循环载荷作用下纳米晶金属晶界迁移滞回行为及其内在机制

Fatigue is an important source of degradation and failure of engineering materials. Analogous to cyclic stress-strain response of ductile materials, the grain-boundary (GB) migration hysteresis was lately observed in repeatedly loaded nanocrystalline (NC) metals. In this proposal, molecular dynamics simulations will be combined with experiments to observe initiation, evolution, saturation and retraction of GB migration, even reversed migration and hysteresis formation. The underlying mechanisms of elementary structure transformation in the observed GB as well as disconnection nucleation and development will be investigated. The variation laws of the critical conditions corresponding to initiation and saturation of GB migration, the area and mid-loop width of hysteresis loop, and GB atom number and energy corresponding to the loop vertex with cycle count, strain ratio and amplitude will be analyzed, as well as cyclic hardening or softening and their influencing factors. The effects of grain size, GB tilt angle, loading rate, loading method and material type on GB migration hysteresis will be evaluated. The generalization of GB migration will be discussed. Finally, the quantitative characterization method of GB migration will be proposed and its mathematical model also established. The research achievements are very important for understanding and improving fatigue performance of NC metals and can extend discipline connotation of strength of materials and fatigue.

疲劳是工程材料劣化甚至失效的重要根源。类似于延性材料的循环应力应变响应，最近在受循环载荷作用的纳米晶金属中观察到晶界迁移滞回现象。项目提出采用分子动力学模拟和实验相结合的方法，观察纳米晶金属在循环载荷作用下晶界迁移启动、演化、饱和与回撤，甚至反向迁移，并形成滞回的全过程，研究晶界基本结构单元转变，脱节形核、扩展等的内在机制，分析晶界迁移启动和饱和的临界条件、滞回环的面积和中间宽度，滞回环顶点对应的晶界原子数和晶界能等随循环次数、应变比和应变幅等的变化规律，以及循环硬化或软化特性及其影响因素，评价晶粒尺寸、晶界倾斜角、加载速率、加载方式和材料类型等因素对晶界迁移滞回性能的影响，讨论晶界迁移滞回行为、形成机制和规律的普遍性，进而提出晶界迁移滞回的定量表征方法，建立合理描述晶界迁移滞回的数学模型。研究成果对于认识和改善纳米晶金属疲劳性能具有重要意义，也是对材料强度学、疲劳等学科内涵的重要补充。

疲劳是工程材料劣化甚至失效的重要根源。类似于延性材料的循环应力应变响应，我们在受循环载荷作用的纳米晶金属中观察到晶界迁移滞回现象。通过对纯Ni以及含不同百分比和不同类型掺杂原子的纳米晶Ni在单调加载、循环加载等条件下的晶界迁移行为的分子动力学模拟，研究了晶界迁移的循环滞回特性和保载效应、晶内和晶间边裂纹在一个周期的脉冲循环载荷作用下的扩展行为，以及迁移中的晶界与层错四面体之间的相互作用等，揭示了纳米晶金属晶界迁移滞回、晶界构型演化的内在机制，以及晶界迁移在愈合材料辐照损伤和缺陷方面的应用前景。进一步，通过分析杂质原子浓度和温度对晶界迁移的影响，提出了针对纳米晶晶界迁移的杂质原子浓度和温度等效原理，构造了在参考温度下的晶界迁移位移主曲线。基于局部原子应力差，提出了保载期间晶界进一步迁移的判据，判据中局部原子应力差的门槛值与温度相关。引入裂纹线和裂纹角的概念，准确刻画了循环加载条件下裂纹尖端的局部钝化和锐化过程。研究成果对于认识和改善纳米晶金属疲劳性能具有重要意义，也是对材料强度学、疲劳等学科内涵的重要补充。