Structure, Dynamics, and Relaxation of Metallic Glasses at the Nanoscale

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

NON-TECHNICAL SUMMARY This project supports research into glasses and liquids consisting entirely of metal atoms. In both glasses and liquids, the atoms are arranged without order - jumbled up, like marbles in jar, not sitting in rows like eggs in an egg carton. That disorder poses special problems for experiments measuring how the atoms are arranged and how they move with respect to one another. This project addresses those problems by studying how nanometer-diameter probe beams of electrons interact with the atoms as the beam is moved from place to place, which reveals how they are arranged, or with a fixed beam position as a function of time, which reveals how the atoms move. Understanding how the atoms are arranged and how they move will enable researchers to understand fundamental questions in the materials physics of glasses, including: (1) How does a liquid cool into a solid glass? Do some parts of the liquid stop moving first, or do all the atoms slow down at once? If some parts stop first, what is different about the atomic arrangements in those regions? (2) How do the atoms in a glass rearrange when they are given just a little heat energy ? not enough to melt, but enough to move around a bit? This process is called aging, and it can make a flexible metallic glass brittle, but it is currently poorly understood at the atomic scale. Answers to these questions will lay the scientific groundwork for scientists to design new metallic glass alloys with desirable properties like very high hardness, high formability into parts, and high corrosion resistance. These new materials may find applications in areas ranging from medical devices and implants to nanomachines with tiny gears or other sliding parts. This project will train scientists and engineers to create new metallic glass materials and understand how they work at the atomic scale, and it will promote science and engineering to K-12 students through live demonstrations of imaging single atoms, the fundamental building blocks of matter, using the power of electron microscopy. TECHNICAL SUMMARY This project will support investigation of the heterogeneous structure and dynamics of metallic glass and glass-forming liquids at the nanoscale using coherent electron nanodiffraction in the form of electron correlation microscopy (ECM) and fluctuation electron microscopy (FEM). Spatially heterogeneous dynamics in supercooled liquids are central to many theories of the glass transition, and temporally heterogeneous dynamics in the glass are central to structural relaxation. ECM experiments on liquids will yield a dynamic length scale and characteristic time for liquid-state dynamics over at least two decades in time, and investigate the glass transition behavior of a recently-discovered near-surface layer with dynamics at least one order of magnitude faster than the bulk. ECM experiments on glasses will reveal the size and spatial distribution of the individual events that create structural relaxation, and FEM experiments will yield insight into the structure of glasses before and after relaxation. New, combined ECM/FEM experiments will enable direct, experimental correlations between nanoscale structure and dynamics. In liquids, the hypothesis that regions with icosahedral local structure have slower dynamics will be tested, and in glasses, tools and approaches will be developed with the goal of measuring the changes in structure associated with single relaxation events. Together, these results will test existing theories of the glass transition and relaxation and drive development of new theories. The insights gained on metal systems with simple bonding have the potential for impact across materials classes, including oxide, chalcogenide, small molecule, and even polymer glasses. The physical insights generated by this project will lay the groundwork for rational design of metallic glass alloys with high glass forming ability and desirable properties including high hardness, high elastic limit, and either high corrosion resistance or biologically benign corrosion products for applications ranging from biomedical devices and implants to micro- or nanomechanical machines. This project will support teaching and learning through ongoing updating of the Electron Microscopy Database website, and it will share the excitement of science through live demonstrations to K-12 students of imaging single atoms using high-resolution electron microscopy. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.