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

最近，R.Busch和他的同事们开发了一种新的含硫(S)合金元素的金属玻璃家族。对于这些合金，在大多数情况下，与类似的合金成分相比，在没有S的情况下，玻璃形成能力有所提高。在某些情况下，S的加入甚至第一次创造了大量玻璃化的可能性。在这种情况下，重要的是要记住，硫具有复杂的配位行为，比任何其他元素形成更多的同素异形体，S8环是环境和近环境条件下最丰富的晶体同素异形体。在纯硫的熔融状态下，还有许多不同的同素异形体共存，包括S8环，而且也知道每转8个原子的螺旋结构的出现。因此，从概念上讲，向金属熔体中添加硫大大增加了拓扑的可变性和复杂性。在玻璃形成金属液体的结构模型中，包括早期的缔合模型，这种情况由于几何受挫而容易玻璃化。因此，新的含S玻璃形成合金系列为系统分析特定合金元素的影响提供了独特的机会，这种合金元素极大地提高了液态抗结晶的稳定性。在这项系统的研究中，已经确定了三组不同的合金，它们的硫含量分别为低、中和高。如果考虑到各自的无S基合金系统，则可以观察到如下关联：高S含量的块体金属玻璃，20at。%或更多与固溶体形成的基合金，如Pd-Ni，根本不形成玻璃；中等S-含量在10at以下。在边缘形成玻璃的基础合金中形成块状金属玻璃需要%，而少量的S添加几个原子百分数量级的元素有助于提高即使在没有硫的情况下也可以形成块状非晶的基础合金的非晶形成能力。因此，不同组的含S玻璃形成合金和块体玻璃形成合金能够以一种独特而史无前例的方式解决与金属系统中玻璃形成有关的长期存在的基本问题。因此，我们提出了一种系统的、相互关联的方法，结合了三个参与小组在玻璃形成金属合金研究中的互补和长期专业知识，以解决以下相互关联的问题：-硫作为合金元素如何改变熔体和玻璃中元素物种的粘度和原子迁移率？-含S体系的增强的GFA是由于微观迁移率的降低，还是由于介观流动行为的改变或热力学约束的改变？-S的加入如何影响液体和玻璃的中期有序？

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?