The nanoscale secret to stronger alloys

Long before they knew they were doing it, as long ago as the Wright Brothers’ first airplane engine, metallurgists were incorporating nanoparticles in aluminum to make a strong, hard, heat-resistant alloy. The process is called solid-state precipitation, in which, after the melt has been quickly cooled,  atoms of alloying metals migrate through a solid matrix and gather themselves in dispersed particles measured in billionths of a meter, only a few-score atoms wide.

 

Key to the strength of these precipitation-hardened alloys is the size, shape, and uniformity of the nanoparticles and how stable they are when heated. One alloy with a highly successful combination of properties is a particular formulation of aluminum, scandium, and lithium, whose precipitates are all nearly the same size. It was first made at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) in 2006 by a team led by Velimir Radmilovic and Ulrich Dahmen of the Materials Sciences Division.

 

These scientists and their colleagues have now combined atomic-scale observations with the powerful TEAM microscope at Berkeley Lab’s National Center for Electron Microscopy (NCEM) with atom-probe tomography and other experimental techniques, and with theoretical calculations, to reveal how nanoparticles consisting of cores rich in scandium and surrounded by lithium-rich shells can disperse in remarkably uniform sizes throughout a pure aluminum matrix.

 

“With the TEAM microscope we were able to study the core-shell structure of these nanoprecipitates and how they form spheres that are nearly the same in diameter,” says Dahmen, the director of NCEM and an author of the Nature Materials paper describing the new studies. “What’s more, these particles don’t change size over time, as most precipitates do. Typically, small particles get smaller and large particles get larger, a process called ripening or coarsening, which eventually weakens the alloys. But these uniform core-shell nanoprecipitates resist change.”

 

 

In the aluminum-scandium-lithium system the researchers found that, after the initial melt, a simple two-step heating process creates first the scandium-rich cores and then the lithium-rich shells of the spherical particles. The spheres self-limit their growth to achieve the same outer dimensions, yielding a lightweight, potentially heat- and corrosion-resistant, superstrong alloy.

 

 “The problem is that, by itself, lithium may not live up to its promise,” says Dahmen, a long-time collaborator with Radmilovic. “The trick is to convince the lithium to take on a useful crystalline structure, namely L1₂.”

 

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 Dahmen explains, “once atoms are in place in L1₂, it’s difficult for them to move.”

 

 

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 brightness. In the shells and cores, however, the corner columns and the face-centered columns differ in contrast – the face-centered columns are pure aluminum but the corner columns are mixed. By supplementing the high-resolution TEAM images with data from other experimental techniques it was possible to use brightness and contrast to calculate the kinds of atoms in each column.

By employing first-principles calculations, team members Colin Ophus and Mark Asta were able to model the effect of lithium on the solid-state precipitation of scandium, stimulating a sudden burst of nucleation, and also to understand why, because of the thermodynamic properties of the two metals interacting with aluminum and with each other, the precipitates are so uniform and stable.

 

Radmilovic says, “Colin and Mark showed that lithium and scandium like each other. They also showed that by using the aluminum columns as a standard, we can calculate the intensity of the scandium and lithium by the brightness of the spot.” In the shells, the corner columns contain aluminum and about 10-percent lithium. In the cores, the corner columns contain all three metals.

 

As good an alloy as aluminum-scandium-lithium is, its use may be limited by the cost of rare scandium, presently ten times the price of gold. By understanding how the alloy achieves its remarkable characteristics, the researchers fully expect that other systems with core-shell precipitates can be controlled by the same mechanisms, leading to new kinds of alloys with a range of desirable properties.