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Important insights into the mechanisms underlying spiral nanostructure formation in solidifying metal alloys have been gained by Ashwin Shahani at the University of Michigan and colleagues. The team’s study combined new advanced imaging techniques with machine learning and their findings could lead to fast and scalable techniques for manufacturing light-controlling metamaterials.
As molten metal alloys cool, solid structures begin to form internally as small atomic clusters crystallize. By fine-tuning the cooling conditions, researchers can steer these crystallization mechanisms to produce a range of different structures in solid materials.
Particularly intriguing structures can arise through non-equilibrium cooling – including “spiral eutectics”, which arise in mixtures of two or more solid metal phases. These materials can spontaneously self-arrange themselves into spirals resembling a DNA helix.
Impractical to manufacture
Despite the exciting technological applications of spiral eutectics, researchers have so far have been unable to isolate their ideal growing conditions. As a result, it is currently impractical to manufacture spiral structures on large scales.
Several, often competing studies have suggested a variety of possible mechanisms through which these spirals could form. Until now, however, none of these mechanisms has been observed experimentally, creating uncertainty over how the overall process unfolds. Any confirmation would require 3D, time-resolved measurements, on length scales ranging from nanometres to microns – which is a tall order for current imaging techniques.
Swirling light beams carve intricate patterns
Now, Shahani’s team have developed an advanced new imaging technique that combines the multiscale, 3D observations of optical and electron microscopes with atomic-scale images of spiral formation. They then used a specialized machine learning algorithm to comb through these data for any notable formation mechanisms.
Using this technique, the team uncovered a two-step pathway to spiral formation in cooling zinc-magnesium alloys. They discovered that both solid metal phases begin to form independently at the defects on solid tetrahedral crystals. As they grow, these phases become strongly coupled, producing intricate nanospirals as they solidify, the team reports in Small.
Importantly, these structures display an inter-phase spacing comparable to the wavelength of infrared light. This property would make them suitable for photonic metamaterials, which can control the flow of visible and infrared light. The team’s discovery reveals for the first time how a molten metal’s cooling conditions can be fine-tuned to bring about nanospiral self-assembly, allowing for the fast, scalable manufacture of these advanced materials. Shahani and colleagues now hope to use their new imaging techniques to explore spiral eutectics in more complex alloys; potentially leading to even more intriguing spiral formation mechanisms.