Kinetic and structural properties of shear bands in bulk metallic glasses

大块金属玻璃(BMG)剪切带中的塑性流动局部化问题引起了人们的广泛关注，但其本质仍存在争议甚至不甚清楚。此外，剪切带作为金属玻璃对塑性应变的内在响应，也是利用这类材料独特的力学性能的技术瓶颈，因为它经常导致在所谓的“灾难性剪切带”的发展过程中早期破坏。为了开发具有导致剪切带分支的固有结构或固有性能的玻璃材料，或者利用其他转变途径来产生宏观假塑性行为，需要更深入地了解剪切带的形成和性质。为此，我们建议将对BMG中扩散动力学的详细研究与诱导剪切带、基于电子显微镜的结构研究和应变测量与研究小组将开展的塑性流动局部化的原子模拟相结合。Pd-Ni-P模型玻璃由于其良好的玻璃形成能力和抗结晶稳定性、大规模样品的制备机会、结构和热力学数据的可获得性以及合适的放射性同位素的存在而被建议进行研究。本项目主要集中在两个主要方面：变形BMG中对应变/应力场和自由体积分布极其敏感的扩散动力学的研究和基于电子显微镜方法的自由体积和应变分布的实验分析。这些剪切带性质的测量相结合是深入了解剪切带形成和传播的原子学性质以及有关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以及原位和身体实验来研究。为了找出剪切带的相关结构对自由体积重分布的响应，将考察热机械处理的影响。实验将集中在剪切带内的过剩自由体积分布作为其热机械历史的函数。此外，还应直接讨论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探测器(HAADF)中卢瑟福散射电子的信号和电子能量损失谱低损耗部分的信息来同时测量剪切带密度。第二个有趣的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模(这是一种内部可用的技术)通过蒸气沉积到样品上。因此，参考晶格的周期性对任何由变形引起的偏差都很敏感。这些实验结果将直接与研究组对模型玻璃形成系统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.