Research On The Microstructure And Property Of Ultrafine-grained Mg-Zn-Y-Nd Alloy For Vascular Stent Application

Cyclic extrusion compression （CEC） is a novel severe plastic deformationtechnology, which has great potential in the preparation of fine-grained metallic inrecent years. In this paper, the research is on Mg-Zn-Y-Nd alloy for vascular stentsapplication of independent intellectual property rights, selecting two severe plasticdeformation （SPD） processes to prepare fine-grained magnesium alloy. Two SPDprocesses are well developed equal channel angular pressing （ECAP） technologyand rapidly developing CEC technology. In the present work, their microstructures,mechanical properties and corrosion behavior in simulated body fluid （SBF） wereevaluated by optical microscopy, scanning electron microscopy, room-temperaturetensile tests, hot tensile, electrochemical measurements and immersion tests. Thegrain refining mechanism and room-temperature mechanical properties for Mg alloysduring SPD was discussed were investigated. The superplastic behavior of thefine-grained magnesium alloys was analyzed. The degradation behavior ofMg-Zn-Y-Nd alloy in SBF is studied.The microstructure observation results show that Mg-Zn-Y-Nd alloy can berefined effectively by CEC. The microstructure of the alloy became finer and morehomogenous due to the dynamic recrystallization occurred during the deformation.The grain size was refined to about1μm and the second phase distributed in thegrains and along the grain boundaries which were brittle and formed into a cluster.The microstructures of as-ECAPed alloy are dynamic recrystallized and containequiaxed grains of α-Mg and fine micrometer sized particles. The average grain sizeof the as-EACPed alloy was~15μm owing to the much higher processingtemperature. At the same time some the second phase which came from the clusterphase uniformly precipitated in grains. The microstructure of the as-extruded alloywas heterogeneous, which was the typical microstructure of incompletely dynamicrecrystallized alloys. Coarse grains of10~15μm in size were surrounded by finedynamic recrystallized grains of3~5μm in size. The second phase distributedgathered in the grains and even along the grain boundaries. The grain refining mechanism of Mg-Zn-Y-Nd alloy during CEC can bedescribed as a Dynamic Recrystallization grain refining mechanism. Owing to theincreasing in the second phase of the types and quantities of magnesium alloy duringextrusion process, the structure is very effective refined.The room-temperature mechanical properties results show that ultimate tensilestrengths of as-extruded and as-ECAPed and as-CECed are280MPa and239MPa and297MPa, compling with the classic formula of Hall-petch. A significant decline inthe yield strength of ECAP samples show that the effect of deformation temperatureon yield strength than ultimate tensile strength. The yield strength of as-ECAPedsample compared to as-extruded and as-CECed samples decrease higher than50%,the ultimate tensile strength is less than20%.It is apparent that the elongation of as-CECed and as-ECAPed samples are29.4%and30.1%respectively, which indicates that the SPD processing plays asignificant role in ductility improvement. The improvement of ductility forMg-Zn-Y-Nd alloy depends on the change of fracture mode. As-extruded alloy withcoarse grains is intracrystalline and shear fracture. The as-CECed and as-ECAPedalloy fractures along grain boundaries and the boundaries between matrix and thesecond phase. A large of deformation twins found in as-ECAPed alloy showeddeformation is mainly through interaction of twinning and slip.The hot tensile study exhibits the superplasticity of ultrafine–grainedMg–Zn–Y–Nd alloy produced by CEC, which brings excellent superplasticity ofMg–Zn–Y–Nd alloy with a maximum elongation of447%at a temperature of350℃and a strain rate of1×104s1. In the present study, the max m value is0.424,indicating that the main deformation mechanism is grain boundary sliding.In vitro experiments showed that the corrosion potential of as-extruded andas-ECAPed and as-CECed alloy are in ascending order. The hydrogen evolution testproved that the as-CECed alloy was the most corrosion-resistant material, with acalculated corrosion rate of1.0837mm/year. Simultaneous improvements ofmechanical and corrosion properties are rarely achieved by CEC processing, and thistype of process has the potential for widespread use in the manufacturing ofcardiovascular stent.