International Journal of Extreme Manufacturing (Jan 2024)
Effect of solution treatment on the microstructure, phase transformation behavior 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 performances of the additively manufactured metallic materials. In this work, a ternary (NiTi) _91 Nb _9 (at.%) shape memory alloy was produced by laser powder bed fusion (L-PBF) using pre-alloyed NiTi and elemental Nb powders. The effect of solution treatment on the microstructure, phase transformation behavior and mechanical/functional performances was investigated. The in-situ alloyed (NiTi) _91 Nb _9 alloy exhibits a submicron cellular-dendritic structure surrounding the supersaturated B2-NiTi matrix. Upon high-temperature (1273 K) solution treatment, Nb-rich precipitates were precipitated from the supersaturated matrix. The fragmentation and spheroidization of the NiTi/Nb eutectics occurred during solution treatment, leading to a morphological transition from mesh-like into rod-like and sphere-like. Coarsening of the β -Nb phases occurred with increasing holding time. The martensite transformation temperature increases after solution treatment, mainly attributed to: ( i ) reduced lattice distortion due to the Nb expulsion from the supersaturated B2-NiTi, and ( ii ) the Ti expulsion from the β -Nb phases that lowers the ratio Ni/Ti in the B2-NiTi 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 1 273 K for 30 min) enhances the ductility and strength of the as-printed specimen, with the increase of fracture stress from (613 ± 19) MPa to (781 ± 20) MPa and the increase of fracture strain from (7.6 ± 0.1)% to (9.5 ± 0.4)%. Both the as-printed and solutionized samples exhibit good tensile shape memory effects with recovery rates >90%. This work suggests that post-process heat treatment is essential to optimize the microstructure and improve the mechanical performances of the L-PBF in-situ alloyed parts.
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