熔体在楔形衬底和纳米狭缝内的结构演变及凝固行为

发布时间：2018-11-27 10:05

【摘要】：在自然界、冶金技术以及材料制备过程中,凝固是一种非常普遍的物理现象。凝固广泛存在于各个领域,物体从液体转变为固体都要经历凝固过程。凝固开始于形核,形核又可以分为均质形核和异质形核。在实际生产中,大多数的形核过程都是异质形核,因此研究固相粒子对熔体结构以及凝固过程的影响有着非常重要的意义。本论文利用分子动力学方法研究了楔形衬底以及受限空间对熔体的结构以及凝固行为的影响,揭示了衬底与熔体结构以及凝固组织之间的内在关联性。研究结果对于通过控制异质形核从而改善材料的宏观性能有着重要的意义。全文的主要内容如下:(1)研究了楔形衬底如何将结构信息传递给正在生长的晶体,进一步影响凝固过程。结果显示,由于衬底的诱导作用靠近衬底的铜熔体在液固界面处分层形成一个"V"形结构,并且有形成孪晶的趋势。但是随着与衬底间距离的增加,衬底的诱导作用逐渐减弱致使铜熔体逐渐恢复其固有的结构。当楔形角为90°时,由于铜原子内部良好的晶格匹配,凝固组织是一个理想晶体。此外,冷却速率以及衬底的原子排列方式对铜熔体的凝固组织也有很大的影响。这些研究结果有利于理解异质凝固过程中,衬底与凝固组织之间的遗传关系。(2)研究了铝熔体在受限狭缝内以及Fe衬底上的异质形核过程。与孤立衬底相比,受限狭缝可以促进铝熔体的结晶,同时狭缝的尺寸对凝固组织有很大的影响。在对称的受限狭缝内,Al原子可以根据衬底的堆垛模型进行排列,而在非对称的受限狭缝内,Al原子的堆垛模型取决于衬底的构成。对于Fe(110)-Fe(111)这种狭缝来说,由Fe(110)衬底诱导的凝固区域要大于由Fe(111)衬底诱导的凝固区域。此外,对于铝熔体在由铜铁两种衬底构成的非对称受限空间我们也进行了研究。这些结果阐明了受限空间对熔体异质凝固过程的影响。(3)探究了在不同压强下以及不同距离的两个石墨烯之间,受限水的结构演变。结果显示,受限狭缝的尺寸以及压强都能够引起受限水的结构演变,具体表现为层数以及堆垛方式的变化。随着压强以及狭缝间距的增加,受限水的分层现象也越来越明显。此外,随着压强的增加,在受限狭缝内,近邻水分子首先形成链状结构,之后演变成正方形结构,最后转变为三角形结构。这些结果有助于了解受限空间内水的结构演变。
[Abstract]:Solidification is a very common physical phenomenon in nature, metallurgical technology and material preparation. Solidification exists widely in various fields, and the transformation of objects from liquid to solid has to go through the solidification process. Solidification begins with nucleation, which can be divided into homogeneous nucleation and heterogeneous nucleation. In practical production, most nucleation processes are heterogeneous nucleation, so it is very important to study the influence of solid particles on melt structure and solidification process. In this paper, the effects of wedge-shaped substrate and confined space on the structure and solidification behavior of melt were studied by molecular dynamics method, and the intrinsic relationship between substrate and melt structure and solidification structure was revealed. The results are of great significance for improving the macroscopic properties of materials by controlling heterogeneous nucleation. The main contents of the thesis are as follows: (1) the structure information of the wedge substrate is transferred to the growing crystal, which further influences the solidification process. The results show that due to the inducement of substrates, copper melts near the substrates form a "V" shaped structure at the liquid-solid interface and have a tendency to form twins. However, with the increase of the distance between the substrate and the substrate, the inductive effect of the substrate gradually weakened, and the copper melt gradually restored its inherent structure. When the wedge angle is 90 掳, the solidified structure is an ideal crystal due to the good lattice matching within the copper atom. In addition, the cooling rate and the atomic arrangement of the substrate also have a great influence on the solidification structure of copper melt. These results are helpful to understand the genetic relationship between substrate and solidification structure during heterogeneous solidification. (2) the heterogeneous nucleation process of aluminum melt in confined slit and on Fe substrate is studied. Compared with the isolated substrate, the confined slit can promote the crystallization of aluminum melt, and the size of the slit has a great influence on the solidification microstructure. In a symmetric confined slit, Al atoms can be arranged according to the stacking model of the substrate, while in the asymmetric confined slit, the stacking model of the Al atom depends on the composition of the substrate. For the slit of Fe (110)-Fe (111), the solidification region induced by Fe (110) substrate is larger than that induced by Fe (111) substrate. In addition, we also study the asymmetric confined space of aluminum melt in two kinds of substrates, copper and iron. These results illustrate the effect of confined space on the process of heterogeneous solidification of melt. (3) the evolution of confined water structure between two graphene at different pressures and distances is investigated. The results show that both the size and the pressure of the confined slit can cause the evolution of the confined water structure, which is manifested by the change of the number of layers and the way of stacking. With the increase of pressure and slit spacing, the stratification of confined water becomes more and more obvious. In addition, with the increase of pressure, in the confined slit, the near neighbor water molecules first form chain structure, then develop into square structure, and finally transform into triangular structure. These results help to understand the structural evolution of water in confined spaces.
【学位授予单位】：山东大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG21

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