Pore formation mechanism and characterization of porous NiTi shape memory alloys synthesized by capsule-free hot isostatic pressing

Porous NiTi alloys with different porosities were fabricated by capsule-free hot isostatic pressing (CF-HIP) with ammonium acid carbonate (NH4HCO3) as a space-holder. The microstructure and porosity of porous NiTi produced with different NH4HCO3 contents and sintering temperatures were determined. Two different creep expansion models are used to explain the pore expansion mechanism during the sintering process, which involves slow and continuous reduction of the argon pressure at high temperatures. When the NH4HCO3 content is 30 wt.% and the sintering temperature is 1050 °C, an ideal porous NiTi alloy with 48 vol.% porosity and circular pores (50–800 μm) is obtained. Compression tests indicate that the porous NiTi alloys with 21–48% porosity possess not only lower Young’s moduli of 6–11 GPa (close to that of human bones) but also higher compression strength and excellent superelasticity. Cell cultures reveal that the porous NiTi prepared here has no apparent cytotoxicity. The porous materials are thus promising biomaterials in hard tissue replacements.