Nice and nano does it: Particulate improvements to alloys

Tuneable alloys

Diffraction, microscopy and theoretical calculations have all been combined to show how aluminium nanoparticles alter the alloy matrix in which they are embedded. The study could lead to approaches for making tuneable materials for engineering, construction and aeronautics based on nanotechnology.

Despite our perception that nanotechnology is a modern discovery, metallurgists have been incorporating nanoparticles into aluminium alloys unwittingly when they heat-treated them to make strong and hard alloys. The process of solid-state precipitation is now well known although certainly not fully understood and by no means entirely controllable. However, US scientists hope to remedy that situation.

The team has combined observations on the TEAM microscope at Berkeley Lab's National Center for Electron Microscopy with atom-probe tomography, diffraction, other experimental techniques and theoretical calculations to reveal how nanoparticles consisting of cores laden with scandium and surrounded by lithium-rich shells can disperse within a solid aluminium matrix. Of particular note is the remarkably uniform size throughout the aluminium matrix of these nanoparticles.

Seemingly, the key to the immense strength of precipitation-hardened alloys is the size, shape and, most importantly, the uniformity of the nanoparticles trapped within. The particularly heat-stable alloy of aluminium, scandium and lithium first created by the Berkeley Lab team led by Velimir Radmilovic and Ulrich Dahmen in 2006 is among the most intriguing.

TEAM effort

"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," explains Dahmen. "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."

The researchers found that after the initial melt, a simple two-step heating process first creates the scandium-rich cores and then the lithium-rich shells of the spherical particles. These spheres are self-limiting in terms of growth so that they ultimately reach the same outer dimensions, which makes for a super-strong alloy.

"Scandium is the most potent strengthener for aluminium," says Radmilovic. "Adding less than one percent scandium can make a dramatic difference in mechanical strength, fracture resistance, corrosion resistance - all kinds of properties." Critically the lithium must adopt the L12 crystalline structure to do its job well. The L12 unit cell resembles a face-centred cubic cell and for alloy inclusions it is among the strongest and most stable structures. "Once atoms are in place in L12, it's difficult for them to move," Dahmen explains.

Alloy ally

Radmilovic developed the idea of alloying both scandium and lithium with aluminium through a series of specific heating and cooling steps. His insight is based on long experience with the separate properties of aluminium-lithium and aluminium-scandium alloys and a deep understanding of how they were likely to interact. The lithium effectively behaves as a catalyst to trigger a "burst of nucleation" in the scandium. Once all three metals are mixed, melted and quenched, the presence of the lithium then lowers the temperature needed to coax scandium to form dense core structures; a colligative effect akin to lowering the freezing point of water by adding salt. The cores formed at this point average a little over 9 nm in diameter but are not uniform in size.

The next step is to heat the alloy again to 190 Celsius. At this temperature the scandium atoms are immobile, but the freely moving lithium can then form a shell around the scandium-rich cores; akin to water crystallizing around a speck of dust in the air to create a snowflake. At this point, the shells average about 10.5 nm but their thickness is not uniform. However, those particles with a thicker core have a thinner shell and those with a thinner core have a thicker shell. Core size and shell thickness are "anticorrelated", the team explains, which results in the particles within the matrix being "size focused."

The aluminium-scandium-lithium alloy has rather desirable properties but scandium is rare and expensive. However, the insights offered by the current research might allow other systems containing core-shell precipitates to be produced using the same methods and so produce equivalent, if not better, analogues.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.