热力载荷下小尺度多晶材料时间相关塑性行为及其多维缺陷耦合动力学机理

The interior grain size and extrinsic sample size of small-sized polycrystalline materials are on the micron scale or below, so the mechanical behaviors of small-sized polycrystalline materials usually display interiorly and extrinsically coupled size effect. Under thermal-mechanically coupled load, the diffusion of the vacancies (point defect), the climb and glide of dislocations (line defect) and the diffusion and slip of grain boundaries (face defect) and their interactions become dominant mechanisms accommodating the plastic deformation, which also are the main causes leading to the interiorly and extrinsically coupled size effect. With the aid of macro-meso-microscopic experiments, multiscale modeling and theoretical analysis, the time-dependent mechanical behaviors of small-sized polycrystalline materials and the underlying micro-/meso-scopic mechanisms leading to the interiorly and extrinsically coupled size effect will be studied in detail and in depth, with a special emphasis on the movement of the vacancies、dislocations and grain boundaries and their interactions. First, the rate-dependent tensile or compressive behavior of small sized polycrystalline materials (such as Al or Ag micro-pillar or thin film) which are applied to temperature in the range of 20oC to 200oC will be experimentally tested, and then the micro-/meso-scopic deformation mechanisms in the small-sized materials will be revealed by high resolution TEM/SEM measurements. Then, MD simulation will be performed to capture the atom-scaled details of vacancy-dislocation interaction, vacancy-grain boundary interaction, dislocation-grain boundary interaction，grain boundary diffusion and slip and etc., and some up-scaled meso-scopic models which can depict these atom-scaled mechanisms will be obtained and incorporated into the simulation framework which couples the 3D vacancy diffusion dynamics and 3D discrete dislocation dynamics (3D-VDD/DDD). On these bases, a 3D-VDD/DDD coupled simulation framework will be developed, which can capture a series of meso-scopic mechanisms which dominate the plastic deformation in the small-sized materials, including the vacancy diffusion, dislocation climb and glide, GB diffusion and slip, vacancy-dislocation interaction, vacancy-GB interaction, dislocation-GB interaction, dislocation nucleation and emission from GB or GBs triple junction and etc. Finally, with the aid of this 3D-VDD/DDD coupled simulation framework, the rate-dependent tensile or compressive behaviors of other small sized polycrystalline materials also will be modeled and compared with the experiment results, and the underlying micro-/meso-scopic mechanisms which lead to the interiorly and extrinsically coupled size effect will be studied carefully and deeply. These researches can provide strong theoretical and technological supports to the increasing application of various small-sized polycrystalline materials in MEMS or flexible electronics.

由于小尺度多晶材料的内部和外部特征尺寸降到微米及以下，其力学行为呈现出内外耦合尺寸效应。在热力耦合载荷下，它们中空位（点缺陷）、位错（线缺陷）和晶界（面缺陷）三种不同维度缺陷的运动及其相互作用是容纳塑性变形的主要机制，也是导致其内外耦合尺度效应的内在原因。本项目围绕"空位-位错-晶界三种缺陷的运动及其相互作用"这一关键科学问题，首先，通过实验研究小尺度多晶材料在不同载荷（温度、应变率或应力）下的时间相关力学行为，揭示不同条件下主导小尺度材料塑性变形的微细观机制；其次，通过MD模拟，揭示不同条件下空位与位错、空位与晶界、位错与晶界间的相互作用及晶界扩散、滑动的物理机制，发展"升尺度"的细观模型；在此基础上，发展三维空位扩散和离散位错动力学耦合算法和程序；最后，利用该程序并结合实验，深入研究小尺度多晶材料在热力载荷下的力学行为及其内外耦合尺度效应，为它们在微型系统中的应用提供理论和技术支持。

随着微型系统制造技术的快速进步，各种小尺度材料和构件在它们中被大量、广泛地应用。在热、力载荷的耦合作用下，小尺度关键构件常常发生蠕变等时间相关塑性变形，引发结构失效和破坏。由于小尺度多晶材料的内、外部特征尺寸降到微米以下，其力学行为呈现出尺寸效应。在热力耦合载荷下，它们中空位（点缺陷） 、位错（线缺陷）和晶界（面缺陷）三种不同维度缺陷的运动及其相互作用是导致其尺度效应的内在原因。认识小尺度材料在热-力耦合环境下的时间相关塑性行为及其内在物理机理，发展有效的定量表征方法和技术手段，能为各种微型系统和柔性电子器件的服役性能、可靠性和耐久性的提高提供理论和技术支撑。 . 本项目通过多尺度计算和理论分析，对热力载荷下小尺度多晶材料时间相关塑性行为及其多维缺陷耦合动力学机理进行了深入、系统的研究和分析，取得的创新性成果包括：（1）采用分子动力学模拟方法，研究了位错与多维缺陷之间的相互作用，揭示了位错与不同维度缺陷相互作用的物理机制，提炼出了位错与不同维度缺陷相互作用的本构法则，为介观尺度下模拟算法的建立提供了基础。（2）基于MD模拟和理论分析，发展了空位动力学和离散位错动力学耦合算法，开发了相应的计算软件，为介观尺度下材料高温塑性行为的模拟提供了新方法和新手段；针对纳米多晶等先进材料在高温下的力学行为进行了深入、系统的计算模拟和分析，揭示了位错攀移/滑移耦合运动对先进高温材料微尺度效应的重要影响。（3）以位错密度为内变量，建立了位错攀移/滑移耦合的高温晶体塑性本构理论，并在ABAQUS的框架下完成了有限元实现；对镍基单晶高温合金及微纳单晶、多晶材料在高温下的力学行为及其尺度效应进行了有限元模拟，得到了与实验一致的结果。. 在项目执行期间，项目组发表期刊论文 14 篇，其中SCI论文13篇；参加学术会议10余次，协办了“固体力学发展战略暨AMSS创刊30周年研讨会”等系列会议；培养青年教师 1名，博士后1名，博士生10名（毕业3人），硕士生5人（3人毕业）。