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

In recent years,degradable biomagnesium alloys have attracted more and more attention due to the increasing number of cardiovascular and cerebrovascular diseases.Compared with other biological alloy materials,such as stainless steel,titanium alloy and polymer materials,the density of the magnesium alloy is small,which gives the biomagnesium alloy material a higher specific strength and specific stiffness.In addition,the magnesium alloy has good biocompatibility and degradability,which avoids the secondary surgery injury caused by other non-degradable biological materials to the human body.However,biological magnesium alloy materials also have some shortcomings that need to be improved.The defects of bio-magnesium alloys include low strength,poor plasticity and poor corrosion resistance which hinder the large-scale application of biological magnesium alloys in clinical medicine,therefore,how to improve the mechanical properties and corrosion resistance of magnesium alloys has become the most popular topic in the field of biological magnesium alloys.Based on the existing research results of my research group and a large amount of literature,we find that Mg-Zn-Mn series ternary alloys have excellent corrosion resistance.Zn is an essential microelement for the human body,so Mg-Zn series alloys are most widely used in the field of biomagnesium alloysand Mn itself is a kind of corrosion resistance element.Adding Mn to Mg-Zn series alloy can effectively improve the corrosion resistance of the alloy.However,the corrosion resistance of the alloy is improving after adding Y element while the overall mechanical properties of Mg-Zn-Mn series alloys are decreasing.Especially the plasticity(elongation after fracture)of the alloy is only about 8%,moreover,the plasticity of the alloy would decreases sharply with the Mn content exceeds 1wt.%.In order to improve the comprehensive mechanical properties of the alloy while maintaining the excellent corrosion resistance of the Mg-Zn-Mn series alloy.In this paper,the Mg-3Zn-1Mn ternary alloy is selected,and then the rare earth element Y is introduced to form the Mg Zn Y ternary alloy,which has an important effect on the mechanical properties and microstructure of the alloy.Through modern analysis methods such as OM,SEM & EDS,XRD,EBSD,TEM,DSC and room temperature tensile and Vickers hardness test methods.The effects of Y element,heat treatment and extrusion deformation on the microstructure and mechanical properties of the alloy are compared and analyzed,which provides effective experimental data for basic biomagnesium alloyresearchand lays the foundation for the next experiments such as cytotoxicity and scaffold preparation.The experimental results show thatthe microstructure of the alloy changes from the original equiaxed grains to dendrites or cell-like crystals with the addition of rare earth element Y.The mechanical properties of the as-cast alloy Mg-3Zn-1Y-1Mn are optimized when the content of Y reaches 1wt.%,especially the ductility of the alloy that mainly refer to the elongation after breaking increases from 9% of the as-cast alloy Mg-3Zn-1Mn to 21% of cast alloy Mg-3Zn-1Mn-1Y.The failure mode of the material changed from the original cleavage fracture to microporous aggregate fracture.The quasicrystalline I phase distributed on the as-cast Mg-3Zn-1Y-1Mn alloy grain boundaries becomes the origin of cracks and micropore formation.In order to further improve the mechanical properties of the as-cast Mg-3Zn-1Y-1Mn alloy,we use appropriate heat treatment(solution treatment)to dissolve the second phase located on the grain boundary into the magnesium matrix to improve the mechanical properties of the alloy.The experimental results show that after being dissolved at 450 °C for 24 hours,the tensile strength reaches 215 MPa,and the elongation after breaking reach to 23%.However,the yield strength of the alloy after solution treatment is lower than that of the as-cast alloy.In addition,in the process of solid solution,a large number of twins appear inside the grains,and the higher the solution temperature,the greater the number of twins.The hot extrusion experiments on as-cast Mg-3Zn-1Y-1Mn alloy have significantly improved the mechanical properties of the alloy.When the alloy structure undergoes dynamic recrystallization during hot extrusion,the grain size is greatly refined compared with the as-cast structure.The experimental results show that the extrusion rate is 40 mm/min and the extrusion temperature is 330 °C,the effect of fine grain strengthening is the most obvious.The tensile strength of the alloy is 313 MPa and the elongation after breaking is 16.8%.However,a large amount of micro residual stress remains in the alloy after hot extrusion reduces the ductility of the alloy.Therefore,we subsequently conduct stress relief annealing to improve the ductility of the extruded alloy.The experimental results show that 280 °C annealing temperature combined with 4h annealing time is the best annealing process at which the comprehensive mechanical properties of the alloy are optimal,the tensile strength is slightly reduced to 305 MPa compared with that before annealing and the elongation is22.8%.