Purdue University (West Lafayette, Indiana, USA) researchers have found two methods to create aluminum alloys rivaling the strength of stainless steel. One application could be to design wear- and corrosion-resistant aluminum coatings for the electronics and automobile sectors, they say.
“Most lightweight aluminum alloys are soft and have inherently low mechanical strength, which hinders more widespread industrial application,” says Purdue’s Xinghang Zhang, materials engineering professor.
The higher-strength aluminum is made possible by introducing “stacking faults,” or distortions in the crystal structure. A metal’s crystal lattice is made up of a repeating sequence of atomic layers. If one layer is missing, there is a stacking fault. Meanwhile, “twin boundaries,” with two layers of stacking faults, can form. While these are easy to produce in metals like copper and silver, researchers say they are hard to introduce in aluminum because of its high stacking fault energy. This energy forms as a result of irregularities in the normal stacking sequence.
One type of stacking fault, called a 9R phase, is particularly promising. “You want to introduce both nanotwins and 9R phase in nanograined aluminum to increase strength and ductility and improve thermal stability,” Zhang explains. Nanotwins are small linear boundaries in a metal’s atomic lattice that have identical crystalline structures on both sides, allowing them to be more resistant to fatigue.
According to Purdue postdoctoral researcher Sichuang Xue, one aspect of their research found a successful shock-induced 9R phase—accomplished by bombarding ultrathin aluminum films with tiny micro-projectiles of silicon dioxide (SiO2). The micro-projectiles are ejected by laser beam at a velocity of 600 m/s (1,969 ft/s), which greatly accelerates the screening tests for various alloys used in impact-resistance applications.
“By using a laser-induced projectile impact testing technique, we discovered a deformation-induced 9R phase with tens of nanometers in width,” Xue says.
Meanwhile, Purdue doctoral student Qiang Li led a group that induced a 9R phase by introducing iron atoms into aluminum’s crystal structure via magnetron sputtering. Iron also can be introduced using other techniques, such as casting, and the route can be scaled up for industrial use. The resulting nanotwinned aluminum-iron alloy coatings were among the strongest aluminum alloys ever created, Li says, comparable to high-strength steels.