纳米多层金属薄膜变形与断裂的界面效应及其位错动力学表征

By alterlating vapor deposition or sputtering of different kinds of metals onto a substrate, a new composite named as nanoscale metallic multilayer (NMM) can be produced. Due to the strong constraint effect on gliding dislocations by phase interfaces and grain boundaries, NMM usually has ultrahigh strength but with remarkably reduced ductility. In this project, NMMs with three different kinds of layer interfaces (the coherent, semi-coherent and incoherent interfaces) are to be considered. Based on macro-meso-microscopic experiments, multiscale modeling and theoretical analysis, the length scale effect on the plastic deformation and fracture behavior of NMMs (induced by layer thickness and grain size) and the intrinsic dislocation-interface interaction mechanism will be investigated systematically. First, by micro-meso-scopic experimental observations, it can be established that the dislocation-interface interaction mechanism, the relationship between interface constraint and length-scale-dependent deformation strengthening, and the relationship between microcrack initiation/propagation and remarkable reduction in the ductility. Then, by performing molecular dynamics simulations focusing on dislocation-interface interaction and dislocation-microcrack interaction, "up-scale" constitutive rules can be obtained, which can be embedded into the multiscale modeling framework for three-dimensional discrete dislocation dynamics (MMF-3D DDD). Finally, employing the so-developed MMF-3D DDD, the effect of length scale and interface constraint on NMMs deformation strengthening and fracture can be revealed. Such research can provide strong theoretical and technological supports to the optimum design of the microstructure of NMMs with both high strength and high ductility, as well as to the strength analysis and safety assessment of NMMs.

纳尺度多层金属薄膜是将两种或多种金属交替溅射或淀积在基体上形成的具有层状结构的复合材料。界面对位错运动的强烈约束使其具有超高的强度和显著下降的塑性。针对三种具有典型界面结构（共格、半共格、非共格）的纳尺度多层金属薄膜，本项目围绕"位错-界面相互作用"这一核心科学问题，通过宏细微观实测、多尺度模拟和理论分析，研究多层薄膜变形与断裂的尺度效应及其界面约束机理。首先，通过微细观观测，获取多层膜中位错-界面作用、界面约束与强度增强、微裂纹启裂与塑性降低之间的关联信息；其次，通过对"位错-界面"、"位错-微裂纹"等相互作用过程的原子模拟，提炼出升尺度的"界面-位错相互作用"和"位错-裂纹粘聚力扩展"等模型，融入并发展三维离散位错多尺度模拟框架；最后，通过三维离散位错模拟，多尺度地揭示多层薄膜变形、断裂的尺度与界面效应。成果将为高强-高韧多层金属薄膜层状结构的优化设计、强度分析提供理论和技术支撑。

新型微纳米厚度薄膜的制备和推广应用，有力地推动着微纳米科学与技术的迅猛发展。在服役过程中，薄膜开裂是各种微纳米结构和器件失效的常见形式。高强度纳米多层薄膜的制备和推广应用，是解决各种微机电系统和柔性电子器件破坏失效的有效途径之一；但是，薄膜高强度的获得常常以牺牲其塑性性能为代价。开发和制备具有超高强度和良好韧性的新型薄膜材料是材料学家和力学家追求的目标之一。. 本项目通过多尺度计算和理论分析，从微观原子尺度和细观离散位错尺度对纳米多晶多层薄膜的塑性增强和断裂行为进行了深入、系统的研究和分析。本项目的主要内容和取得的创新性成果包括：（1）采用分子动力学模拟方法，对多晶多层薄膜的塑性增强行为及其内在物理机制进行了深入、细致的研究，探讨了晶粒尺寸、层厚两个主导尺寸对纳米多晶多层薄膜塑性增强行为的重要影响，揭示了多晶多层薄膜中存在的多种增强机制，给出了不同增强机制相互转换的条件；研究了纳米多晶多层薄膜中孪晶辅助的强化机制，探讨了孪晶片对多层薄膜的增强作用及其层厚效应，揭示了不同厚度孪晶片中塑性增强机制的差异并建立了合理的强度预测模型，同时，还研究了含非均匀厚度纳米孪晶片的多晶多层薄膜中的强化机制并对其强度进行了理论预测。（2）针对纳米多晶多层薄膜的塑性增强和断裂行为，发展了2D/3D DDD-FEM耦合算法、离散位错动力学XFEM算法并开发了相应的计算程序，为细观尺度多层薄膜塑性变形行为及其内在位错机制的研究提供了新方法和新手段。3）运用发展的系列离散位错动力学算法和程序，深入、系统地研究了多晶多层薄膜的变形、断裂的尺度及其及其内在的位错动力学机。本项目的研究成果能为多晶多层纳米薄膜的强韧化设计提供定量表征理论和技术支持。. 在项目执行期间，项目组正式发表SCI期刊论文 6 篇；参加学术会议10余次，协办了“固体的多尺度力学理论、实验与模拟”暑期高级讲习班；培养青年教师1名，博士生6名（其中2名毕业，4名在读）和硕士生3名。