Incipient Nanoscale Plasticity in Metallic Glasses

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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.