高时空分辨率下大块非晶剪切带形成机理的实验研究

Bulk metallic glass has attracted much interest because of its excellent mechanical properties. However, it undergoes inhomogenous flow at room temperature and fracture with shear instability at very small plastic strain,which restricts its otherwise desireable mechanical properties as well as its potential applications. .In this study, we are going to investigate the shear behavior of metallic glass by performing dynamic compression and quasi-static in-situ SEM tension tests on some notched rectangular specimens. High-speed imaging technique combined with Digital Image Correlation(DIC) and fusible coating delomorphic method are employed to obtain the detailed description on the initiation and evolution of metallic glass shear bands with highly temporal resolution. This experiment focuses on verifing the two possible propagating manners of shear bands, i.e. simultaneous manner and progressive manner, and whereafter estimating the propagation speed of shear band. .Besides the dynamic test with high-speed camera, DIC is also combined with highly resolved SEM images obtained at different loading period of in-situ SEM tension test. The purpose of this study is to obtain quantitatively the displacement field of shear banding zone and to investigate the characteristic dimensions of shear band with sufficient spatial resolution..This study aims at developing new experimental methods with highly temporal and spatial resolutions, which can be used to measure and analyse directly the characteristic parameters of shear banding process, such as shear band dimensions and shear band propagting speed. This project has great significance in understanding the initiation and evolution process as well as the intrinsic mechanism of metallic glass shear band.

大块非晶具有优异的力学性能，然而其在室温条件下表现出的非均匀流动和剪切失稳破坏成为制约其应用的一个严重障碍，因此对大块非晶剪切变形机理的研究成为目前国际上的一个研究热点。本项目从实验角度出发，对带初始缺陷的大块非晶试样分别进行动态压缩和静态原位SEM拉伸实验，结合高分辨率超高速照相技术和相关图像显形分析技术获得与剪切带演化特征时间相当的精细时间描述，对剪切带扩展规律及扩展速度进行研究；另外，利用原位SEM加载和DIC相结合的分析技术，在空间域上获得剪切带的高分辨率特征图像，获得分阶段加载大块非晶试样剪切变形区域的位移场及特征尺寸信息。本项目旨在发展一种具有较高时间和空间分辨率，可对剪切带几何尺寸、扩展速度和扩展方式等进行直观测量和分析计算的实验方法，该项研究对于理解剪切带形成和扩展过程，揭示非晶态合金的剪切变形机理具有重要意义。

非晶态合金在室温条件下会发生高度局域化的剪切变形，材料表现出脆性破坏特点，使得非晶合金在工程应用中的可靠性和安全性受到极大限制。目前学术界对于剪切带形成机理已开展了大量的研究工作，但对于剪切带直接客观的实验观察及定量计算，可供参考的结果十分有限。本项目拟通过高速照相机、扫描电子显微镜等图片获取技术和数字图像相关分析技术的结合，开展对非晶合金剪切带变形行为以及断裂行为的实验研究，发展一种高时间和高空间分辨率的剪切带成像技术，以期实现对剪切带发生过程的实时测量和分析计算。. 本项目首先设计开展了动态加载条件下块体非晶的双剪切实验和三点弯曲实验，采用高速照相技术对非晶合金动态裂纹的起始和扩展过程进行了捕获，得到了非晶合金动态断裂的力学响应曲线，并对高速拍照结果进行了初步的DIC分析，对非晶合金剪切和拉伸裂纹在动态加载条件下的扩展速率进行了估计。其次，将高速照相机和金相显微镜结合，对非晶薄带在准静态拉伸条件下的剪切带扩展过程进行原位拍照，估算出剪切带在准静态加载条件下的扩展速率约为几毫米每秒。最后，开展了非晶薄带的原位SEM拉伸实验，采用分阶段加载方式对多条剪切带的渐进式间歇扩展特性进行了研究，并探索了用于SEM图像DIC分析的微小散斑制备技术。