合金元素对镁层错能和孪晶偏析能的影响规律及作用机制

发布时间：2018-04-04 07:55

  本文选题：镁合金　切入点：第一原理计算　出处：《吉林大学》2015年博士论文

【摘要】：镁合金具有密度小、比强度和比刚度高等显著优势，然而室温下的滑移系统少（仅有两个独立滑移系），导致其塑性变形能力很差。传统镁合金无法满足不同环境和工况对使役性能的需求，严重制约了其发展和在工程上的应用。因此，新型高强韧镁合金的研发和制备已成为国内外学者的研究热点。 合金化可促进非基面滑移和孪生等额外变形方式启动，是改善镁基体强度和塑性的有效手段；然而当前有关合金元素对镁基体性质的影响规律和作用机制的系统研究尚不完备。迫于合金设计的需要，亟需建立镁合金“数据库”，以优化出更多具有潜力的合金元素。广义层错能和孪晶界偏析能作为镁合金“数据库”的两个重要物理参数，对预测合金的力学性能具有重要意义。具体来说，层错能降低表明位错和孪晶形核的可能性增加，有助于启动额外变形方式；孪晶界偏析能降低代表溶质原子偏析的可能性增大，即对孪晶性质的改善效果增强。 本论文基于第一性原理计算，研究了合金元素的种类和含量对镁合金广义层错能和孪晶界偏析能的影响规律和作用机制，为新型高强韧镁合金的成分设计提供借鉴和依据，主要结论如下： (1)研究了21种合金元素掺杂下镁合金广义层错能的变化规律。基于基面和非基面滑移系统对塑性变形的贡献，提出了广义层错能权重模型，有助于衡量镁合金的整体变形能力；揭示出随合金元素与Mg原子半径差异增大，层错能降低的共性规律；在原子半径相近情况下，元素电负性越大，层错能越低；与电负性相比，原子半径对层错能的影响更大；该结果为新型高性能镁合金的成分设计提供了借鉴。 (2)构建了广义层错能分布图，为预测合金元素对镁合金塑性的影响提供了依据。揭示出In、Li、Sn和Bi元素使层错能下降且在Mg中固溶度大，有助于提高镁合金的塑性；Cd、Al、Ag、Ga、Zn和Pb元素掺杂可使层错能较大程度降低，但同时引发晶格畸变，因而相应镁合金的塑性依赖于掺杂含量；Be、Si、Ge、Cu、Na、Sr和K元素虽然使层错能大幅下降，然而在Mg中固溶度很小，导致对塑性的改善有限；此外，Al、Sn共掺下I2和T2层错能降低，即形成不全位错和孪晶的可能性增加，有助于改善镁合金塑性；揭示出新型HCP结构Au的基面层错能比纯Mg的低，导致其易于向FCC结构相转变。 (3)选取典型的Al和Zn元素，研究了元素含量对镁合金广义层错能的影响规律。发现随Al或Zn含量升高，基面和非基面滑移系统的广义层错能逐渐降低，同时更多滑移位置的不稳定层错能降低，即滑移机会增加；揭示了形成基面和非基面层错的概率随Al或Zn含量升高而增加，，有助于改善镁合金的塑性；同时，体系晶格畸变严重，阻碍了位错的扩展，引发固溶强化效果；预测出在理论上应存在一临界含量值，在高于该值时表现“强化”效应，反之为“软化”效应。 (4)揭示了Al、Zn对镁合金广义层错能的作用机制，为预测Mg Al Zn合金中最可能形成层错位置提供了依据。Al或Zn元素掺杂下电荷流动性增加，导致滑移面上的Mg Mg键变弱，从而使广义层错能降低；揭示出基面a和锥面c+a滑移在含合金元素的原子层以及邻近掺杂层的Mg层之间均有机会启动，锥面a层错易形成于掺杂原子层之间，而柱面a层错易形成于Mg层之间；为预测多元镁合金的塑性变形能力提供了依据。 (5)研究了19种合金元素掺杂下镁合金孪晶界偏析能的变化规律。发现原子半径比Pb小的合金元素（包括Pb，特例：{1011}孪晶中Ti和Zr元素）倾向占据{1012}和{1011}孪晶界的压缩位置；然而比Pb大的合金元素在扩张位置偏析的可能性更大；揭示出孪晶界处化学键越强，应变缓解程度越大，溶质原子的偏析能力越大；发现随合金元素与Mg尺寸（原子半径、平衡体积）差异增大，孪晶界偏析能降低的共性规律，同时，偏析能对合金元素尺寸的依赖性比对电负性更大；此外，{1012}孪晶界偏析能与平衡体积（除Ti、Zr和Cd元素），{1011}孪晶界偏析能与原子半径（除Ag、Zr、Bi和Be元素）之间大致呈直线关系；该结果为调控镁孪晶性质提供了借鉴。 (6)构建了孪晶界偏析能分布图，为预测合金元素掺杂下镁合金孪晶强化能力提供了依据。揭示出Ga、Zn、Al、Ag、Cd、Bi和Ca元素在Mg中偏析能较小、固溶度适中，可用于提高镁孪晶强化能力；Be、Si、Ge、Sb、Na和Sr元素虽然使偏析能大幅下降，然而固溶度很小，导致孪晶强化效果有限；揭示出Zn（压缩位置）和Ca元素（扩张位置）共同掺杂可使偏析能更大程度下降，有助于稳定孪晶结构；发现随Zn含量增加，孪晶界偏析能逐渐下降，弹性应变量减小，导致偏析可能性增大；有助于筛选多元合金元素调控镁孪晶性质。 总之，本文研究了合金元素对镁合金广义层错能和孪晶界偏析能的影响规律，构建了层错能和偏析能分布图，为预测合金元素掺杂下镁合金的塑性变形和孪晶偏析能力提供了依据；基于电子结构或应变场，揭示了合金元素对层错能和偏析能的作用机制，有助于更好地调控合金元素；对建立镁合金“数据库”，推动新型高性能镁合金研发和制备具有积极意义。
[Abstract]:The magnesium alloy has the advantages of small density , higher specific strength and higher specific rigidity , but less slip system at room temperature ( only two independent slip systems ) , which leads to poor plastic deformation capacity . Traditional magnesium alloys can not meet the requirements of different environments and working conditions on the performance of the service , severely restrict its development and application in engineering . Therefore , the research and development of the new high - toughness magnesium alloy has become a hot spot for scholars at home and abroad .

