Atomic Structural Evolution in Cu-Zr and Ni-Nb Liquids and Glasses: A Measure of Liquid Fragility?
The glass forming ability (GFA) of metallic alloys is widely varied. Bulk metallic glasses (BMGs) have been identified in a number of alloy systems but far more compositions can be formed only when their liquids are rapidly quenched. Understanding the differences between these systems remains one of the most important problems in condensed matter physics. Understanding the structural evolution of metallic liquids as they are supercooled and quenched into glasses is critically important, not only for providing insight into the nature of the glass transition, but also for understanding technical aspects of glass formation and the thermal stability of the glassy solid.
In this talk, we discuss the results of high energy X-ray diffraction studies on Cu-Zr and Ni-Nb liquids and glasses. The temperature dependence of the X-ray structure factors has been measured in the glass from room temperature to above the glass transition temperature by means of stationary diffraction in a capillary while data in the equilibrium and supercooled liquid state were acquired using the Beamline Electrostatic Levitation technique. As will be shown, both the structure factors as well as the calculated total pair correlation functions display an anomalous evolution indicating a rapid acceleration of short-range order above the glass transition temperature. This behavior contrasts sharply with that observed in high glass forming ability metallic alloys suggesting a structural fragility metric distinguishing good glass formers from poor ones. We discuss the implications for this observation on our fundamental understanding of glass formation.