新型高强高韧微/纳米叠层金属材料的界面力学行为与强韧化机理

Laminate materials have superior mechanical properties. To date, the toughening and fracture mechanisms of laminated metallic materials have received extensive attention. Due to the lack of good in-situ characterization methods, the compliant effect of interfaces during plastic deformation is not well understood, and the strengthening and toughening mechanism of laminated metallic materials need investigation furthermore. Based on previous works, this project fabricates micrometer/nanometer laminated metallic materials with sharp and good bonding interfaces. Digital image correlation (DIC) method under SEM and micro/nano indentation techniques are used to characterize the strain distribution and its revolution of interfaces during loading process. Uniaxial tensile load-reload tests combined with DIC technique is used to measure the back stress and its revolution with strain. In-situ synchrotron x-ray diffraction is used to investigate the stress-strain behavior of individual layers. SEM and TEM are used to study the interfacial dislocations and their interaction. The experimental results will real the distribution and interaction of stress and strain in individual layer during compliant plastic deformation, the role and law of interfaces on coordinating the plastic deformation between different layers, the strengthening and toughening mechanisms of laminated metallic materials. The study results will provide reliable theoretical and experimental basis for optimal design and fabrication for high performance laminated metallic materials, strengthening and toughening mechanisms for new materials, which contribute greatly to the development of high strength and high toughness metallic materials.

叠层/层状材料具有优异的力学性能，目前对叠层金属及复合材料的增韧和断裂机理研究较多，但因缺乏适当的在线表征手段而对界面的作用认识不足，叠层金属材料的强韧化机理有待进一步挖掘。基于前期工作，本项目制备具有清晰晶粒尺寸过渡界面的微/纳米叠层金属材料，通过原位SEM-DIC和SEM-EBSD、微/纳米压痕等方法表征加载过程中界面的力学性质变化，通过拉压/加卸载结合DIC研究叠层的包辛格效应和背应力，通过原位同步辐射技术研究加载时的应力应变传递和配分，采用扫描、透射等微观表征方法研究界面位错及其交互作用，揭示耦合变形过程中的包辛格效应和背应力、应力/应变配分、界面在协调微/纳米层变形时的作用与规律，探索叠层金属材料的强韧化机理。研究结果将为发展界面力学理论和探索新型材料的强韧化机理提供可靠的实验和理论依据，对优化高性能叠层金属材料的设计与制备和发展工程化的高强高韧金属材料具有一定的参考价值。

非均匀叠层设计是改善工程结构材料强韧性能的有效途径。在高性能叠层结构设计和制备的基础上，本研究以非均匀界面的力学效应为核心，重点研究叠层结构的变形机理和强韧化机制。通过原位SEM-DIC应变观测，发现跨纳米晶/粗晶界面呈对称分布的塑性应变梯度区。该应变梯度区被定义为界面影响区（IAZ）。IAZ的特征为：约8-10 um的恒定宽度l；内部呈负应变梯度，且应变梯度强度随施加应变增大。考虑IAZ内几何必需位错（GNDs）塞积，建立了基于（近）界面位错源模型的IAZ形成机制：单元层因应变不协调性而交互约束，在近界面区域内激活高密度位错源；位错源发射位错并形成沿界面GNDs塞积，从而形成应变梯度主导的IAZ。基于经典的位错塞积理论，理论推导出IAZ的临界宽度 l≈[(μ/σ_y)]^2 b。该结果为“界面影响区的存在性”的经典问题首次给出了肯定的答案。.力学测试发现IAZ可贡献可观的额外强韧化效应。具体地，IAZ内GNDs塞积诱导产生额外的位错和长程内应力强化及加工硬化。在低应变阶段，非零和的长程内应力（HDI stress）随界面GNDs塞积而快速增长；而后在高应变阶段，界面GNDs塞积保持动态稳定，长程内应力亦趋于稳定。叠层结构的HDI应力、流应力与IAZ体积分数呈线性关系。基于此，量化推导出单位体积IAZ贡献的强韧化效应，并发现该强韧化效应主要源于长程内应力的发展，而不是位错强化。提出了基于界面力学效应的叠层异构优化设计准则：单元层厚度应当接近界面影响区宽度l的2倍。.在梯度叠层结构中，发现其高强度亦源于单元层交互诱导的协同强化效应。经原位应变观测，揭示了梯度结构的独特变形机理：纳米晶表层通过激活高密度、弥散分布、塑性稳定的应变带克服快速应变局部化失稳，获得高均匀延伸率；纳米晶Ni表层的的晶体学变形机制是形变驱动晶界迁移和晶粒粗化。.这些研究结果为以界面为核心的高性能先进叠层结构材料的微结构调控、制备提供理论依据和指导。