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.
The scientific goal of this project was to understand the atomic structure of glass, how it changes when the materials is heated up, and how it influences crystallization. Most materials area crystals, which means that the atoms inside them sit in regular arrays, like eggs in an egg carton. In a glass, the atoms are jumbled up, like marbles in a jar. Measuring the positions of the disordered, jumbled set of atoms in a glass takes special experiments. This project supported application and development of one of those tools, fluctuation electron microscopy. In fluctuation microscopy, we use a an extremely small probe beam of electrons, 1-2 nm across, formed by a powerful electron microscope to probe how the atoms fit together. One of the outcomes of this project is technology for forming electron probes with much greater flexibility in how big they are, which gives us finer control over the type of atomic arrangements we can measure. This project studied glasses made entirely of metallic elements. These metallic glasses can have exceptionally high strength for their weight, good biocompatibility for applications in human health, and are easier to form in micro-mechanical machines than conventional crystalline alloys. However, they only stay glasses at low temperature. At high temperature, some of the atoms spontaneously rearrange to create ordered crystals. We studied exactly how that happens, including how fast it happens, what kinds of atomic arrangements in the glass make it more likely to happen, and how we can control it, in a glass made mostly of aluminum. Glasses are closely related to liquids, which have the same disordered type of atomic structure. The difference is that in liquids the atoms are free to move around. This project also supported work on a new kind of electron microscopy technique for measuring how atoms move around in liquid metal. We have just begun to answer questions like, "How often do the atoms rearrange?" and "Do some kinds of arrangements last longer than others?" As we develop better and better answers to those kinds of questions, based on better and better experiments, we will be able to create new metallic glasses and glasses made of other elements, too, which have useful properties for a variety of practical applications. Finally, this project created opportunities and resources for students interested in learning about materials science and developing new materials science skill. Two graduate students worked on this project. One of them graduated with a Ph.D. and found a job in the semiconductor industry. The other one is still in school. Three undergraduate students gained valuable hands-on engineering experience working with the graduate students to do experiments. This project also supported continued development of the Electron Microscopy Database (http://emdb.msae.wisc.edu/), a web site full of free examples that teachers and students can use to learn some of the advanced electron microscopy techniques that we have developed in this project and in other research.
