| Magnesium alloys are potentially applied in the bone healing process due to their good biocompatibility and biodegradability. Besides, stress shielding effect can be effectively relieved because the integrated mechanical properties of Mg alloys are similar to the bone tissue such as density, modulus of elasticity and so on, compared with other traditional medical implant metal materials. On the other hand, Mg is an essential trace element of the human body. And then, it is not harmful to the human body when magnesium alloys gradually degrade after implantation. Moreover, it is not necessary for the second surgery to remove the implant so that the harm can be reduced to a patient. However, their clinical application has been presently restricted due to the high corrosion rate, because the electrochemical corrosion occurs due to the existence of a large number of second phases in the Mg-based alloys. At the same time, the mechanical properties become worse and worse during the corrosion processes, resulting in being unable to play a fixed or a supporting role. Presently, the corrosion resistance has been mostly improved through the surface treatment of Mg-based alloys, but the bond strength of the coating is poorer. If the Mg-based alloy with a single phase and fine grain can be obtained, the mechanical properties are not only improved, but also it is possible to reduce the corrosion rate, because the electrochemical corrosion can be avoided, resulting from the second phase. However, the study on the corrosion resistance of magnesium alloys only containing the-Mg solid solution is still not reported. Therefore, in this work, the pure Mg, Mg-3Zn, Mg-3Zn-l Ag and Mg-3Zn-3Ag (wt.%) alloys have been prepared at 300℃ by means of indirect hot extrusion. And then, the biocompatibility such as anti hemolytic property and cytotoxicity, microstructure observation, mechanical property, corrosion resistance in the simulated body fluid (SBF) have been investigated. At the same time, the corrosion behavior of as-solution Mg-3Zn (-1～3Ag) alloys in the SBF has been also studied by means of the electrochemical test and weight-loss method. The conclusions are obtained as followed.According to the biological evaluation standards of medical devices, the hemolytic and cytotoxic tests are carried out for as-extruded pure Mg and Mg-3Zn (-1～3Ag) alloys. The results show that the hemolytic rate HR is less than 5% for all materials mentioned above, which means that the hemolysis does not occur after implant. The cytotoxicity is the grade 1, which suggests the materials appear slightly cytotoxic. But the level can be accepted. Therefore, it can be inferred that the Mg-Zn-Ag alloys prepared in this study can fulfill the hemolytic and cytotoxic requirements of the medical materials.The microstructures of as-extruded pure Mg and Mg-3Zn (-1-3Ag) alloys show that the equiaxed grains of -Mg matrix form due to complete dynamic recrystallization during hot extrusion. Average grain size of the samples mentioned above is almost the same, keeping about 20μm. Besides, there is a small quantity of the second phase in as-extruded Mg-3Zn (-1～3Ag) alloys, and the amount increases with the additions of Zn and Ag elements. The tensile tests at room temperature show that the ultimate tensile strength and the elongation increase gradually, such as245MPa and 18.6% for the Mg-3Zn-3Ag alloy, respectively. The similar result is obtained by compression test at room temperature. The ultimate compression strength and compression ratio of the Mg-3Zn-3Ag alloy reach 401MPa and 18%, respectively.The corrosion rate of as-extruded or solution specimens immersed in the simulated body fluid (SBF) at 37℃ have been determined by weight-loss method. The corrosion rate of as-extruded Mg-3Zn alloy slightly decreases, compared with the as-extruded pure Mg, and finally maintains about 4mm/year with the extension of the soaking time, equivalent to that of as-extruded pure Mg. While the corrosion rate of as-extruded Mg-3Zn-(1-3) Ag alloys both increases, and finally keeps about 7 and 17 mm/year, respectively. The corrosion rate of all alloy specimens during solid solution treatment decreases, particularly by 50% for the Mg-Zn-lAg alloy, compared with as-extruded state. The initial corrosion rate of as-solution Mg-3Zn and Mg-3Zn-lAg alloys is less than that of as-extruded pure Mg, and maintains about 3.5mm/year with the extension of immersion time. It is also obtained by the electrochemical test that the corrosion rate of as-solution Mg-3Zn (-1～3) Ag alloys can be reduced compared with pure Mg. The corrosion products mostly consist of Mg(OH)2 and the 2 on the surface of as-solution alloys immersed in the SBF for 14 days.Altogether, it can be suggested by results of corrosion tests mentioned above that the corrosion resistance of Mg-based alloys be improved by solid solution treatment, which makes the amount of second phase decrease, or even removes the second phase. Unfortunately, the mechanical properties probably become worse, because the grain of -Mg matrix grows with the dissolution of the second phase, where the solid solution treatment is carried out at higher temperatures. Therefore, after Mg-Zn-based alloys with the single or quasi-single phase are to be obtained, in order to improve the corrosion resistance, how to further refine the grain of α-Mg matrix so as to improve their mechanical properties, which will be the core of our future work. |
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