Graduate and Undergraduate Education

1. Relations between Structure, Phase Formation and Phase Transitions in Supercooled Metallic Liquids and Glasses Kenneth F. Kelton, Washington University, DMR 0856199 • Primary goal of grant – To study ordering in supercooled liquids and glasses in order to develop a deeper understanding of glass formation, glass stability, and the glass transition. • Method – Used electrostatic levitation facility • constructed at Washington University to obtain • x-ray structural and property data for equilibrium • and supercooled liquids. Density measured • using the shadow method (schematic to right). • Result – First measurements of the density of liquid Cu-Zr as a function of alloy concentration – yield new fundamental information as well as practical insight into bulk metallic glass (BMG) formation. • Thermal expansivity shows local maxima at the best • BMG forming compositions (figure to right). • Ramifications - High thermal expansivity implies a • fragile liquid - in contradiction with widely held view • that BMG forming liquids are strong. • Two directions for resolution – both have • significant impact on fundamental understanding • Either the correlation between fragility and • expansivity is wrong, which would violate • most firmly held theoretical views, • or BMG forming liquids have a fragile to • strong transition below the liquidus • temperature. • Practical consequences – Search for metallic liquids • with a high coefficient of thermal expansion as a new • method for identifying BMG forming systems (supported • by our studies in other glass forming alloys).

2. Relations between Structure, Phase Formation and Phase Transitions in Supercooled Metallic Liquids and Glasses (DMR-0856199) Kenneth F. Kelton, Washington University, DMR 0856199 Graduate and Undergraduate Education Undergraduate sophomore Walter Fu (left) explaining to Professor K. F. Kelton (right) the results of his analysis of the structure factor of liquid Pd-Zr. First year graduate student, Jennifer McKnight measuring the temperature of a Cu-Zr liquid before quenching it to form a metallic glass. Outreach - Kelton is a member and past president of UCSAC, a science advisory council dedicated to improving science education. As Chair of the Physics department at Washington University, Kelton has created an Outreach Committee to encourage and coordinate outreach activities of students (graduate and undergraduate) and faculty. Broader Relevance -Results will be of basic and technological interest, leading to a deeper understanding of phase transitions, an ability to better predict good glass forming compositions and methods for improved control over nanostructure production.

3. First Transparency – Intellectual Merit of Research Kenneth F. Kelton, Washington University, DMR 0856199 Metallic glasses are finding uses in an increasing number of applications from sporting equipment to precision gears to medical implants. The focus of our NSF research is to determine liquid and glass structures and to understand how they impact glass formation, glass stability and phase transitions. In the first year of the grant, we finished construction of a new electrostatic levitation (ESL) facility optimized for x-ray scattering studies of containerlessly processed liquids (WU-BESL, for Washington University Beamline ESL) and completed structural studies on 98 liquids. We recently implemented a density measurement technique on WU-BESL to evaluate a claim made by Li et al., Science 322, 1816 (2008). They demonstrated by careful measurements of density change on crystallization that the best bulk forming glasses (BMGs) had the highest density, indicating the most ordered and best packed structures. They asserted that this was true of the liquids. If true, this would have confirmed some widely held views of glass forming liquids. We demonstrated, however, that this was not true. There were no local maxima in the densities of the liquid at the best glass forming composition. Instead, we found that there were local maxima in the thermal expansivity of those liquids. From free-volume and energy landscape arguments, this implies that these liquids are more fragile than those that do not form BMGs. This is in contradiction to widely held views. There appear to be only two ways to resolve this contradiction. Either the correlation between thermal expansivity and fragility is incorrect, which will require a major rethinking of some of the theoretical underpinnings of this field, or these BMG liquids undergo a fragile to strong transition between the liquidus temperature and the glass transition temperature. If true, this could be a general feature of all BMG forming liquids.. There has been some theoretical and experimental support for this transition, but there has never such a systematic study as we have done. Also, our result provides a new way to look for glass forming liquids before making the glass. All others depend on parameters that can only be known after the glass is formed. Figures Upper Right – A schematic diagram of the shadow technique used to measure the density. Video data are taken from a levitated sample that is backlit. These are analyzed to determine the volume. Calibration spheres are used for volume calibration, allowing the volume to be determined to within 0.3 %. The technique was modeled after one developed by Robert Hyers and his group at University of Massachusetts.

4. Intellectual Merit of Research (cont) Kenneth F. Kelton, Washington University, DMR 0856199 Lower Right – The volume expansivity for Cu-Zr liquids at the liquidus temperature as a function of Zr concentration (dark circles) and the maximum thickness obtained for glass formation (red triangles). (Note that the critical thickness data are taken from Li et al., Science 322, 1816 (2008) and used for comparison with the expansivity data). The expansivity shows peaks that are precisely at the locations of the best glass forming liquids. The increased expansivity indicates that the BMG forming liquids are more fragile rather than more strong, in contradiction to wildly held beliefs and supports the idea of a strong/fragile transition. Such a transition may be a general feature of BMG formation. Ongoing studies by us support this conclusion.