- Scientists have created super-strong aluminum alloy by introducing faults into its crystalline structure.
- It would have a wide range of applications including corrosion resistant coating for vehicles and electronic instruments.
- The study helps us better understand the deformation mechanism in metals with high stacking fault energies.
Most of the aluminum alloys are soft, lightweight and have low mechanical strength, which limits the wide range of industrial applications. However, researchers at Purdue University introduced some faults into the crystalline structure of the metal to improve its strength.
The team has created a strong aluminum alloy that has a strength comparable to stainless steel. Such high-strength and lightweight alloys could be put to work in corrosion-resistant coating, and could revolutionize the automobile and aerospace industries.
Let’s figure out how exactly they have altered the aluminum’s microstructure to impart higher strength and ductility.
Multiple layers of crystal atoms (in repeating cycles), stacked on top of each other, make metals. Usually, when a layer is missing from the pattern, a ‘stacking fault’ occurs. Two layers of such faults are called ‘nanotwins’ or ‘twin boundaries’. If you increase the layers of these faults to nine, it is called a ‘9R phase’. In other words, 9R phase is a fault in a structure that repeats over nine layers.
These types of faults make a substance harder. So the research team tried to introduce 9R phase and twin boundaries into aluminum. However, there was one problem – aluminum has a high stacking fault energy, which means it can correct the faults all by itself.
While introducing twin boundaries and 9R phase is extremely difficult in nanograined aluminum, it’s easier to produce in metals such as silver and copper. To make it work, the scientists used two different techniques to invoke 9R phase in the aluminum.
1. Shock Induced Technique
This one includes using a laser to bombard very thin layers of aluminum with particles of silicon dioxide. The laser beam ejects the particles at a velocity of 600 m/s. The procedure rapidly accelerates the screening tests of numerous alloys for impact-resistance applications.
This results in a deformation-induced 9R phase with tens of nanometers in width in ultrafine-grained aluminium with an average grain size of 140 nanometers.
More specifically, the transmission electron microscopy investigations reveal several tens of nanometers wide 9R phase regions in the impacted ultrafine-grained aluminum, and abundant dislocation networks, along with grain fragmentation and rotation.
Microstructures of Al thin film | Boundary rotation axis (BRA) map reveals the incoherent twin boundary (ITB) & coherent twin boundary (CTB)
Reference: Nature | doi:10.1038/s41467-017-01729-4 | Rice University
To accommodate the plastic deformation under extensive strain rate, the 9R phase formation through dissociation of incoherent twin boundaries can take place even if there is high energy barrier.
Furthermore, the stability of the 9R phase is based on the presence of sessile Frank loops. The study also helps us better understand the deformation mechanism in metals with high stacking fault energies.
2. Magnetron Sputtering
The second technique used a method known as magnetron sputtering to fill the aluminum’s crystal structure with iron atoms. The team discovered that this aluminum-iron alloy is one of the strongest Al-alloy ever built.
Microstructure of the deformed Al–Fe alloy
Reference: WileyOnlineLibrary | doi: 10.1002/adma.201704629 | Purdue University
These alloys induced with 9R phase have flow stress exceeding 1.5 GPa. Molecular dynamics simulations show that high strength and hardening ability of these alloys arise primarily because of nanoscale grain sizes and high density 9R phase.
This alloy could be used at industrial scale, for a wide range of applications, including corrosion resistant coating for vehicles and electronic instruments (coatings can reach a maximum hardness of 5.5 GPa).
However, the extensive plasticity of these alloys under compression doesn’t guarantee a good tensile ductility, an aspect that remains to be examined in future studies.