金属间化合物相稳定性、层错能及力学性质的第一性原理研究

发布时间：2018-06-29 06:48

本文选题：相稳定性 + 层错能　；参考：《重庆大学》2016年博士论文

【摘要】：随着现代工业生产的飞速发展,高性能材料日益成为人们关注的热点。其中金属间化合物具有其它普通金属或合金所不具备的长程有序的超点阵结构,原子间表现出独特的金属共价双重键合特征,宏观上表现出低密度,较高的硬度、熔点,耐腐蚀性强,良好的抗蠕变、抗氧化能力以及优异的高温服役能力,被广泛地应用于航空航天、汽车船舶等诸多领域,有望成为最具发展前景的高性能的高温结构材料。然而,其中最大的不足之处就是它的低温脆性,这极大的限制了其广泛应用。从晶体结构来看造成金属间化合物脆性的主要原因是由于其较低的对称性从而缺少足够多的独立的滑移体系。实验上表明通过合金化手段来稳定高对称的亚稳相从而提供较多的可开动的滑移体系以满足多晶体延性条件的von Mises定则有望提升材料的塑性。然而这些机理并不十分明了,尤其关于微观电子机制的研究还比较匮乏,因此本文将主要通过第一性原理计算和实验方法相结合的手段研究合金化元素对几种具有代表性的金属间化合物材料性能的影响。本文主要基于第一性原理方法研究了MoSi2、TiAl、TiAl3以及Al3Zr这四种比较具有代表性的的金属间化合物在不同类型原子掺杂情况下的相稳定性、力学性能,结合相应的实验观测并从弹性性质、层错以及原子成键角度解释了材料的脆塑性机制,同时给出了的硬度、熔点等相关性质。研究结果对改善金属间化合物的塑性以及为高性能结构材料的设计研发提供了理论参考与指导。本文的主要研究内容如下:(1)MoSi2是由Mo、Si原子组成,表现出特有的金属和陶瓷双重属性,具有高熔点、适中的密度以及极好的高温稳定性,然而低温脆性是一个严重的缺陷。本文基于第一性原理研究了第三元素Al、Mg、Ge掺杂情况下MoSi2的相稳定性及力学性质。计算结果表明Al、Mg元素能够有效地稳定C40亚稳相,并且当Al、Mg元素的浓度分别达到7 at.%、6 at.%时MoSi2将会发生结构相变从C11b相转变为C40相。相应的材料的延展性也得到了很好的改善,这与Dasgupta等人的实验结果符合的很好。此外研究还表明Al、Mg元素的加入能够降低材料的硬度,相应的熔点也有略微的降低,而Ge掺杂并没有引起相结构的转变因而材料的各项性能并没有发生较大的变化。此外还计算了Nb以及Al-Nb、Mg-Nb、Ge-Nb共掺杂情况下对材料性能的影响,计算结果表明只有Mg-Nb共掺杂的情况下,材料的力学性能才有所改善。以上表明,相结构的转变是提升材料力学性能的关键因素。最后从电子层次上研究了掺杂元素对原子键合作用的影响并对相结构稳定性以及上述力学性能做了分析。结果表明掺杂元素的存在能够有效地削弱MoSi2中Mo-Si之间的共价相互作用,相应的塑性得以改善。(2)作为优良的轻质高温结构材料的重要候选者之一,tial金属间化合物表现出其它传统金属或合金所不具备的独特的优良性质。本文基于第一性原计算了过渡金属元素w、mo以及稀土元素sc、yb掺杂情况下tial的相稳定性以及力学性质。计算的选择替位表明稀土元素sc、yb最有可能占据ti位置,而过渡金属元素w和mo在l10相中倾向于占据ti的位置,在b2相中则更倾向于占据al的位置。计算的生成焓表明过渡金属w和mo的掺入能够有效地增强高对称的亚稳相b2相的稳定性,当w和mo的浓度分别达到10.5at.%和11.50at.%时,b2相将取代l10相成为稳定相。基于pugh判据分析了掺杂原子对tial脆塑性行为的影响。脆塑性相图表明由于b2相的产生使得tial由本征脆性材料转变为本征塑性材料。此外,计算结果还表明低浓度的稀土元素sc、yb虽能够提高材料的延展性但效果很有限。这表明塑性改善的本质原因是由于相结构的转变,这和实验中观测到的结果符合的很好。相应的合金化元素对材料的硬度、各向异性等力学性质的影响也进行了详细的研究。电子结构表明tial金属间化合物脆性的根本原因是由于ti、al原子之间较强的方向共价键存在,而塑性的改善是由于w、mo、sc、yb的掺入极大地削弱了ti、al之间的共价键,相应的使得ti、al之间的金属间相互作用得到增强宏观上表现出延展性的极大提高。(3)具有长程有序的d022结构的tial3由于其极低的密度,良好的耐腐蚀性以及抗氧化性,优异的力学性能及高温稳定性,成为一类具有很好发展前景的轻质高温结构材料。但其室温脆性限制了材料的广泛应用。本文基于第一性原理密度泛函理论计算了ds区过渡金属cu、zn、ag掺杂情况下tial3的相稳定性及力学行为。生成焓表明在cu、zn、ag取代tial3中的al位置时能够有效地稳定高对称性的立方l12相从而提供足够多的有效的可开动的滑移体系。pugh比值、cauchy压强以及泊松比表明l12相的产生使得材料的塑性得到改善。层错能计算表明l12相稳定性增强的原因是由于cu、zn、ag的掺入极大地促进了d022相中110{001}滑移系的开动,从而引起结构的转变。此外,计算的l12相的层错能表明塑性提升的原因很大程度上归因于l12相中提供了有效的可开动的110{111}滑移系。基于griffith断裂理论并结合计算的解理能以及层错能引入了关于脆塑性行为的断裂韧性以及rice、zct判据。结果表明造成d022相tial3脆性的根本原因是由于在外加应变下微裂纹极易在材料中萌生。最后电子态密度表明d022相脆性的原因是由于该相中al的3p电子与ti的3d电子的强烈的方向共价键的存在。而l12相的产生能够使共价键得到抑制,主要是由于cu、zn、ag的掺入削弱了al的3p电子与ti的3d电子的共价相互作用,同时增强了ti原子和合金化原子的d-d相互作用,从而使得tial3中原子之间的键合力趋于平衡材料的塑性得到了提升。(4)研究了长程有序结构的Al3Zr金属间化合物。通过实验手段我们用Al-35wt.%Cu、Al-4wt.%Zr以及高纯度的Al制备了直径为12mm长为110mm的Al-Cu-Zr三元合金铸件。热处理后通过透射电镜观察微组织结构。结果表明Al基底中出现L12相Al3Zr沉淀颗粒。能谱分析(EDX)表明其中有Cu的出现,经确认这些颗粒成份为Al2.5Cu0.5Zr,这表明一部分Cu原子取代Al原子的位置生成了稳定相的Al3Zr颗粒。第一性原理计算的生成焓表明Cu确实是容易取代Al位置,理论和实验符合的很好。此外高分辨率电子显微镜(HRTEM)发现沿着Al基底Al001晶带轴出现L12相的Al-Cu-Zr颗粒并且在平行于该相的(001)面出现110(001)层错。根据理论计算发现Cu的掺入会很大程度的降低D023相的110(001)层错能;相反L12相110(001)层错能却会由于Cu的掺入大大的提升。实验和理论相结合表明Cu的掺入能够有效地增强L12相得稳定性。通过计算的弹性性质并基于Pugh判据以及泊松比分析了材料的力学性能,计算表明相比D023相L12相具有较好的塑性。此外,计算结果还表明过渡金属元素Zn和Ag也具有和Cu相似的效果。基于Griffith脆性断裂理论计算了断裂韧性因子IcK并结合Rice以及ZCT判据对相变前后的脆塑性行为进行了分析,不过断裂理论给出的结论和弹性理论给出的结论有一定的矛盾。这些矛盾可能是Griffith脆性断裂理论过于粗糙,因为真实材料中还会存在其它较为复杂位错的影响。因此简单的采用脆性断裂或者不稳定层错能来定义材料的脆塑性行为还不够精确,有必要结合电子结构分析材料的力学性能。最后计算的电子态密度表明材料的脆性本质是由于Al、Zr原子之间强的方向共价性结合,而Cu、Zn、Ag的掺入能够很大程度上削弱Al-3p与Zr-4d的相互作用使得原子之间的键合力趋于平衡是提升材料塑性的本质因素。
