【作者】 黄丹；

【导师】 郭乙木；

【作者基本信息】 浙江大学 ， 工程力学， 2005， 博士

【摘要】 本文建立了适合于模拟纳米金属材料力学行为和性能的分子动力学计算模型,研究了面心立方晶格与体心立方晶格纳米金属丝、纳米薄膜和纳米固体的力学性能、变形机理以及破坏过程:提出了纳米金属力学性能随其尺寸、表面以及加载速率变化的预测公式,并得到验证;采用二维模型和宏-微观耦合的断裂力学理论分析了纳观裂纹的扩展。研究表明本文提出的分子动力学模型和分析方法适合于研究纳米金属的力学行为和力学性能。主要结论有: 自由表面影响纳米材料的力学行为和力学性能。金属纳米丝和纳米薄膜在无外载荷作用时都存在本征应力和高于理想晶体的初始能量,变形都从表面晶格开始向内部晶格深入。纳米丝的变形过程中出现位错和原子台阶;纳米固体的变形表现为内部晶格的原子滑移。 单晶纳米金属材料的断裂强度接近晶格理论强度,符合Griffith理想晶体断裂理论。 体心立方金属的初始弹性模量“软化”严重,体心立方金属纳米丝和薄膜的拉伸模量在屈服阶段后增大,出现“应变刚化”现象;面心立方金属的弹性模量略小于宏观粗晶。 相同条件下体心立方金属的塑性高于面心立方金属,其屈服强度和初始弹性模量均低于面心立方晶格金属,但屈服阶段早于面心立方晶格金属出现,持续时间更长。 面心立方晶格纳米金属丝的拉伸破坏过程中存在短暂的屈服时间,屈服应变只与金属本身有关,与模型尺寸及加载速率等无关。 面心立方金属纳米丝的屈服强度和初始弹性模量与尺寸对数及应变率对数呈现双线性关系,断裂强度与截面尺寸的负指数以及应变率对数呈双线性关系。同一纳米丝的屈服强度、断裂强度和初始弹性模量与加载速率的对数呈线性上升关系;加载速率相同时,屈服强度和弹性模量与尺寸的对数呈线性上升,断裂强度随截面尺寸增加呈一阶指数衰减;屈服强度和弹性模量可以用内部晶格、表面晶格和角落晶格三部分的相应值通过加权线性叠加组成。 相同模型的纳米丝原子能量和应力水平低于纳米薄膜,延性高于纳米薄膜;纳米丝弹性模量仅约为纳米薄膜的一半,拉伸强度为纳米薄膜的三分之二。 结合原子镶嵌势的二维模型能准确地模拟平面纳观裂纹的扩展,揭示在远场应力作用下裂纹区原子的运动规律和应力、能量变化过程;同一问题也可以通过原子镶嵌势函数和二维正三角形晶格常数计算材料的弹性参数并结合宏观断裂理论求解;二者分别得到裂纹的起始扩展时间、裂纹尖端应力场和原子平均能量值,结果吻合。更多还原

【Abstract】 A Molecular dynamics simulation model suitable for mechanical behavior and properties of nanoscale metals was developed, and the mechanical properties, deformation mechanism and failure process of nanowire, nanofilm and nanobulk with face centered or body centered crystals were studied by this model. The relationship between mechanical properties of nano metals and the length size, surface ratio and loading rate was analyzed, and some prediction formula verified by numerical results were proposed to describe the relationship. Moreover, crack propagation in nano plane was studied by both two dimensional MD simulation and continuum-atomistic coupled theoretical method. Study shows that the method and MD model proposed here was proved to be suitable for simulation of metals.Simulation results show that free surfaces take large effect on the mechanical behavior and properties of nano-materials. Initial stress and high energy are found in nanowire and nanofilm without external load, and deformation begins from free surfaces. There are dislocation and lattice steps in nanowire and only lattice slide in nanobulk during their failure process.The fracture strength of monocrystalline metal is close to the theoretical strength of the lattice and agrees well with Griffith’s theory about the fracture of ideal lattices.The modulus of elasticity of nano BCC metal is sharply lower than that of the identical macro metal, and it will arise after the yield period in BCC nanowire and nanofilm. In comparison, the modulus of elasticity of nano FCC metal is a little lower than that of corresponding macro metal.The plasticity of nano BCC metals is stronger than that of FCC metals, but yielding strength and modulus of elasticity are lower than that of FCC metals. Under the same loading condition, BCC metals will enter the yield period earlier and last longer than FCC Metals.There is a short yield period during the tensile failure process of FCC nanowires, which is decided by the metal and independent on external conditions such as loading rate and model size.The yielding strength and modulus of elasticity of an FCC nanowire are linear to the logarithm of both its cross-section size and external strain rate, and the fracture strength is linear to the logarithm of both external strain rate and the inverse exponential of its cross-section size. What’s more, the yielding strength and modulus of elasticity of an FCC nanowire can be divided into three parts: the corresponding property of internal lattices, surface lattices and corner ones, and the weighting linear plus of the three parts may obtain that of the nanowire.The atomic energy and stress in nanowire are lower than in nanofilm, and the ductility higher. The modulus of elasticity of nanowire is about a half of that of nanofilm with the same size, and the tensile strength is about two thirds of that of nanofilm.With Embedded Atom Method, the plane MD model can study the crack propagation in nano metal slab, and the simulation results agree remarkably well with macro fracture mechanics method in which the elastic constants were obtained from the interatomic potential functions and the two-dimensional triangular lattices.更多还原