IMECH-IR  > 非线性力学国家重点实验室
典型原子界面与粘接界面的力学行为
Alternative TitleMechanical behavior of typical atomic interface and bonding interface
彭神佑
Thesis Advisor魏宇杰
2019-06-01
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline固体力学
Keyword界面 位错 位错运动 位错速度 超声速位错 界面塑性
Abstract

界面在自然界与工业界广泛存在,小到晶体的孪晶界面,大到地球的地层结构,从生物中细胞膜界面,到工业上的复合材料。界面作为一类缺陷,它本身的力学性能将对整体结构的性能产生决定性作用。尤其是界面的弹塑性性能,将主导整体的材料强度和韧性。在原子层面上,界面上的位错运动是界面塑性的主要来源,这在金属多晶材料中的作用尤为重要。本文着重考虑孪晶界面上的位错,深入研究了位错运动的速度-应力关系与其超声速行为,这些结果有助于深入了解界面位错塑性的微观机理。

基于以上的考虑,本文的主要研究内容如下:

位错的运动是主导原子界面和晶体强度和韧性的主要因素,但是位错的运动规律仍需深入探索,特别是极端加载情况下完整的速度-应力依赖关系,以及超声速位错的存在性和理论解释。考虑到晶体中的位错主要有两种不同的基本结构:即刃位错与螺位错。首先,构建了孪晶界上的刃型分位错,并通过分子动力学模拟这种位错在不同应变率下、不同金属中等种种情况下的位错运动,得到了完整的位错速度-应力关系,从中发现,刃位错存在两个极限速度:一个亚音速极限和一个超音速极限。当应力足够大时,位错速度从亚音速极限到超音速极限发生速度突变。在理论上,通过简单的声子态密度和分布模型,建立了声子辅助位错滑移的模型。该模型完整地描述了位错速度与外加应力的函数相关性,也解释了两种速度极限的物理起源。

接下来,构建了孪晶界的螺位错模型,为了对比,考虑了三种不同结构的螺位错:孪晶界纯螺型位错、孪晶界混合型位错和完美晶体中的全位错。通过模拟其在外载作用下的运动,得到了三种螺位错的完整的速度-应力关系,证实了铜晶体中的螺型全位错和螺型孪晶界不全位错的超声速运动(超过三个各向异性剪切波速),并且还发现它们都能稳定地以声速滑移。由于螺位错运动过程存在结构不稳定性,超声速螺位错还是首次被模拟发现。同时,理论和模拟表明,位错的运动还与非施密特应力(不贡献分解剪应力)有关,与传统施密特原理相悖。这些结果推翻了连续介质力学中对超声速位错的认知,确认了超声速螺位错的存在。为晶体材料的动态力学行为提供更好的理解。

最后,更一般性地考查粘接界面塑性对分层结构的影响时,采用了简单的双层梁和圆板模型,从中探究其内在的一般性规律,这些规律可以用来解释诸如晶界强化、细胞膜和复合材料中的力学性能。通过考虑双层梁/板的界面弹塑性和初始裂纹,在理论上和有限元模拟中探究了界面的弹塑性性能,比如界面的弹性刚度和塑性强度,如何影响整体结构的弯曲变形响应。总结了界面刚度对结构初始刚度的重要决定性作用,以及界面刚度与强度对塑性阶段结构弯曲刚度的退化作用。同时也考查了界面初始裂纹的位置和大小造成的弯曲刚度的衰减。该结果可用于了解界面分层结构显著的刚度差异,也可用于复合材料界面裂纹的无损检测。

总体而言,该工作从典型原子界面上的位错运动出发,研究了位错的运动性质和超声速行为,从微观上更好地理解了界面的位错塑性。进而更一般性研究了粘接界面塑性对整体分层结构力学性能的影响。为新型多界面材料的设计与力学性能测试提供了重要的指导作用。

Other Abstract

Interfaces exist widely in nature and industry, from the twin boundary in crystalline to the stratigraphic structure, from the biological bilayer membrane to industrial composite materials. As a defect, the interfacial mechanical properties plays a crucial role in the performance of the layered structure. In particular, the elastic-plastic behavior of the interface will dominate the structural deformation response. At the atomic level, dislocation motion in the gain boundary is the main source of interface plasticity, which enhances the ductility of polycrystalline materials. Focusing on the dislocations residing on the twin boundary, we provide a comprehensive research on its velocity-stress relation and supersonic behavior, which is helpful to understand the microscopic mechanism of dislocation plasticity in crystal materials.

Based on the above considerations, the main contents of this paper are as follows:

The dislocation motion dominates the strength and ductility of atomic interface and crystal, but the kinetic response to stress of dislocation still urges further exploration, especially the complete velocity-stress dependence under extreme loading status, as well as the existence and theoretical explanation of supersonic dislocation. There are two basic dislocations in crystal: edge dislocations and screw dislocations. Firstly, we simulated the twinning edge partial dislocation gliding under different strain rates in various FCC metals, and obtained the complete velocity-stress relation. It was found that the dislocation exhibits two velocity limits: one subsonic limit and one supersonic limit. A velocity jump occurs from the subsonic limit to the supersonic limit under large external stress. In theory, a phonon-based interpretation was established using a simple phonon state density and distribution model. The model well described the functional relation between dislocation velocity and applied stress, and explained the physical origin of the two velocity limits.

Next, we investigate the motion of screw dislocation. For comparison, three identical screw dislocation with different structure were simulated: pure screw type twinning partial dislocation, mixed type twinning partial dislocation and dissociated full dislocation in perfect crystal. By simulating the motion under external load, we obtained the complete velocity-stress relationship of three types of screw dislocations, and confirmed the existence of supersonic screw dislocations (exceeding three anisotropic shear wave velocities) in FCC Cu. In addition, they can all glide steadily at the shear wave speed. Because of the instability of moving screw dislocation, it is the first observation of supersonic dislocation in simulation. Furthermore, the simulation and theoretical analysis found that the stress component which does not contribute to the resolved shear stress affects the dislocation motion, in contrast to the conventional Schmid's law. The result overthrows the long-standing conventional theory that the energy dissipation for a screw dislocation moving at the shear wave speed become infinite, and is hence impossible. We confirm the existence of supersonic dislocation and pave the way of better understanding to the dynamic behavior of crystalline materials.

When we explore the influence of bonding interface on layered structure, two-layer beam and circular plate models were employed for simplicity to explore the underlying physical mechanism, which can be used to explain the dramatic mechanical properties of grain boundary, cell membranes and layered composites. By considering the elastic-plastic interface and the initial crack of layered beam/plate, we investigated how the interfacial elastic-plastic property, such as elastic stiffness and plastic strength, affect the bending response of the structure. The theoretical and numerical results showed the important role of interfacial stiffness to the initial structural rigidity, and also showed the structural rigidity degradation effect by interfacial stiffness and strength. The bending stiffness degradation due to the position and size of initial crack is also analyzed. The results can be used to understand the significant stiffness difference of the layered structures.

In general, concerning the atomic interfacial dislocation motion, we studied the kinetic property of dislocations and its supersonic behavior, and provided a better understanding to the dislocation plasticity. Moreover, the effect of interfacial plasticity on the mechanical properties of the layered structure is studied more generally. It provides an important guidance for the designing and testing of multi-interface materials.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/79117
Collection非线性力学国家重点实验室
Recommended Citation
GB/T 7714
彭神佑. 典型原子界面与粘接界面的力学行为[D]. 北京. 中国科学院大学,2019.
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