Incipient Nanoscale Plasticity in Metallic Glasses

金属玻璃（MG）在技术相关温度下的塑性变形是由剪切带的作用介导的。由于其固有的倾向，本地化在变形过程中，MG显示出不希望的宏观长度尺度上的脆性行为。理解剪切带从较小的变形实体中出现，即所谓的剪切转变区（STZ），在过去几年中已成为一个激烈的研究课题，其目标是最终介导MG的脆性行为。然而，由于STZ在时间和长度尺度上的操作几乎无法通过实验获得，因此可以通过小规模的力学测试（例如球形纳米压痕）间接获得。在本项目中，MG将在不同条件下进行球形纳米压痕（外部施加的载荷、应变率、压头尖端半径），目的是获得有关塑性变形开始的信息，即各个STZ相互关联以形成较大的不可逆流动事件的时刻。在载荷-位移数据中，该事件可以被识别为所谓的弹出。通过研究不同条件下的pop-ins统计行为，可以得到MG初始塑性的潜在机制。我们建议通过研究附加外部应力的影响来调查突然发生的根本驱动力。我们将通过改变应变率来研究时间相关性对产生弹出的影响。最后，我们将通过改变压头尖端半径来改变应力体积的大小来探测空间相关性。纳米压痕的结果将补充原位声发射压痕过程中，原子力显微镜调查所得的表面形貌，和波动电子显微镜揭示早期结构变化，由于加载和塑性deformation. Objective的变化，这个项目是为了进一步了解STZs如何在空间和时间上相互作用，形成局部塑性流动事件，并最终组织成剪切带。

Plastic deformation of metallic glasses (MGs) at technically relevant temperatures is mediated by the operation of shear bands. Because of their inherent tendency to localize during deformation, MGs show an undesired brittle behavior on macroscopic length scales. Understanding the emergence of shear bands from smaller deformation entities, so-called shear transformation zones (STZs), has become a topic of intense research in the past years, with the goal of ultimately mediating the brittle behavior of MGs. However, since STZs operate at time and length scales that are barely accessible experimentally, indirect access is gained via small-scale mechanical testing such as spherical nanoindentation.In this project, MGs will be subjected to spherical nanoindentation under different conditions (externally applied loads, strain rate, indenter tip radius) with the aim to gain information about the onset of plastic deformation, i.e. the moment at which individual STZs correlate to form a larger irreversible flow event. In load-displacement data, this event can be identified as a so-called pop-in. The underlying mechanisms for incipient plasticity of MGs can be gained by investigating the statistical behavior of the pop-ins under different conditions. We propose to investigate the fundamental driving forces for pop-ins by studying the effect of additional external stresses. We will study the effect of temporal correlations on the creation of pop-ins by varying the strain rate. Finally, we will probe spatial correlations by varying the indenter tip radius which varies the size of the stressed volume. The nanoindentation results will be complemented by in-situ acoustic emission during indentation, atomic force microscopy to investigate the resultant surface topography, and fluctuation electron microscopy to unveil early structural changes due to loading and plastic deformation.The objective of this project is to further our understanding of how STZs interact in space and time to form local plastic flow events and to eventually organize into shear bands.