Coupling of irreversible plastic rearrangements and heterogeneity of the local structure during deformation of metallic glasses

金属玻璃作为结构材料应用的主要困难之一是其局部化塑性变形的倾向，以及由此导致的载荷作用下的灾难性破坏。然而，尽管两者都有科学和应用方面的兴趣，但对菌株定位的早期阶段还没有很好的了解。因此，本项目旨在研究金属玻璃中剪切局部化的初始阶段。这里的一个重要问题是无定形固体固有的小尺度结构非均质性和局部应力的波动对基本剪切转变事件的发生所起的作用。这些结构应力效应对于金属玻璃从均匀塑性变形到剪切带的转变的开始也是非常重要的。以下问题在这里非常重要。(I)局部结构和局部作用应力如何影响不可逆的粒子重排；(Ii)在什么条件下，它们可能导致跨系统剪切带的形成？为了充分和全面地解决这些问题，本项目汇集了金属玻璃实验研究和模型玻璃系统计算机模拟方面的专门知识。一方面，分子动力学模拟将提供关于小尺度上集体粒子重排的详细信息，从而有助于识别基本塑性事件，即所谓的剪切相变区。此外，应力张量将在局部尺度上确定，与基本剪切事件相关。利用所获得的模拟数据，将在空间和时间上评估应力和塑性应变的相关性。类似地，还将分析塑性应变与局部结构之间的相互联系。另一方面，实验将揭示金属玻璃在桥接尺度上变形时的静态和动态性质，从原子尺度到细观和宏观尺度，在原子尺度上，电子相关显微镜等先进的电子显微镜方法将解决结构不均一性，提供与剪切带化之前的变形局部化早期阶段有关的原子输运增强的样本平均信息。上述实验和计算机模拟将从不同角度阐明局部结构和应力波动对剪切带开始的作用。目前合作工作的首要目标是评估来自互补的实验和理论工作包的结果，以获得金属玻璃中弹性和可塑性之间的转变的物理一致的图像。

One of the major difficulties in the applications of metallic glasses as structural materials lies in their propensity to localized plastic deformation and the resulting catastrophic failure under load. Yet, although of both, scientific as well as application related interest, the early stages of strain localization are not well understood. The present project aims, therefore, at a study of the initial stages of shear localization in metallic glasses. An important issue here is the role played by small scale structural heterogeneity, inherent to amorphous solids, and fluctuations of the local stress for the occurrence of an elementary shear transformation event. These structure-stress effects are also of great importance for the onset of the transition from a homogeneous plastic deformation to shear bands in metallic glasses. The following questions are of key interest here. (i) How do the local structure and the locally acting stress influence irreversible particle rearrangements and (ii) under which conditions may they lead to the formation of system-spanning shear bands? In order to address these issues adequately and in a comprehensive way, the present project brings together expertise in experimental investigations of metallic glasses and computer simulations of model glassy systems. Molecular dynamics simulations, on the one hand, will provide detailed information about the collective particle rearrangement on small scales, thereby helping to identify elementary plastic events, the so-called shear transformation zones. Moreover, the stress tensor will be determined on the local scale, relevant for elementary shear events. Using the thus obtained simulated data, correlations of stress and plastic strain will be evaluated both in space and time. Similarly, an analysis of the mutual connection between plastic strain and the local structure will be performed. Experiments, on the other hand, will reveal static and dynamic properties of metallic glasses under deformation on bridging scales, from atomistic ones, on which advanced TEM methods such as electron correlation microscopy will address the structural heterogeneities, to meso- and macroscopic scales, providing sample-averaged information on the enhancement of atomic transport related to earlier stages of deformation localization, prior to shear banding. The above mentioned experiments and computer simulations will shed light from different angles onto the role of the local structure and stress fluctuations for the onset of shear banding. The overarching goal of the present collaborative work lies in assessing the results from the complementing experimental and theoretical work packages to obtain a physically consistent picture of the transition between elasticity and plasticity in metallic glasses.