[Abstract]:With the rapid development of modern industrial production, high performance materials have become a hot spot of concern. Among them, intermetallic compounds have long range ordered superlattice structures that other ordinary metals or alloys do not possess. The unique metal covalent double bonding characteristics are shown between the atoms, and the macroscopically show low density, high hardness, and melting. With its strong corrosion resistance, good resistance to creep, antioxidation and excellent high temperature service, it is widely used in aerospace, automobile and ships and many other fields, and it is expected to become the most promising high performance high temperature structural material. However, the biggest disadvantage is its low temperature brittleness, which greatly restricts it. The main reason for the brittleness of intermetallic compounds from crystal structure is due to their low symmetry and the lack of enough independent slip systems. Experimental results show that the stability of highly symmetric metastable phase by means of alloying is provided to provide more open sliding systems to satisfy the ductility of polycrystal. The von Mises rule is expected to improve the plasticity of the material. However, these mechanisms are not very clear, especially the research on microelectronic mechanisms is still scarce. Therefore, this paper will study the properties of several representative intermetallic compounds by the combination of the first principle calculation and the experimental method. In this paper, based on the first principle method, the phase stability and mechanical properties of four representative intermetallic compounds with different types of atoms in MoSi2, TiAl, TiAl3 and Al3Zr are studied, and the material brittleness is explained from the elastic properties, the stacking faults and the atomic bonding angles. The research results provide theoretical reference and guidance for the improvement of the plasticity of intermetallic compounds and the design and development of high performance structural materials. The main contents of this paper are as follows: (1) MoSi2 is composed of Mo, Si atoms, showing a unique dual properties of metal and ceramics. There is a high melting point, moderate density and excellent high temperature stability, however, low temperature brittleness is a serious defect. Based on the first principle, the phase stability and mechanical properties of the third elements Al, Mg, and Ge doping are studied. The results show that Al, Mg elements can effectively stabilize the C40 metastable phase, and when Al, Mg element concentration is strong. The structure phase transition from C11b phase to C40 phase will occur at the degree of 7 at.% and 6 at.%, and the ductility of the corresponding materials is well improved. This is in good agreement with the experimental results of Dasgupta et al. In addition, the addition of Al and Mg elements can reduce the hardness of the material, and the corresponding melting point also decreases slightly. The Ge doping does not cause the phase structure transformation and the properties of the materials have not changed greatly. In addition, the effects of Nb and Al-Nb, Mg-Nb and Ge-Nb co doping on the properties of the materials are also calculated. The results show that the mechanical properties of the materials are improved only when the Mg-Nb co doping is codoped. The transformation is the key factor to improve the mechanical properties of the materials. Finally, the effects of doping elements on the cooperation of atomic bonds are studied at the electronic level, and the structural stability and the mechanical properties are analyzed. The results show that the existence of doped elements can effectively weaken the covalent interaction between Mo-Si in MoSi2 and the corresponding plasticity. (2) as one of the important candidates for excellent lightweight and high temperature structural materials, TiAl intermetallic compounds exhibit unique excellent properties that other