【作者】 杨斌；

【导师】 赖文生；

【作者基本信息】 清华大学 ， 材料科学与工程， 2009， 硕士

【摘要】 近几十年来,人们发展出了各种非平衡制备技术并成功地获得了大量具有独特性能的非平衡材料。因此,发展相关的材料科学理论,以阐明非平衡材料的微结构、制备工艺以及性能之间的相关性,就成为材料科学面临的一个巨大挑战。为此,材料科学及凝聚态物理领域的研究者们付出了巨大的努力,发展出基于原子互作用经验势的分子模拟和从量子力学出发的电子结构计算方法,以期能构建一套全新的定量的材料科学理论。在分子动力学模拟中,模拟结果的可靠性取决于原子间相互作用多体势的准确性。对于一些不存在任何中间化合物或者无相关物理性能数据的合金系统,无法拟合出真实有效的原子间多体势。针对这一困难,我们采用了第一性原理辅助构建多体势的方法。首先借助第一性原理计算方法,获取合金系统中可能存在的若干非平衡相的相关物理性能,再通过对这些性能的拟合来确定出多体势中各待定参数的具体数值。遵循这一思路,我们选取了生成热为负但中间相数据缺乏的Cu-Al和Pd-Ti系统,分别构建出Tight-Binding形式的多体势。拟合得到的多体势能够很好地再现系统中重要的物理性能,表明多体势能够正确反应系统中原子间的相互作用。此外,基于多体势的分子动力学模拟对该系统中一些问题的研究结果与实验观测符合得很好,既进一步验证了拟合所得多体势的真实有效性,也证实了第一性原理辅助构建多体势方法的可行性。在Cu-Al系统中,我们采用构建的多体势计算了FCC、BCC固溶体和非晶态的能量随溶质原子的变化,通过比较三者的能量次序揭示了Cu-Al二元系统中很难得到非晶态的原因:即固溶体的能量比非晶相的要小,同时计算表明,BCC固溶体在Al含量32-72at.%范围内能量最低,与在该成分范围内得到单相BCC固溶体的Ball-milling实验结果一致。在Pd-Ti系统中,我们通过计算FCC固溶体和非晶态在全成分范围内的能量次序得到了非晶态的形成范围为21-70at.% Pd,与实验中得到的25-60at.% Pd相符。更多还原

【Abstract】 During the past decades, various nonequilibrium materials processing techniques have been developed and successfully employed to fabricate a great number of nonequilibrium materials with unique properties. Consequently, developing new materials theory to clarify the correlations among the microstructure, processing and property of the nonequilibrium materials has become an urgent demand with a great challenge. In response, researchers in the fields of materials and condensed matter physics have paid much effort to develop atomistic calculation or simulation methods based on quantum mechanics and/or interatomic potential (e.g. ab initio calculations or molecular dynamics simulations). Through these efforts, a solid foundation has been established to formulate new materials theories in a quantitative way.Generally, the reliability of molecular dynamics simulations depends on the interatomic potential adopted. For a binary metallic system, the n-body potential scheme is widely employed, which is usually constructed by fitting the physical properties of various intermetallic compounds in the alloy system. However, when turned to alloy systems without intermetallic compounds or with little physical property information, it is difficult to construct a realistic interatomic potential. Accordingly, we employ a new approach named the first-principle calculation aided construction of n-body potentials. The procedure consists two steps: firstly, the related physical properties of some possible non-equilibrium phases in the alloy system are obtained by the first-principle calculation; secondly, the obtained physical properties are then employed to fit the n-body potential.Following this idea, the Cu-Al and Pd-Ti systems were chosen to fit their respective n-body potentials. The derived potentials were proven to be able to reproduce those important physical properties, which are matched well with those obtained by the first-principle calculations or from experiments. Moreover, based on the constructed potentials, amorphous formation range and some other properties of the alloy phases were studied by molecular dynamics simulations and the results also agreed well with the experimental observations, lending support to the feasibility of the first-principle assisted potential construction approach.In the Cu-Al system, we calculate the energy of FCC solid solution, BCC solid solution and amorphous phase with the change of concentration of Al. It is found that the energy of solid solutions is always lower than that of amorphous phase, explaining why the amorphous phase is hardly obtained in the Cu-Al system. The results also show that for the Cu1-xAlx alloys, the energy of the BCC solid solutions becomes the lowest in the range of 0.32 < x < 0.72 among three phases studied here, in good agreement with the ball-milling experimental observation that the BCC solid solution was obtained in the composition range of 0.3 < x < 0.7. In another binary system, namely the Pd-Ti system, the energy sequence of FCC solid solution and amorphous phase was calculated in the whole composition range with concentration of Pd from 0 to 100 at.%. The results show that the glass formation range in PdxTi1-x is wihtin 0.21 < x < 0.70, in which the energy of amorphous phase is lower than its competitive partner. It is consistent with the experiment result that is in the range of 0.25 < x < 0.60.更多还原