It is an effective means to improve the strength and plasticity of magnesium matrix , which can promote the initiation of non - base slip and twin deformation .
However , the research on the effects of alloying elements on the properties of magnesium matrix is not complete . For the sake of alloy design , it is urgent to establish the magnesium alloy " database " to optimize the alloy elements with potential . The generalized layer fault energy and the twin segregation energy can be used as two important physical parameters of the magnesium alloy " database " , which is of great significance to predict the mechanical properties of the alloy .
Twin - grain segregation reduces the possibility of segregation of solute atoms , that is , the improvement of the twin properties is enhanced .

Based on the calculation of the first principle , the influence law and mechanism of the kinds and contents of the alloy elements on the generalized layer fault energy and the twin segregation energy of the magnesium alloy are studied , and reference and basis for the composition design of the new high toughness magnesium alloy are provided . The main conclusions are as follows :

( 1 ) The change law of the generalized layer fault energy of the magnesium alloy under the doping of 21 kinds of alloy elements is studied . Based on the contribution of the base surface and the non - base slip system to the plastic deformation , a generalized layer fault energy weight model is proposed , which can help to measure the overall deformation ability of the magnesium alloy ;
The commonness law of the reduction of the layer ' s fault energy is revealed with the increase of the difference between the alloy element and the Mg atom radius .
When the atomic radius is similar , the greater the electronegative of the element , the lower the fault energy of the layer ;
The effect of atomic radius on the layer ' s fault energy is bigger than that of electronegative .
The results provide a reference for the composition design of the new high - performance magnesium alloy .

( 2 ) In order to predict the effect of alloy elements on the plasticity of magnesium alloy , the distribution diagram of generalized layer error energy is constructed . In addition , In - , Li , Sn and Bi elements can decrease the layer ' s fault energy and the solid solubility in Mg is large , which can help to improve the plasticity of magnesium alloy ;
The doping of Cd , Al , Ag , Ga , Zn and Pb can greatly reduce the layer error , but also induce lattice distortion , so the plasticity of the corresponding magnesium alloy depends on the doping content .
Be , Si , Ge , Cu , Na , Sr and K elements decrease the layer ' s fault greatly , however , the solid solubility in Mg is very small , which leads to the improvement of plasticity .
In addition , Al and Sn co - doped with I2 and T2 layers can be reduced , that is , the possibility of forming dislocation and twin is increased , which is helpful to improve the plasticity of magnesium alloy ;
It is revealed that the base layer fault energy of the novel HCP structure Au is lower than that of pure Mg , which leads to its easy transition to the FCC structure .

( 3 ) A typical Al and Zn element is selected to study the effect of element content on the generalized layer fault energy of magnesium alloy . It is found that the generalized layer fault energy of the base and non - base slip system decreases gradually with the increase of Al or Zn content , and the dislocation energy of unstable layer at the same time is decreased , that is , the slip opportunity increases ;
The probability of forming base surface and non - base layer is increased with the increase of Al or Zn content , which helps to improve the plasticity of magnesium alloy ;
At the same time , the lattice distortion of the system is serious , the dislocation expansion is hindered , and the solid solution strengthening effect is initiated ;
It is predicted that there should be a critical content value in theory , which exhibits a " hardening " effect when higher than the value , and vice versa .

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