复杂应力条件下金属玻璃剪切变形与断裂的临界条件

The plastic deformation of metallic glass at room temperature usually highly localizes into shear bands. Researches on shear banding deformation and fracture behaviors are of significance for designing new metallic glassy materials or improving the plastic deformation ability of existing metallic glasses. In this project, compositionally different metallic glasses such as Zr- and Ti-based metallic glasses will be chosen for experiments. In situ mechanical experiments (including notch tension, tension of plate with a central circular hole, three-point bending) under a scanning electron microscopy (SEM), as well as mechanical analysis and finite element modeling will be used as the main methods. The critical stress conditions for shear band initiation, propagation and failure in different metallic glasses will be systematically and quantitatively investigated. The physical natures for shear banding deformation and fracture as well as their differences among different metallic glasses will be explored. Based on the experimental results, the formation mechanism of multiple shear bands will be clarified and the validity of the yield or fracture criteria of metallic glass will be examined. By conducting this project, we hope to achieve a better understanding on the shear banding behaviors of metallic glasses, and to provide theoretical instructions for developing new metallic glassy materials or components with good plastic deformation ability.

金属玻璃的室温塑性变形往往高度局域于剪切带中，深入研究剪切带变形与断裂行为对于设计新型金属玻璃材料或改善现有金属玻璃的塑性变形能力有着重要意义。本项目以Zr基与Ti基两种体系不同成分金属玻璃为研究对象，主要通过开展扫描电镜下原位力学实验，并结合弹塑性力学分析与有限元模拟，系统定量地研究复杂应力条件（包括缺口拉伸、含中心圆孔平板拉伸、三点弯曲）下剪切带萌生、扩展与失效断裂的临界力学条件，探索不同金属玻璃剪切带变形与断裂的内在物理规律及其差别，澄清多重剪切带的形成机制，并实验验证金属玻璃屈服或断裂准则的有效性。通过本项目的研究，将对金属玻璃中剪切带行为有更清晰的认识，将为发展新型具有良好塑性变形能力的金属玻璃材料或构件提供理论参考。

剪切带是金属玻璃塑性变形的载体、也是其断裂的主要源头。深入认识剪切带萌生、扩展和开裂的微观机制并确定其发生条件，对于金属玻璃材料的力学性能优化以及构件的安全可靠性设计有着重要意义。本项目以典型金属玻璃材料为研究对象，开展原位/准原位扫描力学实验，研究了多种加载条件下的剪切带萌生、扩展和开裂机制和失效断裂准则。主要取得了如下研究进展：.1、获得了单向加载下剪切带渐进扩展的直接实验证据，发现了金属玻璃表观硬化行为与剪切带渐进扩展的关联，并阐明了表观硬化的机制。直接证实了剪切带对金属玻璃疲劳裂纹萌生的主导作用及循环软化效应，给出了在低于屈服强度的应力下疲劳损伤的原因。.2、实验证实了缺口拉伸时剪切带的萌生受到局部切应力主导，发现了应力-应变曲线的锯齿与多条剪切带协同渐进扩展的对应关系。建立了缺口拉伸塑性与强度、断裂韧性之间的关系。基于椭圆准则，建立了缺口强度比与材料参数、缺口几何之间的关系。.3、首次获得金属玻璃压缩下剪切带内部及其损伤区中裂纹的三维形貌及其演化规律，提出无负压力的应力状态下的三种剪切带开裂机制，并基于断口定量分析提出了金属玻璃压缩下的多孔纯剪断裂机制。.4、提出了材料失效的一般性能量准则，并发现经典的屈特加准则和最大正应力准则均为该准则的特殊形式，且其与椭圆准则是同一物理规律的两种不同表达。也实验验证了能量准则对于预测金属玻璃断裂强度的有效性。