Speaker: Prof. Paul Voyles
Affiliation: Department of Materials Science and Engineering, University of Wisconsin-Madison, USA
Date: December 16th (Monday), 2019
Time: 10:00 – 11:30 am
Contact: Prof. Zhaoping Lu (62332350, firstname.lastname@example.org)
Location: 353 Main Building, USTB
Abstract: Glasses are complex, metastable materials which exist in a variety of thermodynamic states. Electron nanodiffraction probes these materials at the length scales that control their properties and processes. Temporal variations in nanodiffraction form the basis of electron correlation microscopy (ECM), a probe of nanoscale dynamics. Spatial variations in nanodiffraction form the basis of fluctuation electron microscopy (FEM), a probe of nanoscale structure. These techniques enable studies of the process by which a liquid becomes a glass on cooling and the structure of the resulting glass, respectively. We have used FEM to show the structure of Zr-Cu-Al metallic glasses is characterized by a competition between icosahedral and crystal-like nanometer scale order. Increasing icosahedral order either by modifying composition or by changing thin-film deposition parameters increases both thermal stability and glass forming ability of the alloy. This observation is consistent with simulations that show that, compared to other structures, icosahedral arrangements of atoms are lower in energy and slower to rearrange. We have used ECM to probe atomic rearrangements in the supercooled liquid state of Pt57.5Cu14.7Ni5.3P22.5. ECM shows that the structural relaxation time, which measures the rate of atomic rearrangements, varies from place to place in the liquid at a length scale of 0.8-1.4 nm, depending on temperature. ECM also shows that in a ~1 nm thick layer near the surface of the liquid, the dynamics are an order of magnitude faster than in the bulk. The spatial variation of bulk dynamics provides a stringent new test of atomistic theories of the glass transition. The surface layer provides a pathway for homogeneous nucleation of crystals at metal surfaces without catalysts for heterogeneous nucleation. A next-generation, ultrafast pixelated direct electron detection camera will enable combined studies of structure and dynamics using electron nanodiffraction.
Speaker’s short biography: Paul Voyles is Professor of Materials Science and Engineering and Harvey D. Spangler Professor of Engineering at the University of Wisconsin-Madison. He earned degrees in physics from Oberlin College and the University of Illinois, Urbana-Champaign, then worked as a post-doctoral member of technical staff at Bell Labs in Murray Hill NJ. He joined the UW-Madison in 2002 as an Assistant Professor, and served as Chair of the Materials Science and Engineering Department from 2016 to 2018. Since 2018, he has been the Director of the Wisconsin Materials Research Science and Engineering Center. His research specialty is the structure of materials, investigated primarily with electron microscopy, supplemented by simulations and data science. He has worked on metallic and other glasses and on materials for microelectronics, spintronics, and superconductors. He has published over 150 journal articles, book chapters, and conference proceedings.