"Faulty" aluminum alloys sport the strength of stainless steel
Aluminum is nice and light, but as an easily crushable soft drink can shows, that comes at the cost of strength. By introducing faults into the metal's crystalline structure, researchers at Purdue University have now developed new aluminum alloys that are about as strong as stainless steel, and they could be put to work in corrosion-resistant coatings.
On the microscopic level, metals are made up of crystal atoms in repeating cycles of layers stacked on top of each other. A "stacking fault" occurs when a layer is missing from the pattern, and two layers of these faults are known as "twin boundaries" or "nanotwins." When these faults create a structure that repeats over nine layers, it's called a 9R phase.
These stacking faults can actually make a material stronger, so Purdue researchers wanted to introduce both nanotwins and 9R phase into aluminum. The problem is that the metal has a "high stacking fault energy" – essentially, that means aluminum tends to correct the faults itself.
"It has been shown that twin boundaries are difficult to be introduced into aluminum," says Xinghang Zhang, an author on two studies describing the new material. "The formation of the 9R phase in aluminum is even more difficult because of its high stacking fault energy. You want to introduce both nanotwins and 9R phase in nanograined aluminum to increase strength and ductility and improve thermal stability."
The scientists invoked 9R phase in the material using two different techniques. One was "shock-induced," created by using a laser to bombard ultrathin sheets of aluminum with particles of silicon dioxide.
"Here, by using a laser-induced projectile impact testing technique, we discover a deformation-induced 9R phase with tens of nanometers in width," says Sichuang Xue, lead author of this study.
The second study used a process called magnetron sputtering to introduce iron atoms into the aluminum's crystal structure. This aluminum-iron alloy was found to be one of the strongest ever made, and the team says the process could be scaled up to an industrial scale, for eventual applications as corrosion-resistant coatings for electronic devices and vehicles.
"These results show how to fabricate aluminum alloys that are comparable to, or even stronger than, stainless steels," says Zhang. "There is a lot of potential commercial impact in this finding."
The team describes the alloys in the video below.
Source: Purdue University