Superelastic behaviors of biomedical porous NiTi alloy with high porosity and large pore size prepared by spark plasma sintering

A process of combing use of one-step spark plasma sintering (SPS) method and space holder technique was developed to have successfully fabricated the porous NiTi alloy. To optimize porous NiTi for bone implant applications, the effects of pore characteristics and microstructures on mechanical properties and superelastic behaviors of the porous NiTi alloys were investigated by varying sintering temperatures (800-1050 degrees C) and ammonium hydrogen carbonate (NH4HCO3) contents (0-20 wt.%). The results showed that porous NiTi alloys with 18-61% porosity and 21-415 mu m average pore size all consisted of nearly single NiTi phase with few undesired phases Ti2Ni and Ni3Ti. The superelastic recovery strain ratio of the porous NiTi alloy could be improved up to 90% through training, but with further increasing of training cycles, the curves showed an obvious ladder shaped stress plateau, which indicated the collapse of the pores. This enhanced superelasticity will greatly degrade the mechanical mismatch between bones and porous NiTi. Furthermore, with increasing the porosity and pore size, elastic modulus and compressive strength of the porous NiTi alloys decreased. However, the compressive strength was higher or close to those of human bone and the elastic modulus was close to those of human bone. The unique combination of inter-connected pore characteristics, pure phase composition, low elastic modulus, high strength and large superelastic recovery strain made this material a good candidate for ideal long-term load-bearing hard tissue implants. (C) 2015 Elsevier B.V. All rights reserved.