Porous NiTiNb alloys with superior strength and ductility induced by modulating eutectic microregion

Porous metals are lightweight and multi-functional at the expense of strength and ductility, requiring post-heat treatment to elevate mechanical properties. In this study, we report porous NiTi-Nb alloys fabricated via laser powder bed fusion, which achieved a synergistic strength-ductility enhancement by modulating the eutectic microregion through heat treatment. After solution treatment with the aging process, porous NiTi-Nb alloys achieved ultra-high compressive strength (2 GPa) and good ductility (33%). Compared to the as-built sample, the strength was enhanced by 500 MPa, and the ductility was improved by 12%. The effect of heat treatment on the eutectic microregion and the mechanism of strengthening and toughening were revealed. In the as-built sample, β-Nb existed mainly in a mesh-like morphology, while in heat-treated samples, the mesh-like β-Nb was damaged and appeared in an independent morphology (sphere-like or rod-like). Compared to mesh-like β-Nb, independent β-Nb enabled the adjacent matrix to have a more favorable twinning orientation, which could effectively release strain energy through stress-induced martensitic transformation. Moreover, the internal dislocation density of mesh-like β-Nb was relatively low, while independent β-Nb had a superior capacity to store dislocations. During the strain accumulation, the staggered twins and Ti2Ni precipitates together provided a continuous strain hardening capability, which ultimately led to an ultra-high compressive strength. This work can provide inspiration for porous metals to achieve a combination of high porosity and outstanding mechanical properties.