Icosahedral Short Range order and Bulk Glass Formation

1. Icosahedral Short Range order and Bulk Glass FormationKenneth F. Kelton, Washington University, DMR 0856199 Primary goal – To study ordering in supercooled liquids and glasses to develop a deeper understanding of glass formation and the glass transition. WU-BESL – Electrostatic levitation facility constructed at Washington University, which enables x-ray diffraction studies from levitated liquids (left). Icosahedral Order – Based on previous Reverse Monte Carlo studies of liquid diffraction data, we have demonstrated that the structures of many Zr-based liquids that form BMGs are dominated by icosahedral short-range order (ISRO). Based on the presence of a shoulder on the high-q side of the second peak in S(q), which is often taken to indicate ISRO, our most recent data suggest that this is not universally true for all BMGs. The shoulder is present in liquids that form Ni-Nb based BMGs (bottom left), but not in those that form Au-based BMGs (bottom right).

2. Icosahedral Short Range order and Bulk Glass Formation Kenneth F. Kelton, Washington University, DMR Graduate and Undergraduate Education Fourth year graduate student Nick Mauro (left) and first year graduate student Jennifer McKnight (right) taking x-ray diffraction data from levitated metallic liquids at the Advanced Photon Source (APS). Undergraduate junior Austin Hope (back) controls levitation of high temperature liquid at the APS, working with Research Scientist Anup Gangopadhyay (front) 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, he has created an Outreach Committee to encourage and coordinate outreach activities of students (graduate and undergraduate) and faculty. He has recently published a book: Nucleation in Condensed Matter – Applications in Materials and Biology Broader Relevance -Results will be of basic and technological interest, leading to a deeper understanding of glass structure, nucleation processes in phase transitions, and improved control over nanostructure production.

3. First Transparency – Intellectual Merit of Research – Additional Information Kenneth F. Kelton, Washington University, DMR 0856199 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). From data collected during an initial test of the facility at the Advanced Photon Source (APS) in August, 2009, we obtained the first evidence for medium range order (MRO) in a Pt-Zr liquid and confirmed a chemical ordering transition in a CuZr liquid, which likely contributes to easy glass formation in that alloy. In August, 2010, we completed more extensive studies of 98 liquids, including several that form bulk metallic glasses (BMGs), obtaining almost 1 TB of data. A preliminary analysis of these results demonstrates that MRO is much more widespread than thought. On a related issue, icosahedral short-range order (ISRO) is often argued to be important for BMG formation, since it raises the nucleation barrier for the competing crystal phases. However, our most recent scattering data indicate that ISRO is not a general requirement, illustrated in the figures provided with this report. The presence of a shoulder on the second peak in S(q) (generally taken to indicate ISRO) is present in a Ni-Nb-Ta liquid that forms a BMG, but not in a Au-based BMG forming liquid. To further probe this important point the atomic structures in the liquid will be obtained by a Reverse Monte Carlo analysis of the scattering data and the local topology analyzed. Upper Left – Electrostatic-levitated droplet in high-vacuum in WU-BESL. Liquid structural data are obtained from transmission high-energy x-ray diffraction studies at the Advanced Photon Source. Lower Left – S(q) for liquid Ni60Nb30Ta10 (bulk BMG forming liquid) at two temperatures, showing the development of a split second peak that is typically taken to indicate growing icosahedral short-range order (more readily observed in the inset). Lower Right – S(q) for liquid Au49Ag5.5Pd2.2Cu26.9Si16.3 (another bulk BMG forming liquid) at two temperatures, showing the absence of a split second peak, suggesting the absence of significant ISRO.