Aluminium alloys that heal themselves

Aluminium alloys are widely used because they are light and non-magnetic and have great corrosion resistance. But they fatigue badly.

Australian engineers may have solved that problem, however, after creating aluminium alloy microstructures that can heal while in operation. The result, they report in Nature Communications, is demonstrated 25-fold improvements in the life of high-strength alloys.

The team led by Christopher Hutchinson from Monash University showed that poor fatigue performance is due to weak links called precipitate free zones (PFZs).

Hutchison says 80% of all engineering alloy failures are due to fatigue – an alternating stress. As an example, he says, try breaking a metal paperclip. It won’t happen the first time, but bend it back and forth a number of times and it will.  

This failure occurs in stages. The alternative stress leads to microplasticity (undergoing permanent change due to stress) and the accumulation of damage in the form of a localisation of plasticity at the weak links in the material. This causes a crack, which grows and leads to final fracture.

Working with three commercially available aluminium alloys, the researchers used the mechanical energy imparted into the materials during the early cycles of fatigue to heal the weak points in the PFZs. This significantly delayed the localisation of plasticity and the initiation of cracks.

“Our research has demonstrated a conceptual change in the microstructural design of aluminium alloys for dynamic loading applications,” Hutchinson says.

“Instead of designing a strong microstructure and hoping it remains stable for as long as possible during fatigue loading, we recognised that the microstructure will be changed by the dynamic loading and, hence, designed a starting microstructure – that may have lower static strength – that will change in such a way that its fatigue performance is significantly improved.

“In this respect, the structure is trained and the training schedule is used to heal the PFZs that would otherwise represent the weak points. The approach is general and could be applied to other precipitate hardened alloys containing PFZs for which fatigue performance is an important consideration.”