Atomic transport and structural modifications during shear banding of a bulk metallic glass

众所周知，在明显低于玻璃化转变温度的温度下，大块金属玻璃（bmg）的塑性变形局限于剪切带内。由此产生的结构变化以及变形玻璃的量热、动力学和机械响应的相关特征已通过事后分析方法进行了彻底分析，即在施加的机械应力停止后。然而，到目前为止，对剪切带化过程的研究还很少，仅有的几项（准）原位研究还存在争议。与塑性剪切变形同时进行的初步原位扩散实验表明，原子输运异常增强。在类似条件下变形的Ni晶体自扩散未观察到这种增强。因此，本提案的总体目标是通过基于补充原位和准原位实验的策略方法，揭示控制异常快速原子输运和相关结构修饰的机制。这些分析将对剪切带的激活和扩展机制提供更深入和新的认识。特别是，我们将解决一些尚未解决的问题，这些问题对于全面理解bmg的变形响应至关重要，例如原子输运率的异常增强与扩散物质的原子尺寸或变形协议的依赖关系。我们将分析输运增强与剪切带内特定化学再分布或结构变化的相互关系。为此，需要进行放射性示踪剂扩散实验和基于tem的研究。最后，基于专用放射性示踪剂扩散分析的互补方法和专业知识与先进的结构分析方法相结合，应以协调一致的方式应用，这将为块状金属玻璃的变形特性的基本理解提供重大进展。

At temperatures significantly below the glass transition temperature, plastic deformation of bulk metallic glasses (BMGs) is well known to localise within shear bands. The resulting structural changes as well as related signatures of calorimetric, kinetic and mechanical responses of the deformed glasses have been thoroughly analysed by post-mortem approaches, i.e. after the applied mechanical stress has ceased. However, the processes during shear banding have scarcely been investigated up to now and the few (quasi-) in-situ studies available are controversially discussed. Preliminary in-situ diffusion experiments performed concurrently to plastic shear deformation indicated an abnormally large enhancement of atomic transport. No such enhancement has been observed for self-diffusion of crystalline Ni deformed under similar conditions. The overarching goal of the present proposal is thus to uncover the mechanisms governing the abnormally fast atomic transport and the related structure modifications by a strategic approach based on complementing in-situ and quasi-in-situ experiments. These analyses shall provide deep and new insight into the mechanisms of shear band activation and propagation. In particular, we shall address a number of unresolved problems which are critical for a comprehensive understanding of the deformation response of BMGs, such as a dependence of the abnormal enhancement of atomic transport rates on the atom size of the diffusing species or the deformation protocol. We shall analyze the interrelation of the transport enhancement with specific chemical re-distributions or structural changes within the shear bands. For these purposes, both radiotracer diffusion experiments and TEM-based investigations need to be performed. Finally, it is the combination of complementing methods and expertise based on dedicated radiotracer diffusion analyses and advanced structural analyses methods, which shall be applied in a concerted way, that shall provide significant advances in the basic understanding of deformation properties of bulk metallic glasses.