Structure and Dynamics of Metallic Glasses and Glass-forming Liquids
Professor Paul M. Voyles
Char, Department of Materials Science and Engineering, University of Wisconsin, Madison
— All Welcome —
Glasses are complex materials. They exist in a variety of metastable states with different enthalpies, with a wide variety of local atomic arrangements, and those atoms rearrange at different rates in different places. Much of this complexity occurs at the nanoscale, including medium-range structural order and spatially heterogeneous dynamics in the supercooled liquid near Tg. This talk will review my group’s work using electron nanodiffraction in the form of fluctuation electron microscopy (FEM) to study the static structure of glasses and time-resolved nanodiffraction in the form of electron correlation microscopy to study the dynamics of glass-forming liquids.
FEM is the study of fluctuations in nanodiffraction from place to place on a glassy sample. We have used FEM to show that Al-based metallic glasses that exhibit primary Al crystallization have frozen-in, fcc Al nuclei within the glass structure , and that Zr-Cu-Al glasses exhibit two competing structure types , one of which has crystal-like approximate rotational symmetry, and the other of which does not. In high glass-forming ability compositions, the non-crystalline structure is more thermodynamically stable under annealing below Tg. In poorer glass formers, the crystal-like structure is more stable . More recent work has shown that Pd-Si shares this competition between two structures, suggesting that it may be a general feature of metallic glasses.
Electron correlation microscopy (ECM) is the study of temporal fluctuations in nanodiffraction, which are sensitive to structural rearrangements in the supercooled liquid . The time-time autocorrelation function of the diffracted intensity can be fit to obtain the structural relaxation time t and stretching exponent b, both with sub-nanometer spatially resolution. A four-point two-time, two-space correlation function can be fit to obtain a characteristic length x along with t.
We have used ECM to study the dynamics in the supercooled liquid region of Pt57.5Cu14.7Ni5.3P22.5 nanowires from Tg (507 K) to Tg+ 16 K (523 K) . The data constitute the first spatially-resolved images of spatially heterogeneous dynamics in a supercooled liquid. The characteristic time scale t varies from >500 seconds to a few seconds, and the characteristic length scale x varies from 1.4 to 0.8 nm with increasing temperature. The viscosity calculated from the mean relaxation time is in good agreement with bulk viscosity measurements. x(t) over this limited temperature range agrees with all of the major microscopic theories of the glass transition. The nanowires also exhibit a near-surface region with dynamics consistently an order of magnitude faster than the bulk, which may influence the surface-driven crystallization of the wires and may be related to the suppressed Tg observed in nanoconfined liquids.
Paul Voyles is Professor and Chair of the Department of Materials Science and Engineering and Beckwith-Bascom Professor 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. 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 is director of the UW-Madison NSF Materials Research Science and Engineering Center. He has published over 150 journal articles, book chapters, and conference proceedings.
 W.G. Stratton, J. Hamann, J.H. Perepezko, P.M. Voyles, X. Mao, and S. V. Khare, Appl. Phys. Lett. 86, 141910 (2005).
 J. Hwang, Z. Melgarejo, Y.E. Kalay, I. Kalay, M.J. Kramer, D.S. Stone, and P.M. Voyles, Phys. Rev. Lett. 108, 195505 (2012).
 P. Zhang, J.J. Maldonis, M.F. Besser, M.J. Kramer, and P.M. Voyles, Acta Mater. 109, 103 (2016).
 L. He, P. Zhang, M. F. Besser, M. J. Kramer, & P. M. Voyles, Microsc. Microanal. 21, 1026 (2015).
 P. Zhang, J. J. Maldonis, Z. Liu, J. Schroers, P. M. Voyles, Nat. Commun. 9, 1129 (2018).