| The porous NiTi shape memory alloy(SMA) has broad application prospect in hard tissue replacement and surgical implant because of its unique shape memory effect and superelasticity, good biocompatibility, excellent biomechanical property which could be adjusted by pore structure parameters.The traditional powder metallurgy method has a difficulty in pore structure control, what’s more, porous NiTi alloy exists a threaten of leading to cancer in living body because of the release of a large number of Ni2+ion. The method of mixed powder sintering process by adding NH4HCO3as pore-forming agent was used to prepare porous NiTi SMA in this paper. In addition, the proper preparation technology was finalized by analyzing the influence and the mechanism of sintering parameters on porous NiTi SMAs’pore structure, phase composition and compression performance. The shape memory effect of the sintering samples was studied; in the end, the samples were surface-treated by dealloying method at low temperature for the sake of improving the biocompatibility. The results of the study are as follows:(1) The porous NiTi SMAs were prepared with the optimal parameters for50vol.%NH4HCO3in original blended powders, compacting pressure250MPa, sintering temperature1000℃, holding time6h, and heat treatment at300℃for1h. The sample had porosity56.3%, its pore structure met the requirements of bone implant material by exhibiting a uniform distribution and having regular shape with average diameter317μm.The phase of sample was mainly composed of NiTi phase (include B2and B19’), Ti2Ni phase and TiNi3phase, B2phase was the main phase among of them. The sample showed an excellent mechanical compatibility with compressive strength61.38MPa and elastic modulus3.97GPa that were close to compression performance of human bones.(2) As the porosity and average pore size of samples decreased with the increasing molding pressure, the compressive strength and elastic modulus gradually increased on the contrary. The porosity and average pore size of samples increased first and then decreased when the sintering temperature increased in a certain range; the sample had more uniform pore distribution and smoother pore walls gradually with the increasing temperature, at the same time, compressive strength and elastic modulus increased. As liquefaction of sample became increasingly obvious with the increasing holding time, the average pore size decreased, while the compressive strength and elastic modulus increased; when holding time was more than6hours, the porosity of the sample will be reduced. Compression strength and elastic of elasticity of porous NiTi SMA was significantly reduced with increasing pore-forming agent content.(3) The result of XRD indicated that impurities Ti2Ni phase and TiNi3phase couldn’t be eliminated by heat treatment. DSC curves showed that after the heat treatment, impurities in the matrix could reduce the Ni content, thereby the Ms was improved to widen the phase transition temperature range from328.3K～397.7K to323.4K～402.5K.(4) The test results on the shape memory effect of porous NiTi SMA showed that the permanent deformation of sample has occurred at round3%pre-strain, The sample has shape recovery ratio of97.6%when pre-strain was2%, corresponding to a maximum shape recovery force of17.2MPa.(5) The element nickel in surface part of porous NiTi SMA could be effectively removed by dealloying treatment at50℃, at the same time, the smooth surface turned to be network structure composed with TiO2. The results of XRD and spectrum showed that it could form a445.3μm depth nickeless layer after dealloying for11hours. |
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