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

Washington University, Saint Louis MO

Investigators

Abstract

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.