Study On Porous TiNi Shape Memory Alloy

TiNi alloy was used as biomaterials for its unique shape memory effect and excellent mechanical properties. Implantation invalidation is likely to occur as its high Young's module and surface inertia. Porous materials have favorable assembled properties in strength and density, and its cellularity is propitious to bone ingrowth, then good fixation is obtained. In this paper, simple element powders metallurgy technique under inert gas protection was used to prepare porous TiNi shape memory alloy. Powders characters, compact pressure, sintering temperature, sintering time were considered. Pore feature, phases, strengths, module, transformation, electrochemical properties, and biocompability were studied by advanced materials analysis techniques and animal model. Simple mechanical model of porous TiNi alloy was built. Results revealed following conclusions: Porous TiNi alloy with good comprehensive properties could be prepared by element powders metallurgy under Ar gas protection: 45% porosity, 130μm mean pore diameter, 677MPa ultimate compressive strength, 247MPa flexural strength, 11.2GPa compress Young's module, and over 2% restorable strain were obtained from porous TiNi alloy prepared by 300 mesh Ni powders and 300 mesh Ti powders (mixing for 4 hours, compact with 100MPa, sintering 8 hours at 980℃). Small powders resulted in lower porosity, smaller pore diameter, higher ultimate compressive strength, flexural strength and compressive Young's module. With compact pressure increasing, the porosity and pore diameter decreased, and the ultimate compressive strength, flexural strength, compressive Young's module increased. With the increasing of sintering temperature, pore diameters decreased and porosity increased under 980℃, then increased. The ultimate compressive strength, flexural strength and compressive Young's module increased with the increasing of sintering temperature. At 980℃, ultimate compressive strength, flexural strength and compressive Young's module increased with the increasing of sintering time. TiNi(B2), TiNi(B19), Ti₂Ni, TiNi₃,, Ti₃Ni₄ were found in the porous TiNi alloy sintered 8 hours at 980℃. Ni was found in the porous TiNi sintered less 4 hours at 980℃ because of insufficient diffusion. N₂ and H₂ were not appropriate protection gas. Shape of pores was characterized by F_shape and F_RCE. Distribution of pores of porous TiNi had two-segment fractal character. Critical pore diameters were in the range of 60-140μm. General fractal dimensions were in 1-3, and with the increasing of pore diameters, the general fractal dimensions decreased. In 0.9% NaCl solution, the stainlessness of porous TiNi was not good as solid TiNi. Polarization curve indicated that crevice corrosion occurred in porous TiNi. Porous TiNi has lower polarization resistance and higher surface capacitance than solid TiNi. With the increasing of dipping time in Hank's solution, Ni release decreased whether porous TiNi or solid TiNi. But at first stage of dipping, porous TiNi alloy released more Ni than solid TiNi. Animal model showed: there were tight attachment between porous TiNi and bone tissues; new bone tissues were found at the pores inner alloy. These indicated that porous TiNi alloy had good bone conductivity and osteogenetic cell integration.