ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2024 27 levitator to prevent it from reacting with the environment[9]. Schematically, the environment around a sample is shown in Fig. 3. MODERN EXPERIMENTS TO UNDERSTAND GLASS FORMING ABILITY The formation of a glass is fundamentally a competition between the formation of the equilibrium crystalline phase (with inferior material properties) and the glass state with a broad range of technological applications. If the viscosity can be increased rapidly enough as the liquid cools, crystallization can be avoided and the amorphous phase solidifies. However, there are competing theories about how the structural coherence is built up. One such disagreement is whether or not the glass is built up in small structural units or a more global ordering that freezes in place. Neutron experiments are a way to distinguish these theories. However, the right systems must be selected for examination. It is important to find a system that allows for the glass-forming ability to change considerably just by altering the composition a little. One such system is Cu55-xZr45Alx, where changes in the concentration of aluminum substantially affect the glass-forming ability. In one recent experiment, our team prepared a solid crystalline sample of, say, Cu49Zr45Al6, and heated the sample with high powered lasers in a controlled environment at the SNS, preparing the system in a high temperature liquid above 1000°C. Direct pulsed beams of neutrons were then directed at the sample, taking data on how rapidly in time the structure de-coheres at different temperatures. This helps to identify how the different length scales change as a liquid is cooled toward its glass transition temperature[11,12]. After the scattering data were processed, the team was able to measure the likelihood that atoms are located near each other—quantified in the van Hove function, shown on the left in Fig. 4. As the nearest neighbor peak decreases this indicates atoms are leaving the nearest neighbor environment. Focusing on the area under the curve indicated in the van Hove function, a measurement can be taken on how rapidly the environment changes. This is called the “RK metric” and is shown for Cu49Zr45Al6 at about 1000°C in the right panel in Fig. 4. It can be observed that the local ordering, as measured by RK metric, decays rapidly, nearing almost no coherence after just a few picoseconds. This is exciting because for the first time we observe how rapidly atoms leave their local environments. The RK metric is a direct measurement of this. Conventional approaches to understanding order involve a bottom-up construction of the liquid or glassy structure from units, like how one might put a puzzle together. However, a more “top-down” approach is to try to visualize how glasses actually form, which is more like taking all the puzzle pieces and dropping them onto a table and gluing them in place. If the pieces all look the same, an ordered crystalline structure can be formed with long range periodicity. A model of a coherent glass includes very diverse structures/puzzle pieces that are very different. An atom leaving its nearest neighbor environment is critical to ordering in the liquid. These neutron studies are critical to understanding with process correctly mimics the formation of glass. Atoms in the liquid are always moving and the RK metric measures how quickly atoms disconnect from their local environments. This is shown schematically in Fig. 5 for a single green (zirconium) atom Fig. 4 — Left: The van Hove function gives the likelihood that there is an atom around an average atom in the liquid. The region in the box is the nearest neighbor region, showing that the likelihood of a nearest neighbor (the peak height) decreases dramatically over just a picosecond (ps) or so. Right: The RK metric is shown for Cu49Zr45Al6 at about 1000°C found by taking the area under the curve indicated in the van Hove function. Fig. 5 — Schematic of a zirconium atom leaving the immediate environment around a central palladium atom from the simulation of Zr75.5Pd24.5 shown in Fig. 1.
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