多相金属材料纳米尺度下断裂应变尺寸依赖效应研究

The size effect was always observed among the mechanical behaviors of materials. The utilization of the size effect, the smaller the stronger, will promote the design and manufacture of high strength materials. The size-dependent (or grid-size-dependent) effect in fracture strain, the smaller the more ductile phenomena, were observed among the studies on the ductile fracture. In the current proposal, a dual phase high strength steel, DP980, was chosen to investigate the size-dependent fracture strain of multiphase materials at the nanoscale, i.e., 10 nm-200 nm, using the post-necking model which was developed by the applicant before. The Embedded atom method potentials of individual phases and the grain boundaries will be chosen or modified by a new approach including nanoindentation and the corresponding molecular dynamic modeling. Then, the micro/nanostructures of the DP980 will be obtained through the electron microscopy to generate the actual micro/nanostructure-based post-necking models with different model sizes in the molecular dynamic simulation. Then, a comprehensive investigation will be conducted to study the effects of intrinsic features, i.e., dislocations, grain boundaries, grain orientation, and grains from different phases, on the fracture strains. Finally, the obtained size-dependent effect will be implemented and validated through the continuum scale modeling which needs the mechanical properties of materials calculated from the low scale. The proposed project will create a new method to predict the size-dependent ductile fracture strain of multiphase materials at the nanoscale, and illustrate the mechanisms of size-dependent fracture strain observed at the nanoscale.

材料力学行为中广泛观察到尺寸效应。借助于越小越强的尺寸效应，可以设计和制造新型高强度材料。在材料韧性断裂的研究中，也观察到了越小越韧的断裂应变尺寸依赖效应。本项目选择双相高强钢作为研究对象，利用申请人之前开发的后颈缩模型，探索多相金属材料在小尺寸，尤其是在纳米尺度（如10-200纳米）下断裂应变尺寸依赖效应。本项目拟借助纳米压痕实验和分子动力学压痕模拟，选择材料中单组分相和晶界的嵌入原子势；通过电子显微镜下的微纳结构，构造包含材料真实纳观结构的纳米尺寸后颈缩模型；利用分子动力学模拟，系统研究材料内部位错、晶界、取向、异相晶粒等对于断裂应变的影响；最后通过连续介质层面需要小尺寸材料力学性质的模拟应用并验证得到的尺寸依赖效应。本项目将建立一套纳米尺度下断裂应变与研究尺寸的预测方法，预测双相高强钢的断裂应变尺寸依赖性规律，并详细阐明断裂应变尺寸依赖效应在纳米尺度下存在的机理。

本课题中，通过跨尺度模拟-材料结构表征-微纳米力学实验结合，进行了多相金属材料的断裂强度与应变研究，验证了多相金属材料断裂应变的尺度规律，并对纳米尺度下影响断裂应力应变的诸多因素进行了总结和归纳。针对于界面处产生中间相的铝-铜复合体系研究，通过泛函密度计算与多尺度压痕计算，搭建了基于真实结构的分子动力学界面模型。纳米尺度的断裂模拟结果与宏观实验结果基本吻合，证明界面处铝侧的高层错密度是降低断裂强度和断裂应变的主要原因。针对多相高强钢中铁素体-铁素体晶界和渗碳体-铁素体相界的研究，发现晶界转角影响了纳米结构强度和断裂应变，并且转动角度界面处对氢吸附能力的差异，也会导致纳米结构的断裂应变出现不同的增强和减弱。在高强钢中由于存在纳米等级的沉淀颗粒，也存在铁-铜双相的共格和非共格界面，两种界面对于位错运动的阻碍作用有显著差异。同时，界面对氢的吸附能力有显著差异，界面尺寸和吸附氢原子对不同位错运动有着复杂的作用机制，影响纳米结构下的材料强度，也对材料的断裂强度与应变有显著影响。最后，利用得到的断裂应变尺度效应，得到更宏观模型中大网格的断裂应变参数，并应用于板簧断裂模型，通过连续介质有限元模型，成功验证了轻量化板簧由于表面脱碳层导致的断裂失效问题。此外，借助日式剪纸，在连续介质层面设计了一种在拉伸测试中能够实现剪切断裂和剪切拉伸混合断裂的拉伸试样，能够反应不同应力三轴态对材料的破坏