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

Every year,a large number of human deaths occur in the world due to cardiovascular disease.Vascular stents are the most important implants in the treatment of cardiovascular diseases,and their effectiveness determines the success or failure of cardiovascular disease treatment.Mg alloys are often used as biomaterials in humans due to their excellent mechanical compatibility and biocompatibility.However,due to the high corrosion rate of the Mg alloy,the Mg alloy cannot be well controlled in the human body,often failing prematurely,hindering the development of the Mg alloy.Therefore,the development of a Mg alloy with excellent corrosion resistance has become a hot research topic in the world today.Based on the research and development of good corrosion resistance and mechanical properties,this experiment explores a new alloy.The topic adds several common biosafety elements such as Zn、Y、Zr and Sr elements.An alloy of Mg-2Y-1Zn-0.4Zr-xSr（x=0,0.1,0.3,0.5 wt.%）was prepared by a conventional casting method,use the alloy of the optimum composition for solution treatment and extrusion.The effects of Sr element on the microstructure,mechanical properties and corrosion resistance of the alloy were investigated.The addition of the Sr element can significantly refine the grain of the magnesium alloy.The as-cast Mg-2Y-1Zn-0.4Zr alloy is mainly composed of a-Mg、Mg₃Zn₃Y₂ eutectic W phase and granular Mg_0.97Zn_0.03.03 phase;When0.1wt%of the Sr element is added,the grain refines,the second phase precipitated at the original grain boundary decreases,becomes discontinuous,and a part of the granular phase precipitates inside the crystal grain;When0.3wt%of Sr element is added,the grain is refined,and the second phase at the grain boundary exhibits an intermittent state,which is less than that of the previous precipitation,but the point precipitated phase inside the grain is further increased.The addition of a small amount of Sr refines the structure of the alloy,changes the distribution of the original second phase,improves the corrosion resistance,and the corrosion morphology is extremely uniform during the immersion experiment.In addition,the addition of the Sr element increases the potential of the alloy matrix and improves the corrosion resistance of the alloy.When 0.5wt%Sr element is added,the grain size of the alloy grows obviously,the precipitation of the second phase at the grain boundary is severely reduced,and the second phase of the grain inside the grain increases remarkably.The corrosion process begins with intragranular corrosion and the corrosion is intensified.Therefore,the Mg-2Y-1Zn-0.4Zr-0.3Sr alloy exhibits excellent corrosion resistance.During the solution treatment,the grains of Mg-2Y-1Zn-0.4Zr-0.3Sr alloy showed a certain degree of growth;the heat treatment changed the phase composition of the alloy,and the original Mg_0.97Zn_0.03.03 phase dissolves,only a-Mg and a small amount of diffuse W phase.As the solution temperature increases,the intermittent second phase at the grain boundary of the alloy further solidifies and the grains growAfter solid solution at 500℃for 25 h,the granular second phase in the grain and the intermittent second phase on the grain boundary are solidified in a large amount,resulting in an increase in corrosion potential and a decrease in current density after solid solution.The more equiaxed alloy grains,as well as some dispersed W phases,weaken the tendency of local corrosion and enhance corrosion resistance.In this study,the corrosion resistance was optimal at 500℃for 25 h.The average corrosion rate was0.27±0.013（mm/a）,the corrosion potential was-1.4564 V,and the corrosion current was 1.76（uA/cm²）.The requirements for corrosion resistance of the bracket.The alloy after solid solution at 500℃for 25 h was subjected to hot extrusion treatment,and the Mg-2Y-1Zn-0.4Zr-0.3Sr alloy was dynamically recrystallized during hot extrusion,and the crystal grains were remarkably refined.With the increase of extrusion temperature,the recrystallization volume fraction increases,the grain growth is obvious,and the second phase of the alloy increases obviously at 400℃and 450℃.The mechanical properties of the alloy gradually decreased,and the corrosion performance increased first and then decreased.In this study,considering the influence of corrosion performance on the alloy,the alloy after extrusion at 400℃and 60mm/min was selected as the bio-alloy with excellent comprehensive properties.The weight loss corrosion rate was 0.38±0.008（mm/a）,tensile strength is 254 MPa,the yield strength is195 MPa,and the elongation is 27%.