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

Coronary heart disease is one of the major diseases that endanger human health,and the common method of treating coronary heart disease is implant stents in arteries.Magnesium alloy has good biocompatibility and spontaneous degradation,making it become an ideal material for vascular stent.However,magnesium alloy also has the disadvantages of low strength,poor corrosion resistance and so on,which restricts its clinical application.This paper aims to develop a new type of degradable magnesium alloy vascular stent material with excellent performance.Zn,Y and Zr elements with good biosafety as alloying elements were selected in this experiment,Mg-xY-3Zn-0.4Zr(x = 0,1,2,3,4 wt.%)alloys and Mg-2Y-yZn-0.4Zr(y = 0,1,2,4,5 wt.%)alloys were prepared by conventional casting method,and the preferred alloy with the best performances was heat treated.The effects of Y,Zn and heat treatment on the microstructures,mechanical properties and corrosion resistances of the alloy were studied by OM,SEM & EDS,XRD,weight loss method,hydrogen evolution method,electrochemical test and room temperature tensile and so on.The experimental results are summarized as follows:Adding the Y element can refine the alloy grain significantly.Mg-3Zn-0.4Zr alloy mainly consisted of ?-Mg matrix and small granular shaped Mg0.97Zn0.03 phase.The long striped Mg3YZn6 phase(I-phase)appeared in the alloy when Y content was 1 wt.%.The fish-bone structural Mg3Y2Zn3 phase(W-phase)appeared in the alloy when the content of Y was 2 wt.%.The addition of a small amount of Y to promote the formation of I-phase,the potential difference between I-phase and Mg matrix was small,so the number of micro-batteries decreased.In addition,Y element and impurity elements(such as Fe,Mn)to form intermetallic compounds,purifying the melt and improving the corrosion resistance of the alloy.The addition of excess Y to form the hard and brittle W-phase,which distributed in the grain boundary,the bond of W-phase and magnesium matrix was weak,it easy to produce micro-cracks.When the number of the second phases was large,the alloy produced serious segregation phenomenon,reducing the mechanical properties of the alloy.Mg-2Y-3Zn-0.4Zr alloy exhibits the best overall performance.Compared with Y element,the addition of Zn element to the grain refinement of the alloy was limited.Mg-2Y-0.4Zr alloy was mainly composed of ?-Mg matrix and Mg24Y5 phase.Mg-2Y-1Zn-0.4Zr alloy was composed of ?-Mg matrix and I-phase.Continue increasing the content of Zn,alloy appeared W-phase.Trace Zn element could improve the corrosion potential of the matrix,thereby improve the corrosion resistance of the alloy.Zn was melted into Mg matrix produced solid solution strengthening.Excessive amount of Zn element produced a large amount of second phases,increasing the tendency of alloy galvanic corrosion.Hard and brittle W-phase gathered and even split the matrix,reducing the mechanical properties of the alloy.Therefore,with the increase of Zn content,the corrosion resistance and mechanical properties of the alloy showed a tendency to increased first and then decreased.Mg-2Y-1Zn-0.4Zr alloy exhibits the best comprehensive performances.In the heat treatment process,the grain of Mg-2Y-1Zn-0.4Zr alloy had a certain degree of growth phenomenon.Phase composition(?-Mg matrix and I-phase)of heat treated alloys did not change,long striped I-phase was transformed into granular form and evenly distributed in the matrix.I-phase of T4-10 alloy dissolved mostly,reducing the number of micro-batteries and enhancing the corrosion resistance of the alloy.Due to long holding time,the grain size of T4-30 alloy was coarsened seriously,the total grain boundary area decreased,so the barrier effect of grain boundary for dislocation was weakened,and the density of refractory impurity on the grain boundary increased,resulting in reducing performance.After aging treatment,the small dispersed I-phase evenly distributed in the matrix,which playing a role in dispersion enhancement.In addition,the precipitation phase was evenly distributed in the matrix,which reduced the tendency of local corrosion and enhanced the corrosion resistance.In this work,Mg-2Y-1Zn-0.4Zr alloy after T6 treatment shows the best comprehensive performances: average corrosion rate of weight loss is 0.189 mm/a,tensile strength is 258 MPa,yield strength is 144 MPa and elongation is 15.5%.The result basically meets the scaffold material on the mechanical properties and corrosion resistance requirements.