Metals Have the Intrinsic Ability To Heal Themselves, New Research Finds
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During an experiment designed to analyze how metals perform under repeated stress, researchers at Sandia National Laboratories saw something unexpected – a piece of metal that had cracked began to fuse itself back together, without any external intervention.
If this never-before-seen phenomenon can be harnessed, it could be a major breakthrough for engineering, the researchers say. The team has described their findings in a new paper published today in Nature.
An unexpected discovery
When a material is repeatedly subject to stress – such as a bridge with heavy traffic driving over it or an airplane’s wing flexing as it flies – it can begin to develop fatigue damage. These microscopic fatigue cracks formed by repetitive loading and de-loading can grow and spread throughout a material until it reaches a point of catastrophic failure.
“From solder joints in our electronic devices to our vehicle’s engines to the bridges that we drive over, these structures often fail unpredictably due to cyclic loading that leads to crack initiation and eventual fracture,” said Sandia materials scientist Brad Boyce. “When they do fail, we have to contend with replacement costs, lost time and, in some cases, even injuries or loss of life. The economic impact of these failures is measured in hundreds of billions of dollars every year for the US.”
Consequently, understanding how fatigue cracks form and spread is an important area of research for engineers and materials scientists.
This is exactly what Boyce and his team, including Khalid Hattar, now an associate professor at the University of Tennessee, and Chris Barr, who now works for the Department of Energy’s Office of Nuclear Energy, were studying at Sandia when they noticed something unexpected.
Hattar and Barr had been looking at a nanoscale piece of platinum metal through a specialized electron microscope, which had been set into a rig that pulls the ends of the metal sample up to 200 times per second.
As they tracked the growth of fatigue cracks through the metal sample, at around the 40-minute mark, they saw that one end of a crack had begun to fuse over and heal itself. By the end of the experiment, this branch of the crack had completely closed back up, leaving no trace of the former injury. Over time, the crack regrew through the metal in a different direction, leaving the healed area intact.
“This was absolutely stunning to watch first-hand,” said Boyce. “What we have confirmed is that metals have their own intrinsic, natural ability to heal themselves, at least in the case of fatigue damage at the nanoscale.”
Self-healing observation confirms decade-old theory
This is the first time that such behavior has been witnessed in metals. But it is not an entirely new concept.
Back in 2013, Michael Demkowicz, then an assistant professor at the Massachusetts Institute of Technology’s department of materials science and engineering, now a full professor at Texas A&M, was conducting virtual experiments to examine conventional materials theory. In a paper published that year, he presented a theory of his own. Based on computer simulations, Demkowicz argued that under certain conditions metals should be able to autonomously cold-weld fatigue cracks together.
Aware of this theory, the Sandia researchers shared their new findings with Demkowicz.
“I was very glad to hear it, of course,” Demkowicz said. He has since recreated the Sandia team’s findings using his computer model, confirming that this phenomenon is the same one he previously theorized was possible.
Will we have self-healing bridges in the future?
Several types of self-healing plastics have previously been developed with the intent that these could be used to make paints and other coatings that can easily seal up small scratches. With further development, materials engineers say that self-healing plastics could one day be used to make self-repairing seals and gaskets for industrial use in pipelines.
But what about self-healing metals?
Until now, the idea of metals healing themselves was almost unthinkable – unless, of course, your name was Michael Demkowicz.
“Cracks in metals were only ever expected to get bigger, not smaller. Even some of the basic equations we use to describe crack growth preclude the possibility of such healing processes,” Boyce said.
With this new finding, researchers can begin to examine this behavior and unravel what exactly it is about these conditions that promote autonomous cold welding.
“The extent to which these findings are generalizable will likely become a subject of extensive research,” Boyce said. “We show this happening in nanocrystalline metals in vacuum. But we don’t know if this can also be induced in conventional metals in air.”
“My hope is that this finding will encourage materials researchers to consider that, under the right circumstances, materials can do things we never expected,” Demkowicz said.
Reference: Barr CM, Duong T, Bufford DC et al. Autonomous healing of fatigue cracks via cold welding. Nature. 2023. doi: 10.1038/s41586-023-06223-0
This article is a rework of a press release issued by Sandia National Laboratories. Material has been edited for length and content.