【作者】 赵亮；

【导师】 蒋刚；

【作者基本信息】 四川大学 ， 凝聚态物理， 2021， 博士

【摘要】 心血管疾病（CVD）是目前世界上死亡的主要原因,主要表现为心脏和血管功能发生障碍。冠状动脉疾病是第一大心血管疾病,每年导致全球大约1800万人死亡。血管支架植入手术是治疗心血管疾病的有效方法之一。与传统植入材料相比,生物可降解材料是一种极具潜力和前景的新型植入材料。镁合金由于具有低密度、中等弹性、生物相容性、以及比永久性金属支架好的生物降解性、比可降解聚合物支架高的强度等优点,在生物可降解血管支架领域越来越受关注。然而,由于镁的密排六方结构（HCP）以及较低的标准电势,造成镁合金支架材料延展性不够以及在血液中降解速率过快,导致降解后期出现支撑力度不足的现象。因此,如何提高镁合金的力学性能、改善降解速率成为目前面临的主要问题。实验上解决这一问题可行的方法是提高合金的纯度、开发新型合金、变形加工、表面改性。但传统的实验方法成本高、耗时长,在小尺度试样的加工方面一直是一个难题,尤其在镁合金小试样的实验加工中尤为困难。因此,迫切需要寻找一条能够有效提高合金力学性能及开发效率的路径。21世纪以来,随着计算机计算能力的提高以及计算方法的有效改进,基于密度泛函理论的第一性原理计算,分子动力学模拟以及有限元分析已经被广泛用来对材料的物理、化学性能进行计算模拟。这使得在理论上对材料性能的准确预测成为可能,并且避免传统实验中时间、精力、物力的浪费。因此,本文基于材料基因工程思想,开发了高通量计算软件,应用固溶强化机制,从第一性原理计算到分子动力学模拟,研究掺杂对镁的力学性能的影响,寻找表征镁合金力学性能的描述符,为实验上开发高强度、高延展性的镁合金提供理论参考。本论文首先基于几何结构参数,引入对称函数作为指纹表征值,判断掺杂原子与近邻原子所形成局域环境的相似性,开发了镁基多元固溶合金高通量计算软件。快速建立原子取代所产生的初始结构,并且通过指纹函数排除相似结构,筛选得到不重复的镁基二元、三元、四元固溶合金模型。同时,基于泰森多边形法则,开发了多晶掺杂自动化建模软件,构建了不同掺杂元素、不同掺杂方式、不同掺杂比例、不同晶粒尺寸的纳米多晶模型。以上模型的建立,为实现从第一性原理计算到分子动力学模拟来研究镁基合金的力学性能奠定了基础。其次,通过计算纯镁的结构参数以及弹性性质,并与实验值以及其他计算值做比较,验证计算方法的可靠性并确定纯镁的结构参数。利用开发的镁基固溶合金高通量计算软件,考虑生物相容性原理,构建了26289个具有11种不同元素、3种不同浓度的固溶体结构,并对能量最低结构的晶格参数和力学性质进行了计算。研究发现固溶体的弹性模量随同一族或者同一周期溶质原子的半径的减小而增大。掺杂未满3d电子的溶质原子的固溶体的弹性模量比掺杂s电子、满壳层3d电子及4d电子的固溶体的弹性模量大。而表征延展性的B/G比值和泊松比（ν）正好与弹性模量的结论相反。这些现象主要是由原子间的相互作用强弱导致。同时发现镁与溶质原子之间的近邻距离以及原子体积与体积模量都成反比关系。弹性模量值大（小）的溶质原子导致固溶体的弹性模量值大（小）。然后,利用镁基多元固溶合金高通量计算软件与第一性原理计算方法,对Mg-Zn_0.02X_0.02和Mg-Zn_0.02Y_0.02X_0.02（X=Li、Na、K、Ca、V、Cr、Mn、Fe、Cu、Zn、Y）固溶合金的力学性能进行了研究。在Mg-Zn_0.02X_0.02中,溶质原子Mn、Y提高了固溶体的体积模量,而其他溶质原子降低了固溶体的体积模量。MgZn_0.02Mn_0.02的体积模量、剪切模量和杨氏模量值最大。因此,Mn可以添加到那些具有高延展性但是极限强度和屈服强度一般的固溶体中,以此来增加固溶体的机械强度。Mg-Zn_0.02Y_0.02的延展性表现最佳。在Mg-Zn_0.02Y_0.02X_0.02中,体积模量会随着同一族或者同一周期的溶质原子半径的增加而增加。添加溶质原子Cr、Mn、Fe的固溶合金的剪切模量与杨氏模量增大,机械强度增加。相比纯镁,Mg-Zn_0.02Y_0.02K_0.02、Mg-Zn_0.02Y_0.02Ca_0.02不仅拥有相对较好的延展性,而且对外界均一性压缩的抵抗能力较高,可能是潜在的生物可降解血管支架材料。最后,利用多晶掺杂自动化建模软件,构建了Mg-X（X=Nd、Li、Al、Ca、Zn、Y）纳米多晶模型。每种模型有不同的掺杂比例、不同的晶粒尺寸以及晶界掺杂和随机掺杂共210种纳米多晶模型,并对其进行了单轴拉伸分子动力学模拟。在Mg-Nd晶界掺杂中,杨氏模量与平均流变应力都随着晶粒尺寸和掺杂比的增大而增大。说明Nd原子晶界偏析起到强化的作用,增加了抵抗变形和抗拉的能力。而在Mg-Nd随机掺杂模型中,Nd原子起到软化的作用。在Mg-Li、Al、Ca、Zn、Y纳米多晶中,掺杂Li元素后的杨氏模量最大。并且大晶粒尺寸（12.0和17.3 nm）的杨氏模量普遍比小晶粒尺寸（7.17和9.03 nm）的杨氏模量大。在Mg-Li和Mg-Al的平均流变应力随晶粒尺寸的变化中,既表现出霍尔佩奇效应又表现出反霍尔佩奇效应;在Mg-Ca和Mg-Zn中,晶粒尺寸为7.17和12.0nm（9.03和17.3 nm）的时候,掺杂起到强化（软化）纳米多晶的作用。Mg-Y多晶在掺杂比一定的时候,流变应力随晶粒尺寸的增大而增大。通过本文的研究,可以为实验上设计高强度、高延展性的心血管支架用镁基固溶合金提供有价值的理论参考。更多还原

【Abstract】 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.更多还原