Fabrication of porous NiTi shape memory alloy for hard tissue implants by combustion synthesis

Abstract

Porous NiTi shape memory alloy (SMA) with ideal pore characteristics and high strength for hard tissue implants has been fabricated by combustion synthesis in this paper. Microstructure and mechanical properties of the product were studied by optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD) and compression test respectively. The experimental results show the combustion temperature of Ni+Ti reactant system using an improved ignition technique can reach 1310 °C at the pre-heating temperature of 350 °C. Porous NiTi SMA with three-dimensionally interconnected pores has the isotropic pore structure, whose general porosity is about 57.3 vol.% and the open porosity ratio is about 86%. Besides two second phases (Ti 2Ni and Ni 4Ti 3) in small amount, the desired products, such as B2(NiTi) and B19′(NiTi) are the predominant phases in porous NiTi SMA. The undesired Ti 2Ni phase as discrete precipitates formed during the SHS process is often present on or near grain boundaries and difficult to be removed by solution treatment. Porous NiTi SMA has a high compressive strength (208 MPa) and a low Young’s modulus (2.26 GPa), which is similar to natural bones and can meet the mechanical property demands of hard tissue implants for heavy load-bearing applications.