Research On Microstructure And Mechanical Properties Of Mg-Zn-Y-Zr-Ca Alloys For Degradable Cardiovascular Stent Application

Magnesium alloys have been extensively studied in bone implant materials,bone tissue engineering stents and Cardiovascular stents and other fields due to its excellent properties,such as high specific strength,specific stiffness,biodegradability and biocompatibility,low density and elastic modulus close to human bone.However,magnesium alloys also have the disadvantages of low strength and low corrosion resistance when implanted in biological systems,which restricts its clinical application.Therefore,it is necessary to improve the overall performance of biomedical magnesium alloys through further research.In this way,the Mg-3Zn-1Y series bio-magnesium alloys with good mechanical and corrosion properties were selected in this experiment,and appropriate amounts of Ca and Zr elements were prepared as alloying elements.Alloys were obtained by conventional casting method,and the preferred alloys with the best performances were heat treated and extruded.The effects of Ca element,heat treatment and hot extrusion on the microstructure and mechanical properties of the alloy were studied by using OM,SEM&EDS,XRD,EBSD,TEM and room temperature tensile methods.It provides an effective experimental basis for the optimal combination of magnesium alloy components and processes for cardiovascular stents,and lays the foundation for the future steps of biocompatibility and stent preparation experiments.The experimental results are summarized as follows:The as-cast Mg-3Zn-1Y-0.6Zr-xCa alloy mainly consists of?-Mg phase,I phase?Mg₃YZn₆?and a small number of Ca₂Mg₆Zn₃ phase.The addition of appropriate amount of Ca element can further refine the grains of Mg-3Zn-1Y-0.6Zr alloy,and have a certain influence on the second phase in the alloy.When the Ca content is added to 0.5 wt.%,the grain size of the alloy is refined,and the distribution of the second phase in the alloy is more continuous and uniform.This is because Ca element generates constitutional supercooling in the diffusion layer of the magnesium alloy at the front edge of the solid-liquid interface,thereby refining grains and making microstructure uniform.However,the addition of excessive Ca can deteriorate the microstructure of the alloy.The Mg-3Zn-1Y-0.6Zr-0.5Ca alloy exhibits the best mechanical properties.The effects of different solution temperature and solution time on the microstructure and properties of the alloy were investigated.The results show that increasing the solution temperature and extending the holding time can fully dissolve the second-phase solute atoms into the magnesium matrix and increase the solid solubility,and increase the tensile strength of the alloy,but both can not be too high,otherwise there will be over-burning.After the alloy was treated at460°C for 24 h,the tensile strength reached 210 MPa and the elongation reached11.7%,which greatly improved the mechanical properties of the as-cast bio-magnesium alloy.Both the extrusion temperature and the extrusion speed have a certain influence on the properties of the alloy.The lower extrusion temperature and extrusion speed,the finer grains of the alloys.And tensile strength of the alloys will be increased,but the plasticity decreased.The extrusion parameters have a greater influence on the texture of the alloys.When the extrusion speed and extrusion temperature is reduced,the microstructure of the alloy shows a strong basal texture,and the volume fraction of small angle grain boundaries is increased.The higher extrusion temperature and extrusion speed will lead to full dynamic recrystallization for the alloys,which exhibited anisotropy and enhanced the plasticity of the alloy.When the extrusion temperature is 300°C and the extrusion speed is 60 mm/min,the extruded-alloys show the best comprehensive mechanical properties.