Effect of solution treatment on the microstructure, phase transformation behaviour and functional properties of NiTiNb ternary shape memory alloys fabricated via laser powder bed fusion in-situ alloying

Abstract

Post-heat treatment is commonly employed to improve the microstructural homogeneity and enhance the mechanical properties of the additively manufactured metallic materials. In this work, a ternary (NiTi)91Nb9 (at.%) shape memory alloy was fabricated by laser powder bed fusion (L-PBF) using pre-alloyed NiTi and elemental Nb powders. The influence of solution treatment on the microstructure, phase transformation behaviour and mechanical/functional properties was investigated. The in-situ alloyed (NiTi)91Nb9 alloy exhibits a submicron cellular-dendritic structure surrounding the supersaturated B2-NiTi matrix. Upon high-temperature (1273K) solution treatment, Nb-rich precipitates are precipitated from the supersaturated matrix. The fragmentation and spheroidization of the NiTi/Nb eutectics occur during solution treatment, leading to a morphological transition from mesh-like into rod-like and sphere-like. Coarsening of the β-Nb phases occurs with increasing holding time. The martensite transformation temperature increases after solution treatment, mainly attributed to (i) reduced lattice distortion caused by the expulsion of Nb from the supersaturated matrix and (ii) the expulsion of Ti from the β-Nb phases that lowers the Ni/Ti ratio of the matrix, which resulted from the microstructure changes from non-equilibrium to equilibrium state. The thermal hysteresis of the solutionized alloys is around 145 K after 20% pre-deformation, which is comparable to the conventional NiTiNb alloys. A short-term solution treatment (i.e., at 1273K for 30 min) improves the strength and ductility of the as-printed alloy, with the fracture stress increases from 613±19 MPa to 781±20MPa and the fracture strain increases from 7.6±0.1% to 9.5±0.4%. Both the as-printed and solutionized samples exhibit good shape memory effects with shape recovery rates >90%.