Incipient Nanoscale Plasticity in Metallic Glasses

金属玻璃（mg）在技术上相关温度下的塑性变形是由剪切带的作用介导的。由于它们在变形过程中固有的局部化倾向，mg在宏观长度尺度上表现出不期望的脆性行为。从较小的变形实体中理解剪切带的出现，即所谓的剪切转变区（STZs），已成为过去几年研究的热点，其最终目标是调节mg的脆性行为。然而，由于stz在时间和长度尺度上运行，几乎无法通过实验获得，因此可以通过小型机械测试（如球形纳米压痕）间接获得。在这个项目中，mg将在不同条件下（外部载荷、应变率、压头尖端半径）遭受球形纳米压痕，目的是获得有关塑性变形开始的信息，即单个stz相互关联形成更大的不可逆流动事件的时刻。在载荷-位移数据中，此事件可以被识别为所谓的弹出式事件。通过研究弹塑性在不同条件下的统计行为，可以获得mggs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研究弹出的基本驱动力。我们将通过改变应变速率来研究时间相关性对弹出框产生的影响。最后，我们将通过改变压头尖端半径来探测空间相关性，从而改变应力体积的大小。纳米压痕的结果将通过压痕过程中的原位声发射、原子力显微镜来研究所产生的表面形貌，以及波动电子显微镜来揭示由于加载和塑性变形引起的早期结构变化。该项目的目的是进一步了解stz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.