traditional metals or alloys do not possess. Based on the first analysis, the phase stability and mechanics of the transition metal elements, W, Mo, and rare earth element SC, and Yb doped TiAl are calculated. The selected substitutions show that the rare earth element SC, Yb is most likely to occupy the Ti position, while the transition metal elements W and Mo tend to occupy the Ti position in the L10 phase, and are more inclined to occupy the Al position in the B2 phase. The calculation enthalpy of the formation of the transition metals W and Mo can effectively enhance the stability of the highly symmetric metastable phase B2 phase. When the concentration of W and Mo reached 10.5at.% and 11.50at.%, the B2 phase would replace the L10 phase as a stable phase. Based on the Pugh criterion, the influence of the doping atom on the brittle plastic behavior of TiAl was analyzed. The brittle plastic phase diagram showed that the TiAl from the intrinsic brittle material was converted to the intrinsic plastic due to the formation of the B2 phase. Furthermore, the calculation also showed that the low concentration of the TiAl was low. The rare earth element SC, Yb can improve the ductility of the material but the effect is very limited. This indicates that the essential reason for the plastic improvement is the phase structure transformation, which is in good agreement with the results observed in the experiment. The influence of the alloying elements on the hardness and anisotropy of the material is also studied in detail. The basic reason for the brittleness of TiAl intermetallic compounds is that the strong direction covalent bond exists between Ti and Al atoms, and the plastic improvement is due to the incorporation of W, Mo, SC, Yb, which greatly weaken the covalent bond between Ti and Al, and the corresponding intermetallics between Ti and Al can increase the maximum extension of the ductility between the Ti and al. (3) the TiAl3 with long range ordered D022 structure, due to its very low density, good corrosion resistance, antioxidation, excellent mechanical properties and high temperature stability, has become a kind of lightweight high temperature structure material with good prospects, but its room temperature brittleness restricts the wide application of material. This paper is based on the first principle density. The functional theory has calculated the phase stability and mechanical behavior of TiAl3 in the transition metal Cu, Zn, and Ag in the DS region. The enthalpy of formation indicates that the cubic L12 phase of high symmetry can be effectively stabilized when Cu, Zn, Ag take place of Al in TiAl3, thus providing enough effective and open sliding system.Pugh ratio, pressure and Poisson's ratio table. The formation of the L12 phase makes the plasticity of the material improved. The calculation of the stacking fault energy shows that the reason for the enhancement of the stability of the L12 phase is that the incorporation of Cu, Zn, and Ag greatly promotes the opening of the 110{001} slip system in the D022 phase, thus causing the transformation of the structure. In addition, the stacking fault energy of the calculated L12 phase indicates that the reason for the plastic lifting is largely attributable to L1. In the 2 phase, an effective 110{111} slip system is provided. Based on the Griffith fracture theory and the calculation of the cleavage energy and the stacking fault energy, the fracture toughness and the rice, ZCT criterion of the brittle plastic behavior are introduced. The result shows that the root cause of the D022 TiAl3 brittleness is that the micro crack is easily germinated in