块体择优取向纳米孪晶铜的断裂韧性和断裂机理研究

Strength and fracture toughness are two critical mechanical properties required by most engineering structures. Conventional strengthening mechanisms always scarifice the benifical toughness for strength. It is recently established that nanoscale twinned structures produce high strength while keeping strain hardening capability and ductility, which may also imply enhanced fracture resistance; however, there is still lack of quantitative measurement of fracture toughness and in-depth understanding of fracture mechanism of nanotwinned structures. In this project, direct current electrodeposition technique is developed to prepare bulk pure Cu with high-density preferentially oriented nanoscale twins. The specimen size effects and the influences of grain size and twin thickness on the fracture behavior will be investigated, with a special focus on deformation mechanism, crack nucleation and damage evolution in the nanotwinned structures deformed under complex stresses in the crack tip. Through this project, the feasibilites of obtaining optimized strength – fracture toughness combination by designing unique nanotwinned structures will be proved, which would promise to extend the engineering applications of nanostructured metals.

强度和断裂韧性是工程结构材料的两个重要力学属性。传统强化机制往往以牺牲断裂韧性为代价。近期研究发现，纳米孪晶结构可赋予材料高强度、优异加工硬化能力和良好拉伸塑性，这可能也预示其优越的断裂韧性。但是，目前尚缺少纳米孪晶结构断裂韧性的定量分析及其断裂行为的系统认识。本项目拟利用直流电解沉积技术制备具有高密度择优取向纳米孪晶的块体纯Cu样品，重点研究样品几何尺寸及晶粒尺寸/孪晶片层厚度等微观结构参量对纳米孪晶Cu断裂韧性和微观机理的影响，同时揭示纳米孪晶结构在复杂应力条件下的塑性变形机制、裂纹萌生及损伤演化过程，探索利用纳米孪晶结构设计优化材料强度–断裂韧性匹配的可行性。研究成果有望拓宽对纳米孪晶结构变形和损伤机理的理论认识，为发展高强高韧金属材料奠定实验基础。

本项目主要利用直流电解沉积技术制备具有高密度择优取向纳米孪晶块体纯铜样品，并研究试样尺寸以及晶粒尺寸、孪晶片层厚度等微观结构参数对纳米孪晶铜的断裂韧性和断裂机理的影响。主要研究成果包括：(1)研制了非接触式裂纹张开位移规，使得利用微型紧凑拉伸试样测量纳米结构材料的本征断裂韧性成为可能，并证明了形变纳米孪晶束的存在有利于提高高强纳米结构材料的断裂阻力；(2)揭示了样品厚度对择优取向纳米孪晶铜的临界断裂韧性的影响。当样品足够厚时，裂纹尖端处于平面应变状态，微裂纹容易沿晶界萌生和扩展，导致较低的断裂阻力；而当样品较薄时，裂纹尖端处于平面应力状态，晶界开裂被抑制，样品发生穿晶倾斜断裂；(3)揭示了晶粒尺寸等微观结构参数对择优取向纳米孪晶铜断裂韧性的影响。增大晶粒尺寸有利于减少裂纹形核点密度，增强裂纹扩展路径的曲折程度，两者有利于提高材料的断裂韧性。(4)揭示了拉伸应变速率对纳米孪晶Cu拉伸塑性的影响，提高应变速率有利于增强晶粒间应变协调，抑制沿晶断裂趋势，从而显著提高塑性变形能力。本项目研究成果为进一步设计纳米孪晶结构增强材料的断裂韧性提供理论依据。项目严格按照原计划执行，圆满完成任务。