Coupled Nucleation, Nanostructure Formation and Hydrogen Storage in Metallic Glasses and Quasicrystals

Kenneth F. Kelton, Washington University, DMR 0606065

Glass Transition and Icosahedral Ordering• Vanishingly small interfacial free energy between glass and primary crystallizing icosahedral quasicrystal ( = 0.006 ± 0.002 J/m2)• Reverse Monte Carlo analysis of high energy x-ray data from levitated liquids show increasing icosahedral order with cooling through glass transition

Honeycutt-Andersen Indices

Coupled Nucleation, Nanostructure Formation and Hydrogen Storage in Metallic Glasses and Quasicrystals

Kenneth F. Kelton, Washington University, DMR 0606065

Physical Science Orientation for Freshman Undergraduates at Washington University

After Kelton gave an overview of materials physics to a group of freshman undergraduates interested in majoring in the physical sciences, graduate students supported by this grant explain their research.

Left - Graduate student Vic Wessels (back, left) explains hydrogen absorption measurements.Right - Graduate student Lydia Longstreth (back, right) explains how metallic glasses are prepared by rapidly quenching the liquids.

Vic Wessels Lydia Longstreth

First Transparency Our NSF sponsored research is focused on developing a better understanding of liquid and glass structures and how this impacts glass formation and phase transitions in the glasses. The glass transition, a universal phenomena, is still poorly understood after decades of study. In fact, the nature of the glass transition has been deemed by some as one of the fundamental outstanding fundamental problems in condensed matter physics. It has long been thought that the glass transition is connected to the development of local icosahedral order. Over twenty years ago, under the support of of our first NSF grant, we were the first to demonstrate icosahedral short-range order (ISRO) in a metallic glass, from estimates of the interfacial free energy between an Al-based metallic glass and the icosahedral quasicrystal phase that nucleates from it. Those conclusions were based on estimates of the steady-state nucleation rates and required several assumptions and approximations for the thermodynamic and kinetic quantities that were needed to analyze the nucleation data. More recently, we directly demonstrated the growth of ISRO with supercooling in transition metal liquids, based on the development of a second peak in S(q). ISRO is now considered by many to be a key characteristic of the glass structure, although how it associated with the glass transition is still unclear. Recently, we made the first measurements of the time-dependent rate of an icosahedral quasicrystal in a Zr59Ti3Cu20Ni8Al10 metallic glass. These are the first quantitative time-dependent nucleation rate measurements in any metallic glass. They reveal a vanishingly small interfacial free energy (less than 0.01 J/m 2), that indicates extremely strong ISRO in the glass. Under joint NSF and NASA funding, we have also succeeded in measuring the x-ray diffraction from these liquids from above the melting temperature through the glass transition using a containerless technique that avoids heterogeneous nucleation and sample contamination. The structure factors, S(q) extracted from these experimental data also show the growing high-q shoulder on the second peak, signaling icosahedral ordering. We have now constructed realistic atomic structures for the liquid/glass using a Reverse Monte Carlo method. These structures were analyzed in terms of their Honeycutt-Andersen (HA) indices. Left Figure - shows the 1551 HA index, characteristic of icosahedral order. It is shown how this index relates to a perfect icosahedron. It is important to realize, however, that the 1551 is not necessarily indicative of complete icosahedra in the liquid/glass but of a nearest neighbor environment that contains ISRO.Right Figure - shows that the 1551 HA index increases with supercooling of the liquid, saturating out below the glass transition temperature. Not shown are the decreasing density of non-icosahedral indices. This is the first experimental evidence that the frustration underlying the glass transition is indicative of the growing ISRO. This has been theoretically predicted (see for example the theoretical work of Tarjus et. al., J. Phys. Condens. Matter, 17, R1143 (2005)).

Kenneth F. Kelton, Washington University, DMR 0606065

Second Transparency

Graduate Student Presentations of their ResearchThis illustrates some of the educational aspects and outreach aspects of our research. Graduate students are shown talking with a group of freshman students at Washington University who have expressed an interest in possible majors in the physical sciences or engineering. Students in my research group are routinely introduced to such visitors as well as to visiting scientists to the department to talk with them about their research projects.

Outreach The PI is a member and past president of UCSAC, a science advisory council dedicated to improving science education in University City, MO, a racially and socio-economically diverse school system adjacent to

Washington University. Members of this committee work with teachers and students in the school system, giving regular seminars on topic of current scientific interest and arrange for students to work in their labs at Washington University and other local universities.

In addition to invited professional talks, the PI gave two public lectures:1. “Metals and Civilization – From Bronze Swords to Glass Golf Clubs,” Norman Lecture on April 12, 2007 at

Arkansas Tech, Russellville, Arkansas. The Norman Lectures are general lectures suitable for the entire undergraduate body as well as members of the local community.

2. “Metals and Civilization” – Washington University Physics Department Saturday Morning Lecture Series on March 24, 2007. This is a lecture series for members of the local community.

and one plenary lecture for a general scientific body:3. The plenary lecture for the Arkansas Academy of Sciences on April 13, 2007 at Russellville, Arkansas. The

title of the talk was “ The Nucleation of First Order Phase Transitions – Fahrenheit to Arctic Fish”

Kenneth F. Kelton, Washington University DMR 0606065