Investigation On The In-vitro Degradation Behavior Of Medical Magnesium Alloys

Due to excellent biodegradable absorption properties, good mechanical properties and biocompatibility, biodegradable medical magnesium alloy attracts more and more attention. Non-toxic Y, Nd> Gd elements are chosen to prepare biomedical Mg alloys with content of 0.5at% for their good biocompatibility. In addition, pure Mg with content of 99.97wt% is also chosen in the study. Sheets with thickness of 2mm are made by casting, solution, extrusion, hot rolling, annealing and other processing methods. The degradation behavior of four different materials with or without stress in m-SBF under 37? are investigated according to OM, SEM, XRD, tensile test, and in-vitro immersion test. The influence of stress, RE elements and time on the corrosion behavior and mechanical property are investigated.By extrusion and hot rolling process, grain size of the materials decreases obviously, especially for the Mg-Nd alloy with the grain size of about 10?m. The solubility of Nd in Mg alloys is less than Y and Gd, resulting in more second phase particles formed. The second phase particles are broken in to smaller ones after extrusion and rolling, hindering the migration of grain boundaries during the recrystallization process. Microstructure observation and mechanical test show that the addition of Y, Nd and Gd elements into Mg alloys contributes to the improvement of mechanic properties, and enhance the plasticity and molding properties of the alloy.The in-vitro experiment results show that the corrosion behavior of four types of materials are sensitive to stress, and the corrosion rate of alloy sheets under stress are faster. The corrosion resistance of pure Mg is better than the others with no stress loaded, and that of Mg-Y alloy is worst. Under stress, the corrosion resistance of Mg-Nd is the best.According to the results of SEM test, micro-pitting corrosion and macro-uniform corrosion are observed during the corrosion progress. At the beginning of the corrosion, a piece of cracking membrane is formed on the sheet with white corrosion products on it. The XRD results show that the corrosion products are mainly Mg(OH)2 and a little of compounds containing P and Ca at the begging of corrosion. Along with the time, the white corrosion products get more and more, forming pieces in the sheet. And some of them are removed along with the corrosion. The composition of corrosion products is also changed. The content of Mg(OH)2 decreases, but it increases of compounds containing P and Ca.Mechanical test results showed that along with time, the mechanical properties decrease obviously, especially in the early stage of corrosion, and those of material sheets under stress decreases more rapidly. It also can conclude that the tensile strength and yield strength of pure Mg are better than the others,but the elongation is less. The mechanical properties of Mg-Nd are better than those of Mg-Y and Mg-Gd, and those of Mg-Gd are worst.