The ultrastable state of metallic glasses and its role in the structural pathway of ageing

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What are the structural characteristics of ultrastable metallic glasses? The recently discovered ultrastable state of metallic glasses (MGs) exhibits a variety of thermodynamic stability levels in combination with an enhanced kinetic stability. As a hypothesis, observed levels of excess enthalpy originate from faster relaxation contributions that are locally embedded in an otherwise stable structure. Such features remind strongly of novel MG-states of structurally heterogeneous glasses that were recently reported by experiments on conventional MGs and molecular dynamics simulations. The aim of the PathAge project is to test the hypothesis in terms of the ultrastable MG’s relation to these novel structural states. This builds on quantifying the evolution of the ultrastable state in response to thermal stimulus by tracing the structural transformation towards the supercooled liquid or eventual crystallization. Three possible mechanistic routes will be considered: First, a front-initiated process as observed for ultrastable molecular glasses. Second, a homogeneous structural evolution triggered by fast relaxation contributions. Third, a transformation involving an underlying phase transition of a heterogeneous glass state. In order to distinguish between the proposed transformation scenarios, the following novel experimental approaches will be used in addition to traditional methods: The so-called single-parameter-ageing formalism known from the field of molecular glasses, which will allow for predicting and testing the homogeneous ageing scenario. Surface sensitive methods that probe nanoscale heterogeneities revealing the formation of structurally heterogeneous glassy states. Spatially resolved electron diffraction combined with atomistic simulations to identify preferred local structural motifs. In concert, these approaches will significantly enhance the understanding of the unique ultrastable MG-state, thereby unlocking potential for novel applications of MGs.