Metallic glasses and strain localization

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The focus of the MEGALO project is set by the transition between heterogeneous and homogeneous plastic deformation of metallic glasses. So far, this transition is poorly understood even though it has important bearings for basic research as well as glass application. In order to address the underlying basic mechanisms that control this transition, a well-concerted strategy has been devised that consists of closely interlinked experimental and theoretical analyses: structural heterogeneities and their impact will be studied on different length scales, starting at the local scale, through complementing experimental techniques, allowing an evaluation of the main physical parameters of fundamental as well as practical importance. These results will be compared with theoretical predictions, in connection with rheological properties all the way through the transition regime and at various length scales, from the discrete to the continuum regime. This approach relies on and exploits the complementing expertise of the members of the consortium and shall provide a physically solid picture of the mechanisms that govern the transition between localization of deformation and homogeneous flow. Based on only a few experimental studies, recent suggestions in the literature linked the transition to the competition between thermal- and time-dependent structural rearrangements. These few studies, focusing on specific compositions, suggest that the drastic change of the deformation mode would be a footprint of a transition in the elementary mechanisms occurring inside the glass. It is expected to be governed by different structural changes and particularly by the nature and the length scale of the medium range order (MRO) as well as its dynamics. So far, this aspect has not yet been understood. It is the aim of MEGALO project to investigate this transition and to identify what are the first order features that control it. For this reason, two metallic glasses are selected that show, within the range available for metallic glasses, rather distinct characteristics concerning their fragility. High temperature testing will be performed in both compression and tension and a particular attention will be paid to develop appropriate Digital Image Correlation (DIC) techniques to tackle strain localisation as a function of temperature and strain rate in SEM environment. The deformed samples will be characterised to identify structural variations between zones where strain localisation occurs and outside these zones. To this end, complementary experimental techniques, namely fluctuation TEM, diffusivity measurements and nanoindentation, will be used in combination with the modelling of strain localisation in metallic glasses via molecular dynamics and finite element calculations. The close collaboration of the participating groups as well as the exchange of PhD students shall ensure the success of the scale-bridging approach of the MEGALO project.