the material under the external strain. Finally, the electronic density of States indicates that the reason for the D022 phase brittleness is due to the existence of the strong directional covalent bond of the Al 3P electron and the 3D electron of the Ti. The formation of the L12 phase can inhibit the covalent bond, mainly because the incorporation of Cu, Zn, and Ag weakens the covalent interaction between 3P electrons of Al and the Ti electron. The D-D interaction between the atoms and the alloying atoms makes the bonding force between the atoms in the TiAl3 tend to balance the plasticity of the material. (4) the Al3Zr intermetallic compound of the long range ordered structure is studied. By means of experimental means, we use Al-35wt.%Cu, Al-4wt.%Zr and high purity Al to prepare Al-Cu-Zr with a diameter of 12mm as 110mm. The microstructure was observed by transmission electron microscope after heat treatment. The results showed that the L12 phase Al3Zr precipitated particles were found in the Al substrate. The energy spectrum analysis (EDX) showed that there was the emergence of Cu, and it was confirmed that these particles were Al2.5Cu0.5Zr, which indicated that a part of the Cu atoms replaced the Al atoms in the Al3Zr particles of the stable phase. The enthalpy of formation of the calculation of the principle of sexual principle indicates that Cu is really easy to replace the Al position. The theory and the experiment agree well. In addition, the high resolution electron microscope (HRTEM) found the Al-Cu-Zr particles in the L12 phase along the Al001 crystal band of the Al base and appeared 110 (001) layer faults in the (001) plane parallel to the phase. According to the theoretical calculation, the incorporation of Cu is very large. The 110 (001) layer of the D023 phase is reduced to a degree of degree, while the 110 (001) fault energy of the L12 phase will be greatly enhanced by the incorporation of Cu. The combination of the experiment and the theory shows that the incorporation of Cu can effectively enhance the stability of the L12 phase. The mechanical properties of the materials are analyzed by the elastic properties calculated and based on the Pugh criterion and Poisson's ratio. The L12 phase is better than the D023 phase. In addition, the calculation results also show that the transition metal elements Zn and Ag have the same effect as Cu. Based on the Griffith brittle fracture theory, the fracture toughness factor IcK is calculated and the brittle plastic behavior before and after the phase transition is analyzed with Rice and ZCT criterion, but the conclusion and projectile given by the fracture theory are given. There are some contradictions in the conclusions given by the theory of sex. These contradictions may be that the Griffith brittle fracture theory is too rough, because there will be other more complex dislocation in the real material. So it is not accurate to use brittle fracture or unstable layer fault energy to define the brittle plastic behavior of the material. It is necessary to combine the electrons. Structural analysis of the mechanical properties of the material. The final calculation of the electronic density of States indicates that the brittle nature of the material is due to the strong direction covalent bond between Al and Zr atoms, while the incorporation of Cu, Zn, and Ag can greatly weaken the interaction between Al-3p and Zr-4d, making the bond force between atoms balance the essential factor of improving the plasticity of the material.
【学位授予单位】：重庆大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146

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