Scientists have succeeded in making a metal repair tiny cracks independently - without human intervention. The discovery potentially paves the way to 'self-repairing' engines, bridges and aircraft.
The material was developed by Sandia National Laboratories and Texas A&M University. "We have confirmed that metals have their own intrinsic, natural ability to repair themselves, at least in terms of nanoscale damage caused by metal fatigue," explains Brad Boyce, researchers at Sandia National Laboratory.
Repeated stress or movement
Metal fatigue is damage caused by repeated stress or movement, which causes cracks to form in the material at the nanoscale. These cracks can grow over time. This can eventually lead to technical failure and machine downtime. Boyce's team saw first-hand the repair of such a crack at the nano-level.
"From solder joints in our electronic devices to the engine of our vehicle and the bridges we drive over; these structures often fail unpredictably due to cyclic loading, leading to the formation of cracks and eventually fractures," Boyce explains. "If they fail, we face replacement costs, lost time and - in some cases - even injuries or loss of life. The economic impact of these problems amounts to hundreds of millions of dollars a year in the United States (US)."
So far no evidence for self-repairing metal
This is not the first time researchers have succeeded in developing self-repairing materials. Until now, however, these have mainly involved plastics. Until now, however, there was no evidence that metal was also capable of repairing itself.
"Cracks in metal were only expected to get bigger, not smaller. Even some of the basic equations we use to describe the growth of cracks rules out the possibility of such repair processes," Boyce said.
Theory from 2013
However, there have long been indications that this should be possible. For instance, Michael Demkowicz, a former assistant professor in the Massachusetts Institute of Technology's department of materials science and engineering and now a professor at Texas A&M University, has been working on such material since 2013. He published in 2013 a new theory based on computer simulations that metal can, under certain conditions, repair cracks caused by wear and tear.
Demkowicz's theory has now been confirmed by the team from Sandia National Laboratories and Texas A&M University. The team made this discovery by chance at the Center for Integrated Nanotechnologies, in which Sandia collaborates with national laboratories from the US Los Alamos. They conducted an experiment at the centre that aimed to map the formation of cracks in a piece of platinum at the nanoscale. In doing so, they used a specialised microscopy technology. This allowed the researchers to pull on the ends of this piece of platinum two hundred times per second to create metal fatigue.
Cracks disappear again
About 40 minutes into the experiment, however, the team discovered that the cracks created by their technique disappeared again. In the process, the material re-melted on one side of the crack, leaving no trace of the crack. At a later stage, the material 'regrown' further and repaired the entire crack.
Demkowicz is pleased with the discovery. The professor has since recreated the researchers' discovery in a computer model. He confirms that the phenomenon the team observed is similar to the one he developed a theory about in 2013. "My hope is that this discovery encourages materials researchers to consider that - under the right conditions - materials can do things we don't expect," Demkowicz said.
Further information is available in the scientific journal *Nature*, where the researchers describe their findings.
Author: Wouter Hoeffnagel
