纳米多晶金属的弹性和热力学性质研究

发布时间：2018-11-05 08:30

【摘要】：纳米材料指晶粒尺寸在1-100nm之间的物质,纳米材料正成为一种新型材料,广泛地应用于各个领域。纳米金属由于其区别于粗晶的特殊性质而引起了人们广泛关注,例如其增长的硬度、屈服应力、超强的韧性,与粗晶相比变大的扩散系数、优良的耐磨性等。相比于粗晶,纳米多晶材料的杨氏模量减小、定容热容增大,了解纳米多晶金属的弹性及热力学性质有助于提升纳米材料在各个领域的应用价值。本文主要包含两部分研究内容,首先用Voronoi几何法搭建不同晶粒大小的纳米多晶金属铝和铜样本,并对初始结构进行弛豫来降低由大角度晶界带来的高应力和高能量。在弛豫过程中监测了体系结构、平均能量、平均内应力等在各个过程的变化情况,对比分析了不同晶粒样本在同一弛豫过程的变化趋势。画出了初始结构与弛豫后样本的结构图,发现晶界原子所占比例发生了变化,最终获得了稳定的结构;其次,用经典分子动力学模拟方法对比了施加微小应变时纳米多晶与单晶的冷能曲线,计算分析了晶粒大小对纳米多晶金属铝和铜的弹性常数和一些热力学参数的影响。通过对体系施加微小的拉伸或压缩应变,画出了对应的冷能曲线,并与相应的单晶曲线图进行了对比,发现图形形状基本一致,微小差别可能由于晶界引起的,说明了模拟方法的可行性。计算弹性常数选用的是恒压分子动力学方法,在晶粒粒径小于10nm时,发现体系的硬度随着尺寸的减小而变软,与反Hall-Petch效应吻合。样本的拉伸强度随着晶粒的增大变得更难,压缩程度则相对变得容易。根据热力学参量与独立弹性常数的关系,得出剪切模量、泊松比、德拜温度等热力学值,使我们更深入的认识小晶粒粒径纳米材料的抵抗形变、塑性变形、刚性和延性等性质。通过详细分析小粒径纳米多晶金属的弹性和热力学性质,可以使人们更全面的认识微小纳米材料,为更好的应用纳米材料提供了有力保障。
[Abstract]:Nanomaterials refer to materials with grain size between 1-100nm. Nanomaterials are becoming a new type of materials and are widely used in various fields. Nanometallic materials have attracted much attention due to their special properties different from coarse grains, such as their increasing hardness, yield stress, super toughness, large diffusion coefficient compared with coarse grains, excellent wear resistance and so on. Compared with coarse crystals, the Young's modulus of nanocrystalline materials decreases and the heat capacity of nanocrystalline materials increases. Understanding the elastic and thermodynamic properties of nanocrystalline metals is helpful to enhance the application value of nanocrystalline materials in various fields. This paper mainly consists of two parts. Firstly, the samples of nanocrystalline aluminum and copper with different grain sizes are constructed by Voronoi geometry method, and the initial structure is relaxed to reduce the high stress and high energy caused by large angle grain boundary. The changes of system structure, average energy and average internal stress in each process were monitored during the relaxation process, and the variation trends of different grain samples in the same relaxation process were compared and analyzed. The structure diagram of the initial structure and the relaxed sample is drawn. It is found that the proportion of the grain boundary atoms has changed and finally the stable structure has been obtained. Secondly, the cold energy curves of nanocrystalline polycrystalline and single crystal are compared by classical molecular dynamics simulation method, and the effect of grain size on elastic constants and some thermodynamic parameters of nanocrystalline aluminum and copper are calculated and analyzed. The corresponding cold energy curves are drawn by applying small tensile or compression strain to the system, and compared with the corresponding single crystal curves, it is found that the figure shape is basically the same, and the slight difference may be caused by the grain boundary. The feasibility of the simulation method is illustrated. The constant pressure molecular dynamics method is used to calculate the elastic constants. When the grain size is smaller than 10nm, it is found that the hardness of the system softens with the decrease of the size, which coincides with the inverse Hall-Petch effect. The tensile strength of the sample becomes more difficult with the increase of grain size, and the compression degree becomes relatively easy. According to the relationship between thermodynamic parameters and independent elastic constants, the thermodynamic values such as shear modulus, Poisson's ratio, Debye temperature and so on are obtained, which makes us know more about the resistance to deformation, plastic deformation, rigidity and ductility of nano-materials with small grain size. Through the detailed analysis of the elastic and thermodynamic properties of nanocrystalline metal with small particle size, it can make people fully understand the micro-nano materials and provide a strong guarantee for the better application of nanocrystalline materials.
【学位授予单位】：山西大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG111;TB383.1

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