【作者】 郑勇刚；

【导师】 张洪武； 陈震； 顾元宪；

【作者基本信息】 大连理工大学 ， 工程力学， 2008， 博士

【摘要】 近年来,纳米材料在越来越多的领域得到应用。因此对纳米材料的制备工艺、变形及破坏机理的研究受到了广泛的关注。大多固体材料都是由晶粒构成的,其或为单晶体或为多晶体。晶粒边界上原子的不规则排列使得材料内部储存的能量很高,因此晶界易成为缺陷的源头。一方面在能量驱动下,晶粒会逐渐长大以减小晶界的面积;另一方面,晶粒的尺寸、分布情况和变形方式与材料性能及其使用寿命密切相关,不同尺寸的晶粒在外载作用下其变形机理不尽相同。因此,深刻地了解和剖析晶粒生长的基本特征和变形机理对于材料设计具有重要的指导意义。针对单相多晶材料中的晶粒生长情况,本文提出了一个理论的生长模型,其中考虑晶粒的生长来自于两方面的作用:系统的平均场效应和邻域的局部效应,即将传统的扩散控制机制和曲率引导机制相结合,并对此模型的求解给出了相应的有限差分格式。计算结果表明,晶粒在生长过程中具有一个尺寸效应,当晶粒尺寸很小时,如纳米级别,生长主要是由扩散作用引起的。当尺寸增大,晶粒生长逐渐变为一个由曲率控制的过程,这与实验观测一致。这两个阶段的生长分别具有自相似的特征。与其它模型的结果相比,基于此模型所预测的晶粒尺寸分布函数与实验结果吻合的更好。在多相材料中(通常以两相为原型),针对其晶粒生长机理和特征的认识尚不完整,且很难导出相应的理论模型,因此晶粒生长过程主要采用模特卡罗方法进行模拟研究。本文通过扩展传统的Q态Potts模型,将其运用到两相不相溶固体材料晶粒生长的研究中。主要考虑了具有不同晶格方向的相同组分的晶格重定向和不同组分之间的扩散作用。蒙特卡罗模拟结果表明两相材料中晶粒尺寸与生长时间满足一个幂指数关系。由于附加相的存在,晶粒生长受到了很大程度的抑制。这时候生长指数为0.16,即为单相生长情况的1/3,这与实验结果相一致。同时研究发现为了控制单相材料生长而添加另一相物质时,附加相的最佳体积百分比约为7%～11%,实验中常用的比例为5%～10%,两者吻合很好。两相材料中晶粒尺寸基本上满足对数正态分布,表明所研究系统中的晶粒属于正常形态的生长模式,且此分布与时间无关。分析结果还显示了晶界能与表面能之比存在一个临界值(～2.6),当实际比值大于临界值时,其中一相材料更趋于形成网状结构附着在另一相的边界上,否则其将更容易以颗粒形式散布在系统中。在对材料微结构的变化,即晶粒生长,有了一定的认识的基础上,本文展开了对具有特定微结构的纳米晶体材料的变形机理研究。基于分子动力学方法,研究了单相单晶铜纳米线的变形机理。对纳米线的弯曲加载模拟结果表明,纳米单晶铜材料的塑性变形主要是由原子在密排面内的滑移所引导。塑性变形过程中所形成的形变孪晶将相互作用和影响,导致原子局部微结构的转化,从而形成两个相连的五折孪晶,这一过程是第一次从计算机模拟的角度观测到。而相连五折孪晶的出现对铜纳米线有很好的增强作用,从而使得供给纳米线塑性变形的位错运动受到抑制。对纳米单相多晶铜材料,同样采用分子动力学方法模拟了其在拉伸载荷作用下的变形机理。数值实验的试件首先用前面所建立的扩展Potts模型结合蒙特卡罗方法模拟获得,这样的系统满足尺寸的对数正态分布,与实验更相符。然后采用单轴拉伸加载,同时控制其它两个垂直方向的应力。模拟结果表明,纳米多晶铜的变形主要存在两种方式,即小晶粒的变形主要是通过晶界滑移和晶粒旋转引导的,而大晶粒的变形则主要是由位错运动控制。此外,研究还发现这两种机理控制着不同的变形阶段,如晶粒旋转主要发生在塑性变形的初期,而塑性变形中后期则主要由位错运动引导。随着系统晶粒平均尺寸的改变,这两种机理所占的比重不尽相同,从而导致了纳米材料与传统粗晶材料的物理力学性能具有较大的差别。此外,研究结果还表明由于外载的作用,系统内部所出现的高应力将导致晶粒的快速生长。本文的研究工作是国家自然科学基金项目(10721062,10640420176,50679013)、长江学者和创新团队发展计划以及国家基础发展规划项目(2005CB321704)资助的一部分。更多还原

