Elastic and inelastic scattering studies of supercooled metallic glass-forming liquids - the connection between ordering and fragility

非技术概述：除了金属玻璃的新特性，包括强度大于钢，高硬度和耐腐蚀，它们可以热塑性成型，在很大程度上与玻璃鼓风机形成传统硅酸盐玻璃的方式相同。这使得它们可以用一种传统合金无法实现的工艺被塑造成复杂的形状。然而，为了充分发挥其潜力，需要新的方法来快速识别潜在的金属玻璃，以满足新兴的技术需求。一种与液体粘度随温度的变化相对应的性质，即脆性，经常被认为与玻璃成形性有关。然而，脆弱性的微观含义尚不清楚；它似乎与冷却到熔化温度以下时液体结构变化的速率有关。这些结构变化可以通过从悬浮在高真空环境中的液体散射高能x射线来测量，避免液体与其容器之间的反应。使用一种新的设备，这些扩展到散裂中子源的中子散射研究，这是地球上最强烈的脉冲中子源，允许直接比较结构变化和液体流动。从这项工作中获得的见解提高了预测玻璃形成的能力，并加深了对玻璃形成过程本身的理解，这仍然是凝聚态科学中尚未解决的关键问题。根据材料基因组计划的指导方针，研究结果还为计算机建模提供了新的结构和物理性质数据，这是先进材料开发所必需的。这项研究是研究生和本科生科学训练的一个组成部分，并有助于PI对地区中学教师的持续推广活动，这些活动的重点是讨论常见材料，向学生传达基本的科学原理。技术总结：虽然金属玻璃现在有许多新的技术应用，但需要新的方法来快速识别和开发新的玻璃，以满足不断变化的需求，以及探索玻璃形成的基本性质。脆性与粘度的温度依赖性有关，经常与玻璃的成形性有关。然而，脆性的微观起源以及为什么它对玻璃形成很重要，人们知之甚少。对液体结构和流动之间的关系以及它们与玻璃转变和玻璃形成之间的关系的基础研究可以阐明这一点。根据该研究小组最近的工作：(i)脆性反映在结构随温度的有序率上；（ii）粘度的高温交叉Tcoop与协同流动的开始有关；（iii） Tcoop与玻璃化转变温度Tg密切相关，证实玻璃化形成是协同过程的最后一步，实际上始于更高的温度。从这些结果中出现的一些关键问题是：(i)化学或拓扑排序与脆弱性的关系更密切吗？（二）合作流动发生的结构关系是什么？（iii）如何才能最好地利用Tcoop和高温测量液体易碎性来开发新的搜索方法，以确定良好的玻璃形成液体？这些问题可以通过对悬浮液体的弹性和非弹性中子散射研究来解决，这些研究使用的是该团队为散裂中子源建造的新的静电悬浮（ESL）设备。SNS的高强度使得测量可以在更短的时间内完成，因此可以进行更深的液体过冷。这些数据可以与高温下液体粘度的测量相关联，通过更好地理解拓扑/化学排序过程与液体动力学之间的联系，以及将Tcoop与玻璃化转变联系起来的过程，可以更深入地了解脆性。在高温液体中也可以发现玻璃形成的线索，提供新的搜索方案来识别良好的玻璃形成物，超越了通常使用的洞察力引导的试验和错误或组合方法。液体有序性和玻璃可成形性之间的关系引起了玻璃界的广泛关注，从金属玻璃延伸到硅酸盐和硫系玻璃。

NON-TECHNICAL SUMMARY:In addition to the novel properties of metallic glasses, including strengths greater than steel, high hardness and resistance to corrosion, they can be thermo-plastically formed, in much the same way that a glass blower forms traditional silicate glasses. This allows them to be molded into complex shapes using a process that is not possible for conventional alloys. To realize their full potential, however, new methods are needed for the quick identification of potential metallic glasses to meet emerging technological needs. A property, fragility, corresponding to the change in the liquid viscosity with temperature, is frequently taken to correlate with glass-formability. However, the microscopic meaning of fragility is unclear; it appears to correlate with the rate of structural change of the liquid with cooling below the melting temperature. These structural changes can be measured by scattering high-energy X-rays from liquids that are levitated in a high-vacuum environment, avoiding reactions between the liquid and its container. Using a novel facility these are extended to neutron scattering studies at the Spallation Neutron Source, the most intense pulsed neutron source on Earth, allowing a direct comparison between structural changes and liquid flow. The insights gained from this work lead to an improved ability to predict glass formation and a deeper understanding of the process of glass formation itself, which remains a key unsolved problem in condensed matter science. Following the guidelines of the Materials Genome Initiative, the results also yield new structural and physical property data for computer modeling, necessary for advanced materials development. The research is an integral part of the scientific training of both graduate and undergraduate students, and contributes to the PI's ongoing outreach activities to regional secondary school teachers, which are focused on discussions of common materials to convey basic scientific principles to students. TECHNICAL SUMMARY:While metallic glasses are now poised for many novel technological applications, new methods are needed to rapidly identify and develop new glasses to meet changing demands, as well as to probe the fundamental nature of glass formation. Fragility, related to the temperature dependence of the viscosity, frequently correlates with glass formability. However, the microscopic origin of fragility and why it may be important for glass formation are poorly understood. Fundamental investigations of the relations between liquid structure and flow and how these are related to the glass transition and glass formation can illuminate this. Based on recent work from this research team: (i) fragility is reflected in the rate of structural ordering with temperature, (ii) a high temperature crossover in the viscosity, Tcoop, is related to the onset of cooperative flow and (iii) Tcoop is strongly correlated with the glass transition temperature, Tg, confirming that glass formation is the final step in a cooperative process that actually begins at a much higher temperature. Some key questions emerging from these results are: (i) Is chemical or topological ordering more closely related to fragility? (ii) What is the structural relation to the onset of cooperative flow? (iii) How can Tcoop and a high temperature measure of liquid fragility be best used to develop new search approaches to identify good glass-forming liquids? These questions can be addressed by elastic and inelastic neutron scattering studies on levitated liquids using a new electrostatic levitation (ESL) facility constructed by this team for use at the Spallation Neutron Source. The high intensity of the SNS allows measurements to be made in shorter times and hence to deeper liquid supercoolings. These data can be correlated with measurements of the liquid viscosity at high temperature, giving a deeper understanding of fragility from a better understanding of the connections between topological/chemical ordering processes and liquid dynamics, and the processes that connect Tcoop with the glass transition. Clues can also be found for glass formation in the high temperature liquid, giving new search schemes to identify good glass formers, beyond the insight-guided trial and error or combinatorial methods commonly used. Relations between liquid ordering and glass formability are of broad interest to the glass community, extending beyond metallic glasses to the silicate and chalcogenide glasses as well.