as a result of the increased constitutional supercooling (ΔTCS) and thermal undercooling (ΔTt) ahead of the solidification front. The L-PBF fabricated (NiTi)91Nb9 alloy shows comparable transformation characteristics to the conventional Ni47Ti44Nb9 alloy, with martensite start temperature (Ms) of 190 K and thermal hysteresis up to ca. 170 K after 20% pre-deformation. Achieving low Ms temperature with near-equiatomic Ni/Ti composition (i.e., Ni/Ti=1.024) is attributed to severe lattice distortion caused by Nb solution in the supersaturated matrix and the depletion of Ti to form the Ti-rich β-Nb phase, which resulted from the intrinsic rapid solidification associated with L-PBF. The in-situ alloyed (NiTi)100–xNbx alloys exhibit good mechanical and functional properties (total fracture strain >7%, and shape recovery rates >85%). This work may provide insights into in-situ alloying of NiTi-based ternary SMAs through L-PBF.