【Abstract】 In the past few decades,nanocrystalline materials have attracted much attention because of their superior physico-mechanical properties.It is well known that the properties of a material are closely tied to its microstructure.Until now,a lot of research work has been conducted to investigate the microstructure evolution and the related deformation mechanisms, and significant successes have been achieved.Our knowledge on grain growth and deformation mechanisms of nanostructure,however,is still far from complete.Thus,a better understanding toward grain growth processes and deformation details is needed,which is the main goal of this work and will provide an effective way to tailor and optimize the fabrication of nanostructured materials.The research work is arranged as follows:In chapter 1,some backgrounds of nanocrystalline materials are introduced and the related research work that has been done before is surveyed,which include the experimental observations,theoretical studies and numerical simulations of grain growth and deformation in nanocrytalline materials.In the last section,a brief description of this work is presented.In chapter 2,a combined stochastic diffusion and mean-field model is developed for a systematic study of the grain growth in a pure single-phase polycrystalline material.A corresponding Fokker-Planck continuity equation is formulated,and the interplay/competition of stochastic and curvature-driven mechanisms is investigated.Numerical results show that when the grains are smaller than several tens of nanometres the dominating mechanism is stochastic diffusion-controlled of boundaries.As the grains grow the influence of the deterministic curvature-driven mechanism increases and finally controls the process.The transition ranges between these two mechanisms are about 2-26 and 2-15 nm with boundary energy of 0.01-1 J m^-2 in two- and three-dimensional systems,respectively.The grain size distribution of a three-dimensional system changes dramatically with time,while it changes a little in a two-dimensional system.The grain size distribution from the combined model is in good agreement with experimental observations.In chapter 3,a modified Potts model is proposed to systematically study normal grain growth in a single- or two-phase volume-conserved system.In this model,the driving forces for grain boundary migration are the interfacial energy between two phases and the boundary energy inside each phase.Simulation results show that the grain growth kinetics follows a power law with a temperature-independent exponent and the normalized grain size distribution is lognormal and time-invariant.The optimal volume fraction for the second-phase is found to be 7-11%for a well control of the primary grain growth,which is consistent with experimental data.Also a simple theoretical model is used to predict the potential microstructure in a two-phase system due to competition between interfacial and grain boundary energies.A critical energy ratio（～2.6） of grain boundary and interfacial energies is found for a common two-phase system and is supported by Monte Carlo simulations.In chapter 4,the basic theories of the molecular dynamics method are introduced, including the motion equations,integration scheme,temperature/pressure control algorithm, empirical potential functions.In addition,some practical methods,such as periodic boundary condition,time step selection and neighboring list method,are also addressed.In chapter 5,the deformation of single-crystalline copper nanowires under bending is studied using molecular dynamics simulations.The length and thickness effects on the stability and deformation of wires are also discussed.The results suggest that the plastic deformation is dominated by atomic slip on close-packed planes,and the twinning deformation is a primary mode in copper nanowires with sizes of～10 nm.It is found that an intermediate icosahedral phase is formed to facilitate the transformation from a low dense （010） plane in a face-centered-cubic lattice to a {111} close-packed fashion,forming tri-junctions composed of three deformation twins.These tri-junctions can easily interact with other deformation twins,forming two conjoint fivefold deformation twins.In chapter 6,a combined Monte Carlo and molecular dynamics scheme is established to investigate the deformation mechanisms of polycrystalline nanostructured copper with grain size in the range of 6-18 nm.The results show that grain boundary mediated plastic deformation,such as grain boundary sliding and grain rotation,mainly occurs in small grains; while large grain deformation is dislocation-accommodated through nucleation,propagation and absorption of partial/extended dislocations.It is also found that these two mechanisms operate in the different stages.With the evolution of deformation,stress-assisted grain growth could also occur due to atomic diffusion and grain boundary migration.Finally,the main contributions of this work are summarized and the further work is suggested.This research work is supported by the National Natural Science Foundation of China under Project Nos.10721062,10640420176 and 50679013,the Program for Changjiang Scholars and Innovative Research Teams in Universities of China,and the National Key Basic Research Special Foundation of China（2005CB321704）.更多还原