Snuffing out a cigarette butt with a 10-ton boot would be excessive, but using the equivalent on certain metals can yield amazing results. By smashing an aluminum alloy between two anvils, researchers have created a metal that's as strong as steel but much lighter. If the process can be commercialized, it could yield better components for aircraft and automobiles, as well as metal armor light enough for soldiers to wear in battle.
Aluminum's main advantage is its lightness. But the most abundant metal in Earth's crust is also a weakling: It breaks apart under loads that heavier metals such as steel shoulder easily. For decades, scientists have been looking for a way to manufacture the aluminum equivalent of titanium, a lightweight metal that's stronger than steel, but without titanium's high cost.
In the new study, an international team of materials scientists turned to an emerging metal-processing technique called high-pressure torsion (HPT). Basically, HPT involves clamping a thin disk of metal to a cylindrical anvil and pressing it against another anvil with a force of about 60,000 kilograms per square centimeter, all while turning one anvil slowly. The researchers also kept the processed samples at room temperature for over a month, in a common metallurgical process called natural aging. The deformation under the enormous pressure plus the aging alters the basic structure of metals at the nanoscale—or distances measured in billionths of a meter.
And indeed, when the team subjected an alloy of aluminum called aluminum 7075 (which contains small percentages of magnesium and zinc) to the process, the metal attained a strength of 1 gigapascal, the researchers report in the current issue of Nature Communications. That's equal to some of the strongest steels and more than three times higher than conventional aluminum. A meter-square plate of the processed alloy could withstand the weight of a fully loaded aircraft carrier.
To find out why the alloy had gotten so much stronger, the team examined samples using a technique called atom probe tomography. Resembling a combination of an electron microscope and a CT scanner, the method showed that HPT had deformed the lattice of atoms in the alloy into an unprecedented arrangement. Instead of the normal structure found in the conventional metal, HPT had created what the researchers call a hierarchical nanostructure: the size of the aluminum grains was reduced, and the zinc and magnesium atoms clustered together in groups of various sizes, depending on whether they were located inside the aluminum grains or on the edges (see photo).
Exactly how this arrangement creates stronger aluminum is unclear, says co-author Simon Ringer, director of the Electron Microscope Unit at the University of Sydney in Australia. He says the atoms at the edges of the grains seem to be bonded tightly to atoms at adjoining grain edges. Whatever the physics, he says, the hierarchical structures are "very potent for strengthening."
Ringer adds that even though the experiments produced only laboratory quantities of the superstrength alloy, the process could quickly be adapted to produce small components that require high strength but low weight, such as biomedical implants. Co-author and materials scientist Yuntian Zhu of North Carolina State University in Raleigh says there is strong incentive to scale up the process because the alloy could be useful for "many lightweight, energy-efficient applications such as aerospace, transportation, and body armor."
The experiments "have achieved remarkable strength" in a conventional commercial aluminum alloy, says materials scientist Terence Langdon of the University of Southern California in Los Angeles. The research team has also demonstrated "the exceptional capabilities provided through processing by high-pressure torsion," a technique that Langdon and others have been working with for several years.
Materials scientist Yuri Estrin of Monash University in Melbourne, Australia, calls the results exciting and agrees that the hierarchical nanostructures "appear to be crucial to the spectacular enhancement of [the alloy's] strength."