Superelastic behaviors of additively manufactured porous NiTi shape memory alloys designed with Menger sponge-like fractal structures

Additively manufactured porous NiTi alloys hold unprecedented promise in metallic implants due to their low elastic modulus and superelastic behavior. Such porous structures are usually topologically ordered and designed with periodically-repeating unit cells. However, the superelastic behaviors of fractal porous structures have never been studied. The Menger sponge-like fractal structures consisting of non-periodic fractal pores mimick the architectures and biomechanical properties of human bone. In this investigation, porous NiTi alloys designed with Menger sponges were fabricated by additive manufacturing using selective laser melting technology, and their superelastic behaviors were systematically characterized for the first time. Additively manufactured bulk NiTi alloys exhibit fully recoverable superelastic responses with slim stress hysteresis. The mechanical properties of bulk NiTi alloys match the properties of human cortical bones. The Menger sponges display excellent superelastic recovery strain ratios even at high porosity levels. The mechanical properties of highly fractalized Menger sponges are almost identical to the properties of human cancellous bones. The deformation mechanism undergoes a transformation from bending-dominated to stretching-dominated mode when the porous structures are highly fractalized. The unique combination of fractal topology, nanostructured microstructure, highly controllable elastic modulus and large recoverable deformation make the NiTi Menger sponge a promising candidate for metallic implants.