#eutectic

Unraveling the fundamental principles of eutectic solidification with real-time, nanoscale imaging

During eutectic solidification, a mixture of two or more solids self-assemble, forming composite microstructures ranging from ordered layers to intricate maze-like patterns that underlie properties like tensile strength or ductility. Up to this point, researchers have not understood what conditions drive eutectics to form certain patterns, which is critical for designing reproducible next-generation eutectic composites. Capturing real-time solidification of an aluminum-nickel eutectic alloy (Al-Al3Ni) in nanometer resolution revealed that increasing the solidification velocity shifts microstructure from irregular and faceted to regular and rounded, according to a study led by University of Michigan researchers recently published in Acta Materialia.

Graphic granite.

Looking like an ancient Sumerian clay tablet engraved with cuneiform writing, this rock formed during the crystallisation of a magma. As the temperature dropped below a tipping point, the pink feldspar and grey quartz separated out from each other in a process called exsolution, and grew simultaneously at a constant gentle rate to form this texture. Graphic texture is not limited to the magmas of silica rich granites (known as leucogranites, the prefix leuco indicating siliceous richness), but is most commonly found in them, particularly in pegmatites, the last silica and rare element rich remnants of already crystallised granites, accounting for the usual absence of any dark minerals such as mica or hornblende.

Loz

Sample from Knickel's feldspar quarry at Bedford, NY, originally in the USGS Petroographic Collection, now in the Smithsonian's.

Image credit: Ken Larsen/Smithsonian Institution.

A paper on its origin: http://www.minsocam.org/ammin/AM71/AM71_325.pdf

Small molecule organic eutectics show potential for replacing plastics

Plastics have long been a mainstay in modern manufacturing, but their environmental impact has increased demand for eco-friendly alternatives. Researchers at The University of Warwick have made significant progress in the search for sustainable alternatives to conventional plastics. In response to growing environmental concerns, the move towards a circular economy and changing consumer preferences, the research team has identified that certain mixtures of small organic molecules—materials created by mixing crystalline components—form interesting glasses and viscous liquids. These so-called organic eutectics are promising candidates for replacing polymers in various products.

Al, Si 12, Na 0.02 (wt%), eutectic alloy - refined microstructure

Processing As-cast (metal mould) [...] Technique Reflected light microscopy Length bar 400 μm / 80 µm Further information This sample is a casting alloy of eutectic composition. The microstructure has been refined by modification through the addition of a small amount of sodium to the alloy composition. The large silicon plates have been altered to a more favourable fibrous form, leading to an enhancement of the mechanical properties due to the refinement of the microstructure. There is also a change to a planar interface during solidification, which minimises porosity in the casting. Contributor Prof T W Clyne Organisation Department of Materials Science and Metallurgy, University of Cambridge

3D pattern generation via chemical vapor deposition of ceramic eutectic system for novel solid-state phosphors

The eutectic structure of metals and ceramics occurs when multiple solid phases solidify from a liquid phase, forming a three-dimensional (3D) pattern through a self-organizing phenomenon. Traditionally, it was believed that eutectic structures could only be obtained through a melt-solidification process. Researchers at Yokohama National University (YNU) have developed a chemical vapor deposition (CVD) process that allows precursor gases to react and generate solid-state composites with ordered structures in a YAG (yttrium alumina garnet)-alumina ceramic eutectic system. In their study, they observed the growth of spatially ordered rod- and lamellar-shaped YAG crystals within an alumina matrix on a sapphire wafer under Al-rich conditions. Conversely, under Y-rich conditions, they observed the growth of ordered alumina crystals with a YAG matrix. The choice of sapphire seed crystal and the composition of the precursor determined the 3D patterns. Compared with the melt-solidification process, the CVD process expanded the range of chemical compositions that could generate such patterns.

Al 67, Cu 33 (wt%), eutectic alloy

System: Al-Cu

Composition: Al 67, Cu 33 (wt%)

[...]

Processing: Unidirectionally solidified

[...]

Sample preparation: Etched in sodium hydroxide solution

Technique: Reflected light microscopy /  Scanning electron microscopy (SEM)

Length bar: 80 μm / 15 μm

Further information: This alloy is of the eutectic composition and has solidified with a lamellae eutectic structure. The Al and q phases form co-operatively. The eutectic lamellae grows in the principal direction of heat flow; the lamellae structure is stabilised by the high temperature gradient. In the lower part of the micrograph the lamellae structure breaks down. /  This SEM image shows the lamellar eutectic very clearly. The interlamellar spacing is about one micron. There are several imperfections in the lamellar structure, which have arisen from irregularities and disturbances during growth.

Contributor: Prof T W Clyne

Organisation: Department of Materials Science and Metallurgy, University of Cambridge

Nanospirals that form as molten metals solidify could be key to new materials—and invisibility

Humans have been cooling metal mixtures from liquid to solid for thousands of years. But surprisingly, not much is known about exactly what happens during the process of solidification. Particularly puzzling is the solidification of eutectics, which are mixtures of two or more solid phases.

Ashwin Shahani, an assistant professor of materials science and engineering at the University of Michigan, is working to solve the mystery of eutectic solidification, and his research has revealed an intricate and beautiful universe of nanoscale rods, sheets and spirals that form spontaneously in cooling metal alloys.

We sat down recently to talk with him about his latest paper, "Multi-Step Crystallization of Self-Organized Spiral Eutectics," and how it could lead to a new generation of lightweight alloys and optical products with properties superior to monolithic materials.

Researchers gain control over internal structure of self-assembled composite materials

Composites made from self-assembling inorganic materials are valued for their unique strength and thermal, optical and magnetic properties. However, because self-assembly can be difficult to control, the structures formed can be highly disordered, leading to defects during large-scale production. Researchers at the University of Illinois and the University of Michigan have developed a templating technique that instills greater order and gives rise to new 3-D structures in a special class of materials, called eutectics, to form new, high-performance materials.

The findings of the collaborative study are published in the journal Nature.

Eutectic materials contain elements and compounds that have different melting and solidification temperatures. When combined, however, the composite formed has single melting and freezing temperatures—like when salt and water combined to form brine, which freezes at a lower temperature than water or salt alone, the researchers said. When a eutectic liquid solidifies, the individual components separate, forming a cohesive structure—most commonly in a layered form. The fact that eutectic materials self-assemble into composites makes them highly desirable to many modern technologies, ranging from high-performance turbine blades to solder alloys.