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铝基金属间化合物电子结构和力学性质的第一性原理研究
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摘要
铝基金属间化合物有诸多优异的性能如低密度、高的比强度、优异的抗氧化和耐腐蚀性,在航空航天、国防军工及日常生活用品中得到广泛的应用。近年来的大量研究表明,电子结构决定材料的力学性质。考虑实验条件的限制如实验方法上的缺陷、实验设备的昂贵,实验操作的复杂性,这些因素使得测试结果不准确,实验上对铝基金属间化合物的研究较少,从电子层次上对铝基金属间化合物的力学性质进行研究则更少。而在原子、电子尺度上基于密度泛函理论的第一性原理方法能准确地预测物质的各种性质。本论文运用了基于密度泛函理论的第一性原理计算方法研究了铝基金属间化合物的晶体结构、电子结构和力学性质。论文主要分为两部分:第一部分详细分析了Al-Cu金属间化合物的电子结构及力学性质之间的关系;第二部分研究了点缺陷对立方结构铝基金属间化合物电子结构和力学性质的影响。具体内容如下:
     1.基于密度泛函理论的第一性原理方法研究了铝铜系列中几种重要金属间化合物(Al2Cu、Al3Cu2和AlCu3)的晶体结构和电子结构。通过计算它们的表面电子功函数、延展性、电子态密度和电荷密度,定性地解释了其延展性的差异。并进一步探讨了合金电子结构与延展性之间的关系,特别是电子功函数同合金材料脆性或延展性之间的关系。结果发现:延展性随着金属间化合物电子功函数的增加而减小,除了AlCu3。这主要归因于电荷的转移,一方面由于铜的电负性大于铝的电负性,在原子弛豫的时候,会使原子从电负性小向电负性大的铜原子移动,从而引起电荷分布的变化,电荷分布的变化又引起表面偶极子势的增加,因而使得电子功函数增加;另一方面材料的力学性能由电荷密度分布的均匀性和对称性来决定,对称性越好,说明材料的可塑性越好。
     2.基于第一性原理研究点缺陷对立方结构铝基合金电子结构和力学性质的影响。计算了空位下的晶格常数和平衡体积,发现计算得到的平衡晶格常数与实验值吻合较好;点缺陷形成能的计算表明AlCu3比AlNi3更易形成空位;形成能和结合能的研究表明Al7Cu24和Al7Ni24具有较好的结构稳定性和延展性;杨氏模量、切变模量、泊松比和各向异性因子等力学参量的计算发现,对于空位结构计算得到的晶格常数和弹性常数与已报道的实验及计算值相吻合,Al7Cu24和Al7Ni24满足立方结构机械稳定性条件;态密度和电荷密度合理地解释了空位对合金的结构稳定性和力学性质的影响。
Al-based intermetallics have wide application,because of their low density, high mechanic strength, corrosion resistance, simple productionprocess, low costs in comparison with other alloys, particularly the significant advantages of easy-recycling. Recent theoretical investigations have revealed that the primary origin of brittleness in intermetallics should be attributed to their electronic structures or chemical bond nature. However, data are very limited due to the complications of sample preparation and the limitations of experimental testing techniques, and the electronic nature of the mechanical properties of Al-based intermetallics is also not clear. Therefore,it is crucial to investigate the electronic structures in detail in order to understand brittleness completely.In this paper, we use the first–principle calculation based on the density functional theory to investigate the crystal structure, electronic structure and mechanical properties of Al-based intermetallics. Firstly, we report the relationship between the electrical and mechanical properties of the Al-Cu intermetallics. Secondly, we carry out a systematic first-principles study on the electronic structure and mechanical properties of Al-based intermetallics with cubic structure and vacancy point defects. The main contents of our work are:
     1. We use the first–principles calculation based on the density functional theory to investigate the crystal structure and electronic structure of Al-Cu intermetallic compounds (Al2Cu、Al3Cu2 and AlCu3). We calculate work function, ductility, density of states and charge density. The result shows that density of states and charge density can qualitatively affect the mechanical properties of material. And further explore the relationship between the electronic structure and ductility, that is, the relationship of work function and the brittleness. We find that the ductility decreases with increase of work function of intermetic compounds, except AlCu3. It mainly change charge distribution. When the atoms relax, atoms can move from aluminum to copper as the electronegativity of copper is greater than the electronegativity of aluminum. This can cause changes in charge distribution and form the surface dipole potential which can increase the electronic work function and lead to changes of mechanical properties. On the other hand, the electronic structure and mechanical properties is the charge density distribution of the material uniformity and symmetry; the better symmetry the better plasticity of materials which corresponds to the symmetry of charge distribution.
     2. Electronic structure and mechanical properties of Al-based alloys with cubic structure and vacancy have been investigated by using the first–principles calculation based on the density functional theory. Equilibrium structural parameters were derived and agreed well with experimental value. AlNi3 is easier to form vacancy point defects than AlCu3; the heat of formation and cohesive energy were calculated and showed that Al7Cu24 and Al7Ni24 had the better alloying ability and structural stability. Elastic constants and mechanical parameters such as bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio and anisotropy have also been calculated. There are a little difference between the imperfect crystal structures and perfect crystal structures from the calculated results and the experimental data. Investigation of the mechanical properties shows that cubic structures with vacancy point defects obey well the requirement of mechanical stability for cubic crystals. Cubic structure with vacancy point defects has a good alloying ability and structural stability due to an increase in the bonding electron numbers below the Fermi level. The effects of the alloying ability, structural stability and mechanical properties with vacancy point defects could be reasonably described and explained in terms of the electronic structure (the density of states and charge density distribution).
引文
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