| NiTi shape memory alloy has been applied in medical fields as hard tissue implants for its unique shape memory effect and excellent mechanical properties. The implanted NiTi alloy is likely to become loose from the human tissue owing to its high Young's module and surface inertness. Porous materials have favorable combined properties in strength stiffness and density, and its three-dimensionally connected porous structure allows the ingrowth of new tissue and the transportation of body fluids. In this paper, the powders metallurgy method and the thermal explosion mode of Self-propagating high-temperature synthesis were used to prepare porous NiTi shape memory alloys with good porosity and large pore size. In order to improve its biocompatibility, corrosion resistance and restrain Ni release, the porous NiTi alloy was anodized and deposited with hydroxyapatite (HA). Pore characteristics, microstructure, mechanical properties, phase transformation, surface composition and electrochemical properties were studied by advanced materials analysis techniques. Models to correlate compressive properties of porous NiTi alloy with process parameters were established by the artificial neural network method.For the porous NiTi alloys prepared by the powder metallurgy method, pore morphology and mechanical properties can be tailored by suitably controlling processing parameters such as compaction pressure, sintering temperature, sintering time and NH4HCO3 amount. With the increase of compaction pressure, porosity of the porous NiTi alloy decreases, and compressive strength and Young's modulus increase. With the increase of sintering temperature or holding time at 980℃, porosity of the NiTi alloy increases first and then decreases, and compressive strength and Young's modulus both increase. Porous NiTi alloy with 53.2% porosity,178μm mean pore diameter,228MPa ultimate compressive strength,4.8GPa compress Young's module, and 2% restorable strain was prepared using a suitable fabrication process with a compaction pressure of 100MPa, a sintering temperature of 980℃and a sintering time of 8 hours. The gradient porous NiTi alloy was obtained by designing and controlling the amount and distribution of NH4HCO3, and its compressive strength and Young's modulus both increase compared with the uniform porous structure. XRD analysis showed TiNi(B2), TiNi(B19'), Ti2Ni and TiNi3 exist in the porous NiTi alloy sintered at 980℃for 8 hours. Thermal explosion reaction and pore characters of products were affected by three crucial processing variables, namely hearing rate, Ti particle size and green density. When increasing heating rate and green density or decreasing Ti particle size, thermal explosion reaction became more severe with more heat released, and porosity and mean pore size of the as-reacted products increased. The porous NiTi alloy with 55.3% porosity,287μm mean pore diameter,192MPa ultimate compressive strength,4.2GPa compress Young's modulus, and 2% restorable strain was prepared with a heating rate of 15℃/min, Ti particle size of 44μm, green density of 50%. XRD analysis shows that the thermal explosion products are mainly composed of TiNi(B2) and TiNi(B19') phases, with a little Ti2Ni and TiNi3 phases.During the anodic oxidation process with direct current (DC) and pulse current (PC), if the electrolyte temperature and current density are too high, more heat is released and the oxide film tends to be eroded. Uniform oxide film with a thickness of about 180nm on surface and cell walls of the porous NiTi alloys prepared by the powder sintering or thermal explosion methods was obtained with pH2SO4 of 200ml/L, T of 0～10℃, and I of DC 3A and PC 6A. The anode oxidation process is very effective to obtain a uniform and comparatively thick oxide film. After immersing in simulated body fluid for 15 days, the surface of the two kinds of NiTi alloys with anodic oxidation is covered with a continuous HA layer about 480nm thick. According to the electrochemical analysis, compared with initial samples, the anodically oxidized and HA deposited porous NiTi alloys have less icorr density, indicating their improved corrosion resistance after the chemical treatments. Due to the HA layer, Ni release can be significantly reduced, and biocompatibility of the porous NiTi alloys is improved.Compared with the gradient descent BP model and the gradient descent BP model with momentum, the gradient descent 3-7-2 BP model with momentum and adaptive learning rate is more efficient and precise for predicting mechanical properties of the porous NiTi alloy fabricated by thermal explosion method. The artificial neural network method is superiors in predicting materials properties with less experimental data and high efficiency and preciseness.
|
PDF Full Text Request