11-08 :: August 2011
nanotimes News in Brief
Aluminum Alloys // The Nanoscale Secret to Stronger Alloys
© Text: LBL
cientists and their colleagues at Lawrence Ber- keley National Laboratory (Berkeley Lab) have
combined atomic-scale observations with the po- werful TEAM microscope at Berkeley Lab’s Natio- nal 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 U. Dahmen, the director of NCEM and an author of the Nature Materials paper descri- bing 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 parti- cles get larger, a process called ripening or coarse- ning, which eventually weakens the alloys. But these uniform core-shell nanoprecipitates resist change.”
In the aluminum-scandium-lithium system the resear- chers 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 achie- ve the same outer dimensions, yielding a lightweight, potentially heat- and corrosion-resistant, superstrong alloy.
“Scandium is the most potent strengthener for alu- minum,” says NCEM’s V. Radmilović, who is also a professor of metallurgy at the University of Belgrade, Serbia, and an author of the paper. “Adding less than one percent scandium can make a dramatic diffe- rence in mechanical strength, fracture resistance, corrosion resistance – all kinds of properties.” Becau- se scandium diffuses very slowly through the solid aluminum matrix, the solid mix must be heated to a high temperature (short of melting) before scandium will precipitate.
Lithium is the lightest of all metals (only hydrogen and helium are lighter) and brings not only lightness to an aluminum alloy but, potentially, strength as well. Lithium diffuses much more rapidly than scan- dium, at much lower temperature.
“The problem is that, by itself, lithium may not live up to its promise,” says U. Dahmen, a long-time col- laborator with Radmilović. “The trick is to convince the lithium to take on a useful crystalline structure, namely L12.”