ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2024 26 transition temperature”). The atoms are not organized into repeating units, like a crystal, but there is measurable structural coherence. For instance, if we make a metallic glass by rapidly cooling a liquid, we can take a high energy x-ray diffraction image of it and observe that there are regions on the detector where constructive and destructive interference occur—a hallmark of order[4]. Glasses have significant short range and medium range order, however, that ordering does not change measurably with time and the viscosity is enormous—greater than 1013 Poise[5]. That ordering has been shown to be extremely diverse and extremely difficult to characterize with any precision. A common approach is to build up the structure from preferred local units, as schematically shown in Fig. 1 for a zirconium-based liquid. The red circles help the eye to observe possible clustering around palladium atoms in the liquid. The preferred structures to focus on are often icosahedral, which results in a disordered global structure, producing a geometrically frustrated state incompatible with the long-range periodicity of a crystal. This approach does not fully explain why medium range order exists in not only glasses but also liquids, where the structures are rapidly changing, often on the order of picoseconds (10-12 seconds). The fact that structures in the liquids are constantly changing is the other major difference between metallic glasses and liquids. The similarity in scattering signals between the two states of matter is remarkable and leads us away from considering structures alone. In Fig. 2, a schematic is shown of a liquid metal sample levitated between electrodes[7] and a beam of x-rays scatters from the sample producing a pattern on a far screen. The different patterns can be analyzed producing a scattering signal, S(q), whose details provide information about the static structure. Details are available elsewhere on the interpretation of these signals[4]. A more useful approach is to consider how rapidly structures in the liquid change. Of all the physical properties that can be measured (density, heat capacity, etc.), the viscosity perhaps changes the most dramatically over experimentally accessible temper- atures, some 17 orders of magnitude. To provide a sense of scale, the ratio of the highest to lowest viscosity accessible in an experiment is roughly the ratio of earth’s distance to the sun against the width of a human hair. There are fantastic examples of utilizing the temperature dependency of the viscosity for technological applications, such as using silica glass near the glass transition temperature to create glass art and heating Ti-based metallic glasses[8] to form dental implants with superior performance compared to other metals, ceramics, and composite materials. The example scattering data shown in Fig. 2 are representative of many different metallic glass forming systems. It is remarkable that small changes in structural coherence correspond to enormously large changes in viscosity. A modern approach to understanding glass forming ability is to consider how quickly structures in the liquid fall apart or “de-cohere.” To do this requires another tool with a unique capability to exchange energy with the atoms: neutrons. NEUTRON SCATTERING Liquids are dynamic, with the atoms moving extremely quickly, e.g., on the order of picoseconds (10-12 seconds). X-rays only provide “static” information, that is, an average snapshot of the liquid. Neutrons are extraordinary probes in that they can behave like waves if cooled so that they only have a few millielectron-volts (meV) of energy. This is a very small amount of energy. When carrying a box of books up the stairs, a person expends about a million, billion, billion (1024) times this energy. However, in modern pulsed neutron sources such as the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory in Tennessee, scientists can direct beams of 20 meV neutrons on a liquid metal sample and use the information to not only measure the structure but also how it changes. In an inelastic neutron scattering experiment from a pulsed source—called a time-of-flight experiment—researchers can investigate how quickly the atoms around a central atom are changing. To conduct these experiments, a metallic sample is separated in an electrostatic Fig. 2 — Schematic of a levitated metallic liquid sample (left) and the associated scattering signal for a zirconium-based metal (Vit 106a). Fig. 3 — A metallic sample levitated in the neutron electrostatic levitator. Neutrons enter from the left and scatter from the sample inelastically allowing technicians to measure the structure and observe changes over time. Image reproduced with permission from Ref. 10.
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