Achieving excellent strength-ductility-superelasticity combination in high-porosity NiTiNb scaffolds via high-temperature annealing

Metallic scaffolds with lightweight, low elastic modulus, and high energy-absorbing capacity are widely utilized in industrial applications but usually require post-heat treatment to enhance their comprehensive mechanical properties. However, it is unclear how to utilize the impact of β-Nb on the surrounding matrix for NiTiNb ternary alloys to achieve strength-ductility-superelasticity enhancement. Here, we prepared rhomboidal dodecahedral NiTiNb porous scaffolds with a porosity of 85.9% by additive manufacturing. Subsequently, annealing treatment was employed to drastically reduce the phase transformation temperatures and expand the thermal hysteresis. Interestingly, the 850°C annealed scaffold exhibited exceeding double compressive strength of the as-built sample, with a remarkable improvement in ductility and superelasticity. From the microstructure perspective, high-temperature annealing caused a further eutectic reaction of the unmelted Nb particles with the NiTi matrix and the transformation of mesh-like β-Nb into diffuse spherical β-Nb particles. The microstructure evolution after deformation indicated that stress-induced martensitic transformation occurred in the matrix away from the NiTi-Nb eutectic region whereas almost no martensite formed nearby β-Nb particles. Atom probe tomography characterization revealed an element diffuse zone in several nanometers surrounding the β-Nb particle, where the substitution of Nb with Ti led to a higher Ni: Ti atomic ratio, lowering transformation temperatures. Molecular dynamics simulations illustrated that β-Nb particles can not only entangle dislocations internally, acting as reinforcements but also hinder the twin growth, contributing to strain hardening. This work elucidates the influence of β-Nb particles on the deformation mechanism of the NiTi-Nb eutectic region through in-depth atomic-scale investigation, which can provide inspiration for the improvement of comprehensive mechanical properties of NiTiNb alloys.