| Biodegradable magnesium implants were first applied to the human body in1878,many researchers had gradually focused on the design and application of biodegradablemagnesium alloys in recent years. It was due to magnesium alloys had more sufficientload-bearing ability and better biocompatibility than those of polymers; magnesium alloyshad degradation ability compared with traditional biological metal materials, then, it is notnecessary to be removed the implants through a second surgical operation; however,magnesium alloys had worse mechanical properties and faster corrosion degradable velocitythan those of titanium alloys and Co-based alloys, which restricts it from wide application inthe field of biological materials. For improving the mechanical properties and corrosionresistance of medical degradable magnesium alloys in this paper, based on a few amount andmulti-element principle, Mg-Zn-Ca alloys are treated in modification, thus, Zr, Nd and Yelements which have good biocompatibility, improve the mechanical properties andcorrosion resistance of medical degradable magnesium alloys are selected as alloy elements,then, Mg-Zn-Ca-Zr/Nd/Y biological magnesium alloys are designed. It was investigated thatZr, Nd and Y elements and solid solution treatment influence on the microstructure,mechanical properties and corrosion behavior of the biodegradable magnesium alloys. Theresults are summarized as follows:1. The as-cast Mg-4Zn-0.5Ca-xNd alloys mainly consist of-Mg matrix, MgZn,Ca2Mg6Zn3and Mg41Nd5compounds. The microstructure is gradually refined and thenumber of Mg41Nd5compounds increases with the increasing of the content of Nd. When thecontent of Nd is2wt.%, eutectic structure of Ca2Mg6Zn3+Mg41Nd5can be found at grainboundaries. Compounds precipitated at grain boundaries and in grain interior graduallyincrease and strengthen the-Mg matrix and grain boundaries as the content of Nd increasesfrom1wt.%to2wt.%, meanwhile, the grains are obviously refined and alloys’mechanicalproperties are improved. If the content of Nd continues to be increased, brittle phases areaggregated at grain boundaries, forces among grains and alloys’mechanical properties areboth decreased. The experiment’s result indicates that Mg-4Zn-0.5Ca-1Nd alloy has thebest corrosion resistance of these alloys in SBF fluid at37oC. When the content of Nd is more than2wt.%, even if the sizes of grains are decreased, compounds precipitated at grainboundaries and in grain interior gradually increase, which lead to the increase corrosioncurrent density of alloys, the corrosion resistance properties of alloys decreases.2. The as-cast Mg-4Zn-0.5Ca-xZr alloys mainly consist of-Mg matrix, Mg0.97Zn0.3and Ca2Mg6Zn3compounds, The Ca2Mg6Zn3compound precipitates in grains interior andgrain boundaries, but XRD pattern of the Mg-4Zn-0.5Ca-xZr alloys has no diffraction peaksof Zr particles. The alloys’grains decrease from65m to30m as the content of Zrincreases from0.1wt.%to0.7wt.%, that is, the grains are obviously refined, the strength andplastic deformation are improved, especially, the as-cast Mg-4Zn-0.5Ca-0.7Zr alloy has thebest mechanical properties. The curves of the rate of the evolved hydrogen and polarizationrepresent that corrosion resistance of these alloys increases with the increasing of as thecontent of Zr is less than0.5wt%. The reason is that the grains are refined, the distribution ofthe grains are uniform, the corrosion current density of alloys are effectively decreased andthe corrosion resistance properties of alloys are increased after adding Zr element. If thecontent of Zr is more than0.5wt%, the corrosion resistance properties of alloys decreasewith the increasing of the content of Zr. It dues to the Zr particles congregate at the grainboundaries as impurities to dissever the continuity of the compounds, which can noteffectively stop corrosion spreading from one grain to another grain, therefore the corrosioncurrent density of alloys increases, the corrosion resistance properties of alloys decreases andthe as-cast Mg-4Zn-0.5Ca-0.5Zr alloy has the best corrosion resistance properties. The phasecomposition of alloys is unchanged after solid solution treatment. The mechanical propertiesof alloys are improved since the sizes of the grains are still decreased with the increasing ofcontent of Zr and some of solute atoms solubilize in the-Mg matrix; Meanwhile, the molefraction of compounds is decreased, then the electrochemical properties are improved. Thesolution-treated Mg-4Zn-0.5Ca-0.5Zr alloy has the best comprehensive properties, the yieldstrength is153MPa, the rate of corrosion is only0.451mm/year.3. The as-cast Mg-6Zn-0.5Ca alloys mainly consist of-Mg matrix, MgZn2andCa2Mg6Zn3compounds. After adding the rare earth element Y, the granular compoundMg12ZnY dispersive distributes in the matrix， and as the increase of the content of elementY, the sizes of grains decrease and the mole fraction of the Mg12ZnY compound increasegradually. The sizes of grains of the as-cast alloys are decreased from118m to79m withthe increasing of Y content from0wt.%up to1.5wt.%. The mechanical properties are greatlyimproved after adding Y, which has relationship with solid solution strengthening resulting from element Y, grain size refinement strengthening and the precipitation strengtheningresulting from compound Mg12ZnY. The phase composition of alloys is unchanged aftersolid solution treatment, but the sizes of grains become increase. In the same way, solidsolution treatment could improve the mechanical properties of alloys, namely, thesolution-treated Mg-6Zn-0.5Ca-1.0Y alloy has the best mechanical properties, the ultimatetensile strength, yield strength and elongation whose values are284MPa,166MPa,14.7%,respectively.4. The as-cast Mg-2Zn-0.5Ca alloy mainly consist of-Mg matrix and CaZn3compounds. After adding the rare earth element Y, the rare earth compound Mg24Y5distributes in the matrix. The compression test at room temperature indicates that thecompression fracture of solution-treated Mg-2Zn-0.5Ca alloy is relatively flat and bright, thefracture surface exhibits brittle fracture pattern. Because solid solution is strengthenedresulting from element Y and precipitation is strengthened resulting from compound Mg24Y5,the mechanical properties of alloys improve. It indicates that the pH value of the corrosionliquor increases with the increasing of the immersion time, the pH value of the corrosionliquor immersed with the as-cast alloys reaches the maximum of10.4, and the pH value ofthe corrosion liquor immersed with the solution-treated alloys reaches the maximum of8.7under the condition of experiment after the as-cast and solution-treated Mg-2Zn-0.5Ca-xYalloys of immerse in SBF fluid for30days. Compared with the as-cast alloys, the corrosionrate of solution-treated is obviously smaller. As for the Mg-2Zn-0.5Ca-1.5Y alloy, the seriesof as-cast Mg-2Zn-0.5Ca-хY alloys have worse corrosion resistance abilities than those ofthe solution-treated alloys, the maximum corrosion rate of as-cast Mg-2Zn-0.5Ca-1.5Yalloys is9.748×10-4g/cm2/day, the minimum corrosion rate of solution-treatedMg-2Zn-0.5Ca-1.5Y alloys is0.2917×10-4g/cm2/day. |
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