Kinetic and structural properties of shear bands in bulk metallic glasses

大块金属玻璃剪切带塑性流动局部化引起了广泛的关注，但其性质仍存在争议，甚至尚不清楚。此外，剪切带作为金属玻璃塑性应变的固有响应，也是利用这类材料独特力学性能的技术瓶颈，因为它经常导致所谓的“灾难性剪切带”发展的早期破坏。为了开发具有导致剪切带分支或利用其他转化途径产生宏观伪塑性行为的固有结构或固有性质的玻璃材料，需要更深入地了解剪切带的起始和性质。为了做到这一点，我们建议将bmg中扩散动力学的详细研究与诱导剪切带、结构调查和基于电子显微镜的应变测量与塑性流动局部化的原子模拟结合起来，这将由研究小组进行。模型玻璃原体Pd-Ni-P由于其优异的玻璃形成能力和相关的高晶体形成稳定性，有机会生产大规模样品，结构和热力学数据的可用性以及合适的放射性同位素的存在而被提出研究。本项目主要研究了对应变/应力场和自由体积分布极为敏感的变形bmg的扩散动力学，以及基于电子显微镜方法的自由体积和应变分布的实验分析。这些剪切带性质测量的结合是深入了解剪切带起始和传播的原子性质以及剪切带性质的有力工具。剪切带的动力学性质将通过放射性示踪技术和现场及死后实验进行研究。为了确定剪切带的相关结构对自由体积重分布的响应，我们将考察热力学处理的影响。实验将集中于剪切带内的多余自由体积分布作为其热力学历史的函数。此外，还应直接解决自由体积和应变分布的作用。因此，我们计划利用高角环形暗场（HAADF）探测器中的卢瑟福散射电子信号和电子能量损失谱中的低损耗部分信息同时测量剪切带的密度。感兴趣的第二个方面是金属玻璃在变形过程中的应变发展。我们的新方法是通过参考晶格测量应变，参考晶格通过纳米尺寸的自组织氧化铝掩膜（一种内部可用的技术）气相沉积带到样品上。参考晶格的周期性对变形引起的任何偏差都很敏感。这些实验结果将直接与研究组在模型玻璃成型系统上进行的原子模拟进行比较。

The plastic flow localization in shear bands of Bulk metallic glasses (BMGs) attracted a lot of attention although its nature is still controversially discussed or even obscure. Additionally, shear banding as the intrinsic response to plastic straining of metallic glasses presents also a technological bottle neck for utilizing the unique mechanical properties of this class of materials, since it often leads to early failure during the development of so-called “catastrophic shear bands”. In order to develop glassy materials with intrinsic structures or inherent properties that lead to branching of shear bands or that utilize other transformation pathways to yield macroscopic pseudo-plastic behavior, the initiation and properties of shear bands need to be understood in more depth. In order to do so, we propose combining detailed studies of the diffusion kinetics in BMGs with induced shear bands and structural investigations and strain measurements based on electron microscopy with atomistic simulations of plastic flow localization that are to be carried out by the Researchgroup. The model glass former Pd-Ni-P is proposed to be investigate due to its excellent glass forming ability and the associated high stability against crystal formation, the opportunity to produce large-scale samples, the availability of structural and thermodynamic data and the existence of suitable radioisotopes.The present project is focused on two mayor aspects: the investigation of the diffusion kinetics in deformed BMGs which is extremely sensitive to the strain/stress fields and the free volume distribution and the experimental analysis of the free volume and strain distribution based on electron microscopy methods. The combination of these measurements of the shear band properties is a powerful tool to render deep insight into the atomistic nature of shear band initiation and propagation as well as concerning the properties of the shear bands.The kinetic properties of shear bands will be studied by the radiotracer technique and in-situ and postmortem experiments. The effect of thermomechanical treatments will be examined in order to figure out the response of the pertinent structure of shear bands on the free volume redistribution. The experiments will be focused on the excess free volume distribution inside the shear bands as a function of their thermomechanical history. Additionally, the role of the free volume and strain distribution shall also be addressed directly. Therefore it is planned to measure the density of shear bands by a new technique using the signal of the Rutherford-scattered electrons in the high-angle annular dark-field (HAADF) detector and the information of the low loss part of an electron energy loss spectrum simultaneously. The second aspect of interest is the strain development in a metallic glass during deformation. Our new approach is to measure the strain via a reference lattice, which is brought onto the sample by vapour deposition through nanometre-sized, selforganized Alumina masks (a technique which is available in-house). The periodicity of the reference lattice is then sensitive to any deviation introduced by deformation. These experimental results shall be directly compared with atomistic simulations on a model glass forming system performed in the Researchgroup.