高累积循环应变下微纳米尺度金属薄膜位错演化行为及机理

To understand mechanisms of the fatigue damage behaviors and dislocation evolution in the micro/nano-scale metals is crucial to the development of high-fatigue-performance metals. For this key scientific problem, ultrafine-grained and nanocrystalline Cu, Cu-5at%Al and Cu-16at%Al films are selected as the model materials. High elastic substrates are chosen to confine the metal films, so as to enhance the ability of cyclic plastic accumulation. Cyclic deformation behaviors of Cu and Cu-5at%Al characterized by wavy slip, and Cu-16at%Al characterized by planar slip will be investigated systematically by the characterization techniques (transmission electron microscope (TEM) and the atomic scale observation) and theoretical analysis methods. Then we can clarify mechanisms of dislocation evolution and interaction between dislocation and grain boundaries in different FCC metal films under the micro/nano-scale, and explore the competition and cooperation among full dislocations, partial dislocations, grain boundaries and the surface in the nanoscale materials. It is expected that the basic ideas and the research results from the present project will not only get insight into basic mechanisms on dislocation evolution and fatigue damage formation in the micro/nano-scale materials, but also expand our knowledge on the fatigue deformation in the micro/nano-scale. It is hoped that it would finally provide experimental and theoretical foundations for the design of the high-fatigue-performance materials.

深入理解微纳尺度金属材料的疲劳损伤行为及微观缺陷演化机制不仅是高疲劳性能材料发展，且是微纳米尺度下疲劳理论发展的迫切需求。项目拟选取900 nm厚的超细晶和纳米晶Cu、Cu-5at%Al和Cu-16at%Al三种薄膜作为模型材料，通过弹性基体的约束对三种金属薄膜施加高累积循环塑性应变，结合透射电镜和原子尺度的微观结构表征与理论计算，系统研究在具有波状滑移特性的超细晶/纳米晶Cu和Cu-5at%Al薄膜以及具有平面滑移特性的超细晶和纳米晶Cu-16at%Al薄膜的疲劳损伤行为；通过对比研究，澄清面心立方金属薄膜中疲劳位错演化行为及位错与晶界交互作用机理；探究超细晶/纳米晶金属薄膜位错（不全位错）、晶界及表面在循环变形行为中的竞争与协调关系。项目学术思想的实现和研究结果对于深入理解微纳尺度疲劳位错组态形成与演化机理、疲劳损伤早期形成机制及进一步发展微纳米尺度金属的疲劳新理论具有重要意义。

本项目设计制备了一系列微纳尺度金属薄膜和层状薄膜材料，探索了薄膜材料力学性能测试方法，开展了不同尺度与组元种类组合的金属薄膜和层状薄膜材料的拉伸、机械疲劳、热疲劳变形机制和损伤行为等方面的实验研究工作；结合理论计算，探究了尺度控制的金属薄膜和层状薄膜材料的机械疲劳和热疲劳性能、疲劳变形和损伤微观机制。项目获得如下主要创新结果:. 提出了一种测量受柔性基底约束薄膜断裂应变的方法；设计并搭建了受基底约束薄膜动态弯曲疲劳测试平台和受柔性基底约束金属线的热疲劳测试平台。. 发现了通过适当的退火处理工艺和添加超薄钛结合层，可以有效地抑制应变局部化，提高微米厚度薄膜的断裂应变和疲劳性能。. 930nm到20nm厚纳米晶Au薄膜拉伸和疲劳行为研究结果表明：纳米晶Au薄膜的拉伸断裂应变随薄膜厚度的减小而逐渐降低，但是其疲劳性能随薄膜厚度的减小而逐渐升高。当薄膜厚度大于90nm时，薄膜疲劳损伤行为表现出与外加载荷相关的竞争损伤机制-沿局部滑移带的穿晶断裂和沿晶断裂。当薄膜厚度减小到几十纳米时，纳米孪生不仅辅助了纳米晶有限的晶粒长大，且晶粒长大消耗了部分循环塑性应变，一定程度上提高了疲劳性能。. 发现了930nm厚Au薄膜在拉-拉疲劳载荷作用下异常长大晶粒主要由孪晶界和小角晶界组成。新形成的孪晶是由裂纹尖端高应力驱动的晶界发射不全位错形成的变形孪晶，而且孪晶界主要是非共格孪晶界而非低能共格孪晶界。提出了点阵旋转模型来澄清动态弯曲疲劳载荷作用下异常长大晶粒的择优取向微观机制。. 不同线宽和厚度的Au互连线热疲劳行为研究结果表明：薄膜的热疲劳寿命表现出明显的尺寸效应，建立了与厚度和外加载荷/温度相关的Au互连线的热疲劳变形机制图。. 不同尺度和组元组合的层状薄膜材料的研究结果表明：多层膜的疲劳寿命随着薄膜单层厚度的减小而升高，异质界面和柱状晶晶界是影响其疲劳行为的重要因素。