Materials Science Research Lecture

Webinar Link:

https://caltech.zoom.us/j/95708772987

Webinar ID: 957 0877 2987

Abstract:

Glasses belong to daily-life structural and functional materials, spanning a large number of materials classes, such as amorphous polymers, oxide glasses, molecular glasses, and metallic glasses. All these systems have some degree of metastability, which drives structural evolution with time. This is typically referred to as aging or relaxation. Whether or not aging is of relevance for a given application is determined by the time scales of the underlying structural processes. For the case of metallic glasses, relaxation time scales are sufficiently short to immediately affect engineering applications, which may have detrimental consequences for load bearing systems.

Being a classical topic, aging of metallic glasses has primarily been studied through thermal protocols that trigger thermally-activated relaxation dynamics. Such investigations have for a long time been restricted to substantial time- and volume averaging in narrow temperature regimes. This has recently been overcome by coherent scattering methods, notable x-ray and electron correlation spectroscopy techniques.

Here we exploit the x-ray based-approach to gain unprecedented insight into the structural dynamics induced by mechanical and thermal stresses in the glassy solid regime. Heating towards the glass transition temperature reveals accelerated and monotonic dynamics considerably faster and of different aging dynamics in regions that have a deformation history [1]. Highly complex cross-correlation data and non-monotonic changes in the time-dependent structural dynamics are revealed in the glassy solid regime. We highlight this by considering both mechanical loading in the elastic regime at room temperature [2] and through the introduction of thermal stresses via cooling-heating cycles [3]. Applying a mechanical stress within the nominally elastic regime introduces deviations away from the universally known time-temperature rescaling of relaxation times in amorphous materials. Atomistic simulations are used to guide the interpretation of the highly complex cross-correlation data. Our insights are discussed in the context of microstructural plastic excitations in metallic glasses that occur heterogeneously in space and time. We conclude with discussing current challenges and future opportunities of the used method.

[1] S. Kuechemann, C. Liu, E. Dufresne, J. Shin, R. Maass, Physical Review B 97 (2018) 014204.

[2] A. Das, P.M. Derlet, C. Liu, E.M. Dufresne, R. Maass, Nature Communications 10 (2019) 5006.

[3] A. Das, E.M. Dufresne, R. Maass, Acta Materialia 196 (2020) 723-732.

More about the Speaker:

Robert Maass received a triple diploma in Materials Science and Engineering from the Institut National Polytechnique de Lorraine (INPL-EEIGM, France), Luleå Technical University (Sweden) and Saarland University (Germany) and in 2009, he obtained his PhD from the Materials Science Department at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. He was a postdoctoral researcher at the Swiss Federal Institute of Technology (ETH Zurich), after which he joined the California Institute of Technology as an Alexander von Humboldt postdoctoral scholar. After working as a specialist management consultant for metals at McKinsey & Co., he transferred to the University of Göttingen as a junior research group leader. He joined the faculty of the University of Illinois at Urbana-Champaign as Assistant Professor of Materials Science and Engineering in 2015, where he still holds an appointment. In 2020 he became the head of department 5 and a member of the directorate at the Federal Institute of Materials Research and Testing (BAM). Currently, he has published about 80 international peer-reviewed articles in the area of mechanical metallurgy of metals. His honors include the prestigious Emmy Noether award, the NSF Career award, and the Masing memorial medal. His research interests include microstructure-property relations, size effects, strain localization and defect structures of amorphous and crystalline metals, defect dynamics, mechanical properties, microplasticity, glass transition phenomena, and test system development.