| Abstract:Magnesium alloy exhibits good biocompatibility, biodegradability and excellent mechanical properties, and is considered to become one of the ideal biomedical materials for the next generation. Then it is concerned by the growing number of researchers, and lots of experimental studies have been performed. However, it hasn’t been applicated in the clinical until now because some key problems such as the low mechanical property, rapid corrosion rate and no clear biocompatibility for some elements has not been settled. In view of the above mentioned questions, the present thesis is to develop a new biodegradable Mg-Zn series alloy exhibiting high plastic, moderate strength and excellent corrosion resistance, which can provide the materials basis for the development of the biodegradable stent made by magnesium alloy. The Mg-6Zn alloy is taken as the base alloy. The effects of addition of Gd and Ca on the microstructure and tensile mechanical properties of as-cast and extruded alloys and the corrosion resistance of extruded alloys in the simulated body fluid (Hanks solution) were studied systematically by experimental means such as alloy composition design, materials preparation and extrusion deformation and test methods of inductively coupled plasma analyzer (ICP), optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), electronic tensile test and the electrochemical workstation. The main results are as follows:When0～3.41wt%Gd and0～1.35wt%Ca are added into the Mg-6Zn alloy respectively, as-cast microstructure is refined gradually with increasing the Gd and Ca content. Mg-Zn-Gd and Mg6Zn3Ca2new phases increase gradually, while MgZn2phase decreases gradually until to disappear completely. The second phase tends to distribute along grain boundary by continuous reticulation. As-cast tensile mechanical property on the whole first increases and then decreases gradually. Mg-6Zn-0.66Gd and Mg-6Zn-0.085Ca alloys exhibit the best tensile mechanical property, where tensile strength σb, yield strength σ0.2and elongation δ are215MPa,78MPa and6.5%for the former alloy and230MPa,84MPa and14%for the latter alloy, respectively.After extrusion, dynamic recrystallization occurrs for all the alloys, and the microstructure is refined obviously. Broken second phase, inhibiting the growth of recrystallization grains, tends to distribute by zonal shape in extrusion direction. So the average grain size gradually decreases from15μm for Mg-6Zn alloy to2μm for Mg-6Zn-3.41Gd alloy and10μm for Mg-6Zn-0.47Ca alloy, respectively. Extruded tensile mechanical property is enhanced obviously, σb and σ0.2increase gradually from295MPa and193MPa for Mg-6Zn alloy to350MPa and325MPa for Mg-6Zn-3.41Gd alloy, respectively.6first decreases, and then increases, which is not lower than10%, especially the best elongtation of16.5%for Mg-6Zn-0.66Gd alloy. Among the extruded Mg-6Zn-xCa alloys, Mg-6Zn-0.085Ca alloy exhibits the best tensile mechanical property, where σb, σ0.2and δ can reach290MPa,183MPa and19%, respectively. It is seen that Mg-6Zn-xGd and Mg-6Zn-xCa alloys, developed by the present thesis, exhibit the Characteristic of high plastic and moderate strength.For the extruded Mg-6Zn-xGd alloys, Mg-6Zn exhibits the lowest corrosion rate, but its corrosion mode is local corrosion. When little Gd content (0.66%) is added, the corrosion rate increased slightly, however the corrosion becomes more uniform and changes to the uniform corrosion. When more Gd content (1.66%and3.41%) is added, the corrosion resistance deteriorates sharply. For the extruded Mg-6Zn-xCa alloys, with increasing the Ca content, the corrosion rate and corrosion current become larger, and while corrosion potential becomes lower, which indicates that the corrosion resistance becomes worse. |
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