Theoretical Study Of Mechanical Properties Of Magnesium Alloy For Vascular Stent

Cardiovascular diseases（CVD）mainly present vascular and heart dysfunction,which are the primary reason of death and morbidity in the world.Coronary artery disease is the number one CVD and gives rise to around 18 million of death globally.One of the effective methods to treat the CVD is the implantation of vascular stents.Biodegradable materials are new potential and promising implanted materials compared with the traditional implanted materials.Magnesium alloys as potential biodegradable materials have attracted increasing attentions in cardiovascular stents fields due to their low density,moderate elasticity and biocompatibility,especially higher strength compared with degradable polymers and better biodegradability in comparison with permanent metal stents.However,Mg alloys reveal limited formability and poor corrosion resistance as a result of HCP crystalline structure and low corrosion potential.Thus,corrosion resistance can’t be effectively matched the vascular lesion recovered,resulting in the mechanical is not enough.At present,the feasible method to solve this problem in the experiment is to improve the purity of alloy,develope new alloys,use technology of deformation machining and surface modification.However,the traditional experimental methods to develop alloy are a costly and time-consuming endeavor.Furthermore,to process the sample with small scale is always a difficult challenge,and especially for Mg alloys.Thus,finding a new way to enhance efficiency for alloy development is urgently necessary.Since the beginning of the 21 st century,with the effective improvement of computing methods and the rapid development of supercomputer facilities,firstprinciples calculations in the framework of density functional theory（DFT）,molecular dynamics simulation and finite element method（FEM）have been widely used to multi-scale calculate and simulate the physical and chemical properties of crystalline materials.These make it possible to predict the physical and chemical properties of crystalline materials accurately with theoretical calculations and save significantly time and cost in the trial-and-error experimental procedures.Thus,the software of high-through calculations was estibilished based on the idea of Materials Genome Engineering（MGE）.The effects of doping on the mechanical properties of Mg alloys were studied from the first pinciples calculations to molecular dynamics simulation by means of the solid solution strengthening.The valuable theoretical refrence was provide for the design of high strength and high ductility magnesium based solid solution alloyFirstly,based on the geometric structure parameters,the symmetric function was introduced as the fingerprint value to judge the similarity of the local environment formed by the doped atoms and its nearest neighbor atoms,and the software of highthroughput calculations for the multielement Mg alloys was developed.In this way,not only the initial structure of atomic substitution can be established quickly,but also the same substituted structure can be eliminated by the fingerprint function.The models of magnesium based binary,ternary and quaternary solid solution alloy were constructed by using the software of high-throughput calculations for the multielement Mg alloys.And,the software of polycrystalline doping automatically was developed based on the voronoi tesselation to bulid the nanocrystalline polycrystalline models rapidly with different doping methods,doping ratios and grain sizes.The establishment of the above model has laid a foundation for the study of mechanical properties of magnesium based alloys from first principles calculation to molecular dynamics simulation.Secondly,the parameters and elatic properties of pure Mg were calculated and the results are in good agreement with the experimental values and other calculated values,which validate the first-principles methodology employed.Satisfying the premise of biocompatibility,the solid solution structures of Mg_54-NX_N（X = Li,Na,K,Ca,Al,V,Cr,Mn,Fe,Cu,Zn and Y.N = 1,2 and 3,corresponding to 1.85,3.70 and 5.55 at.%X,respectively）were built through the software of high-throughput calculstions.And,the parameters and elastic properties of the stable structures were calculated.We find that on the addition of the same groups solute atoms to Mg,the elastic modulus increase with the decrease of atomic radius.Solute atoms belonging to the d-block of the periodic table with d electrons partially filled result in a larger bulk,shear and Young’s modulus than the s-block solute atoms and d-block solute atoms with d electrons fully filled.However,the results of B/G ratio and Possion’s ritio（ν）are contrary to the elastic modulus.These phenomena are mainly caused by the strength of the interaction between Mg and solute atoms.We also find that the distance between Mg and solute atoms and the atomic volume are inversely proportional to the bulk modulus.The solute atoms with a large（small）elatic modulus result in a large（small）elatic modulus of the solid solutions.Then,the mechanical properties of Mg-Zn_0.02X_0.02 and Mg-Zn_0.02Y_0.02X_0.02 were calculated throuth the software of high-throughput calculations combined with VASP.All the Mg-Zn_0.02X_0.02 solid solutions except Mg-Zn_0.02Mn_0.02 and Mg-Zn_0.02Y_0.02 demonstrate a lower bulk modulus than the Mg-Zn_0.02 although to different extents.The elastic modulus of Mg-Zn_0.02Mn_0.02 is the largest.Thus,solute atom Mn can be used as addition to add to solid solutions exhibiting high initial ductility but mediocre ultimate or yield strength to significantly increase the mechanical strength.The ductility of Mg-Zn_0.02Y_0.02 is the best.For the quaternary Mg-Zn_0.02Y_0.02X_0.02 solid solutions,the bulk modulus increases with the increase of atomic radius of solute atoms belonging to the same group or the same period.When the solute atoms Cr,Mn and Fe were added to Mg-Zn_0.02Y_0.02,the shear and Young’s modulus significantly increase and much larger than pure Mg,indicating those solute atoms can be used to increase the mechanical strength.Mg-Zn_0.02Y_0.02K_0.02 has a relatively large bulk modulus.Thus,Mg-Zn_0.02Y_0.02K_0.02 and Mg-Zn_0.02Y_0.02Ca_0.02 not only has high strength,but also possesses good ductility,demonstrating it may be very suitable material for biodegradable cardiovascular stent.Finally,the Mg-X（X = Nd,Li,Al,Ca,Zn,Y）nanocrystalline polycrystalline model was built using the software of polycrystalline doping automatically.There are 210 kinds of nanocrystalline polycrystalline models with different doping ratios and grain sizes,and uniaxial tensile molecular dynamics simulation is carried out.Both Young’s modulus and average flow stress increase with the increase of grain size and doping ratio when the Nd atoms ware grain boundary,indicating that grain boundary segregation of Nd atom plays a strengthening role and increases the resistance to deformation and tensile strength.While the Nd atom plays a softening role when the Nd atoms were doped randomly.In the Mg-Li,Al,Ca,Zn and Y nanocrystals,doping Li element results in the largest Young’s modulus.The Young’s modulus of large grain sizes（12.0 and 17.3 nm）is generally larger than that of small grain sizes（7.17 and9.03 nm）.The average flow stress of Mg-Li and Mg-Al varied with grain size,representing both ‘Hall-Petch effect’ and ‘inverse Hall-Petch’.Doping Ca and Zn atoms plays a strengthening（softening）role when the grain sizes are 7.17 and 12.0nm（9.03 and 17.3 nm）.For Mg-Y polycrystalline,the average flow stress increases with the increase of grain size when the doping ratio is constant.This study can provide valuable theoretical refrence on design of high strength and high ductility magnesium based solid solution alloy for cardiovascular stents.