Topological Frustration and Glass-Forming Ability of Sulfur-containing Metallic Glass-Formers

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Very recently, a new family of metallic glasses that contain Sulfur (S) as alloying element has been developed by R. Busch and co-workers. For those alloys, in most cases the glass-forming ability was improved as compared to similar alloy compositions where S was absent. In some cases, the addition of S even served to create the possibility for bulk vitrification for the first time. In that context it is important to recall that Sulfur has a complex coordination behavior and forms more allotropes than any other element, with the S8-ring being the most abundant crystalline allotrope under ambient and near-ambient conditions. In the molten state of pure sulfur, also a large variety of largely different allotropes coexist, including S8-rings, and the occurrence of a helical structure with eight atoms per turn is also known. Thus, conceptually, adding sulfur to a metallic melt increases considerably the topological variability and complexity. In the view of structural models of glass-forming metallic liquids, including early association models, such a situation is prone to vitrification due to geometrical frustration. Thus, the new family of S-containing glass-forming alloys offers the unique opportunity to systematically analyze the impact of a specific alloying element that largely enhances the stability of the liquid state against crystallization. For this systematic study, three distinct groups of alloys with minor -, medium - and high Sulfur content have been identified. If the respective S-free base alloy systems are regarded, the following correlation is observed: bulk metallic glasses with high S-content of 20 at. % or more are related to solid-solution-forming base alloys such as Pd-Ni that do not form glasses at all; medium S-contents below 10 at. % are required to form bulk metallic glasses in base alloys that are marginal glass-formers and minor S additions of the order of a few atomic percent serve to enhance the glass-forming ability (GFA) of base alloys that are already allowing for bulk glass formation even without the addition of sulfur. Thus, the different groups of S-containing and bulk glass-forming alloys allow addressing long-standing and basic issues concerning glass formation in metallic systems in a unique and unprecedented way. We thus propose a systematic and strongly interlinked approach that combines the complementing and long-standing expertise of the three participating groups in research on glass-forming metallic alloys to address the following interconnected questions: - How does sulfur as alloying element modify the viscosity and the atomic mobility of the elemental species in the melt and the glass?- Do the enhanced GFAs of S-containing systems result from a decreased microscopic mobility, from a modified mesoscopic flow behavior or from changed thermodynamic constraints?- How does the S-addition affect the medium-range order of the liquid and the glass?