Fluctuations of Amorphous Metals in Time and Space from Electron Nanodiffraction

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

TECHNICAL SUMMARY: Fluctuations in the intensity diffracted from materials illuminated with coherent probes contain unique information about the structure of disordered materials. In this project, a new, state-of-the-art scanning transmission electron microscope will be used to study spatial and temporal fluctuations in metallic glasses and glass forming liquids with nanometer spatial resolution. Fluctuations in space give information about nanoscale structural order, which will be used to study nucleation and growth during the primary crystallization reaction exhibited by some high Al-content metallic glasses. The size distribution of protocrystalline clusters as a function of time and temperature will be measured through the phase transformation. Fluctuations in time give information about atom dynamics, which will be used to study the glass transition of bulk-glass forming alloys with varying fragility as a function of temperature. In particular, direct experimental evidence of the nanoscale spatially heterogeneous dynamical domains that underlie some models of the glass transition will be sought. For both these efforts, coherent electron probes spanning 0.1 to ~1000 nm in diameter will be developed. These probes will also be useful for nanodiffraction and diffractive imaging. NON-TECHNICAL SUMMARY: Metallic glasses are a potentially useful new class of metal alloys. They have exceptional strength and springiness, and they can be used as a stepping stone to create new nanostructured metals with, for example, even higher strength or useful properties as magnets. One of the stumbling blocks to exploiting these materials is understanding their structure, particularly at a length scale around 1 nanometer, which is a cluster of 20-50 atoms. This project supports research using a new technique in electron microscopy to measure that nanometer-scale structure. The new structural data will be used to explore how metallic glasses form in the first place during cooling of a molten metal alloy, and one of the ways metallic glasses can be used to create nanostructured metals. Understanding how metallic glasses form from metallic liquids may shed light on the glass transition in general, which is one of the grand challenges in materials physics. This project will also enhance the competitiveness of the U.S. technical workforce by disseminating advanced electron microscopy techniques through an online database of examples (http://tem.msae.wisc.edu/emdb/). Examples from the database are used in classroom teaching and by working scientists and engineers who want to upgrade their skills.