【作者】 陈平；

【导师】 唐壁玉；

【作者基本信息】 湘潭大学 ， 材料物理与化学， 2010， 硕士

【摘要】 镁合金密度低、力学性能优异、电磁屏蔽能力强,在航空航天、交通运输、电子等领域具有重要的应用价值和应用前景。然而,镁合金绝对强度低、高温抗蠕变能力差,这些都大大限制了其应用。含稀土镁合金具有强度高、抗蠕变能力强和热稳定性好的优点,得到了材料研究者的广泛关注。本文利用基于密度泛函理论的第一性原理计算方法,研究了稀土镁合金部分中间相的结构、力学和电子属性。所得主要结果如下:1.理论上确定了Mg–Gd系合金中β’相的晶格参数,并计算了正交晶体9个独立的弹性常数。Mg15Gd不符合正交晶体的力学稳定性准则,是一种力学不稳定结构。采用Voigt–Reuss–Hill（VRH）近似,得到了Mg7Gd的多晶体模量B,剪切模量G,杨氏模量E和泊松比ν。通过分析电子态密度和电荷密度,发现镁和钆之间形成了方向性的共价键。最后,还计算得到了Mg7Gd的密度、波速度和德拜温度。2.通过理论计算,得到了Mg–Gd系合金中β"相的结构参数,从形成能的角度说明它是能够稳定存在的。计算的九个独立的弹性常数符合力学稳定性准则,并从单晶弹性常数得到了β"相的多晶体模量B,剪切模量G,杨氏模量E和泊松比ν。从这些参数可以看出,它的力学性能并不十分优异。它的延展性较好,弹性各项异性行为也比较显著。同β’相一样,体系内也存在较强的方向性共价键。3.理论上调查了Mg₉₇Zn₁Y₂合金系统中6H型ABACAB长周期堆垛（LPS）结构的微结构、稳定性和电子属性特征。Y和Zn原子在浓度较低时主要分布于层错缺陷层,这个结果与实验观察一致。Y和Zn原子倾向于紧密排列,这样可以抵消原子半径不同带来的应变,使的6H型结构能量上更稳定。通过计算不同固溶原子含量时的结合能,发现Y元素增强了6H结构的稳定性,在改善合金的抗蠕变能力方面起到了重要作用。Zn原子位于两个不匹配的层错层时将导致晶格畸变,进一步的分析表明Y和Zn原子紧密排列将会削弱了畸变的幅度。通过分析电子态密度和电荷密度,发现Mg原子和Y原子之间形成了较强的共价键,这将增加合金强度。Mg原子与Zn原子之间的杂化作用广泛而均匀存在,这对改善合金延展性有帮助。更多还原

【Abstract】 Magnesium alloys have important application value and prospect in the aerospace, transportation, electronics fields due to their low density, excellent mechanical properties, and electromagnetic shielding ability. However, the applications are still limited because of the lower strength, inferior fatigue and creep resistance at elevated temperature. Mg-rare earth alloys have received material researchers’great interest because of their high strength, good creep resistance and thermal stability. In this paper, we carried out first-principles calculations based on density functional theory to study on the structural, mechanical and electronic properties of the intermediate phase in Mg alloys. The obtained main results are as follows:1. We determined the lattice parameters ofβ’ phase in Mg–Gd alloy theoretically and calculated the nine independent elastic constants of the orthogonal crystal. The nine independent elastic constants did not meet the mechanical stability criteria, which suggest the reported Mg15Gd is a mechanically unstable structure. The polycrystalline bulk modulus B, shear modulus G, Young’s modulus E, and Poisson’s ratioνfor Mg7Gd are calculated within the scheme of Voigt–Reuss–Hill（VRH） approximation. We found the covalent bond was formed between the Mg and Gd atoms based on the analysis of the density of states and charge density. Finally, the density, wave velocity and Debye temperature of Mg7Gd are calculated.2. The structural parameters ofβ" phase in Mg–Gd alloy are determined. The obtained formation enthalpy indicates that theβ" phase Mg3Gd could be formed energetically. The nine independent elastic constants are calculated, which suggest the Mg3Gd is mechanically stable. The polycrystalline bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratioνforβ" phase Mg3Gd are calculated from the elastic constants. It seems that the improvement of mechanical properties is limited after alloying from these parameters. Theβ" phase Mg3Gd is a ductile material and exhibits the elastic anisotropy apparently. Just as theβ’ phase, the directional covalent bonding was formed in the system.3. The microstructure, stability and electronic properties of the 6H-type ABACAB LPS phase in Mg₉₇Zn₁Y₂ are investigated theoretically. The enrichment of Y and Zn atoms occurs in the stacking fault defect layers, which is in accordance with the experimental observations. The present calculated results still show clearly that the Y and Zn atoms are preferably arranged closely. It is reasonable that the two atoms arrange closely to cancel strain due to the different atomic radii, and then lead the structure energetically more stable. The Y element enhances the structural stability from the calculated cohesive energy. It is clearly that the Y element would improve the creep property of alloy. The Zn atoms located in two different misfit fault layers would lead to lattice distortion. Further analysis shows that the intimate arrangement of Y and Zn atoms would weaken the distortion. Based on the analysis of the density of states, we found the covalent bonding exists between the Mg and Gd atoms which would enhance the strength of alloy. The hybridization between Mg and Zn is clear in entire region and the variation of hybridization is small. So this is helpful for improving the ductility of the 6H-type LPS structure.更多还原