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nanotimes News in Brief

The L12 unit cell resembles a face-centered cubic cell, among the simplest and most symmetric of crystal structures. Atoms occupy each corner of an imaginary cube and are centered in the cube’s six faces; in the L12 structure, the kinds of atoms at the corners may differ from those at the centers of the faces. For alloy inclusions it’s one of the strongest and stablest of structures because, as U. Dahmen ex- plains, “once atoms are in place in L12, it’s difficult for them to move.”

Dahmen credits V. Radmilović with the “intuition” to alloy both scandium and lithium with aluminum, heating and cooling the material in a specific series of steps. That intuition was based on Radmilović’s long experience with the separate properties of aluminum-lithium and aluminum-scandium alloys and a deep understanding of how they were likely to interact. He drew up a recipe for the proportions of ingredients in the initial melt and how to cool and rewarm them.

The key to the process was to use lithium as a kind of catalyst to force a “burst of nucleation” in the scan- dium. After the three metals are mixed, melted, and quickly cooled or quenched, lithium serves to lower the heating needed to coax scandium to form dense core structures – although the solid mix must still be heated to 450° Celsius (842° Fahrenheit) for 18 hours to form these cores, made of aluminum, lithium, and scandium. The cores average a little over nine nano- meters in diameter but are not uniform in size.

Next the alloy is heated again, this time to 190˚ Cel- sius (374˚ F) for four hours. At the lower temperature the scandium is immobile; the freely-moving lithium forms a shell around the scandium-rich cores, much

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as water in a cloud crystallizes around a speck of dust to make a snowflake. The shells average about 10.5nm in thickness, but their thickness is not uni- form.

What’s remarkable, though, is that when a core is thicker than the average, the shell is thinner than the average, and vice versa: the smaller the core, the faster the shell grows. Core size and shell size are “anticorrelated” and the result is “size focused.” Whole spheres still vary somewhat, but the diffe- rences are much less than among the cores alone or the shells alone.

The structure of the cores and shells embedded in aluminum seems equally remarkable. Pure aluminum itself has a face-centered-cubic structure, and this structure is seamlessly repeated by the L12 structure of both the cores and the shells, perfectly joined with no dislocations at the interfaces between core, shell, and matrix.

U. Dahmen says, “It’s the scandium-rich cores that convince the lithium to take on the useful L12 struc- ture.”

Using the TEAM microscope and a special imaging technique to look down at the tops of the regular rows of columns of atoms, the L12 structure reveals itself in groups of interlocking squares, with four columns of atoms at the corners and five columns of atoms at the lined-up centers of the faces.

In pure aluminum, all the dots are the same bright- ness. In the shells and cores, however, the corner columns and the face-centered columns differ in contrast – the face-centered columns are pure alumi-