Scientists for the first time witnessed pieces of metal crack, then reunite without any human intervention, overturning basic scientific theories in the process. If the newly discovered phenomenon can be harnessed, it could lead to an engineering revolution — one in which self-healing machines, bridges and planes can reverse damage caused by wear and tear, creating it is more safe and sustainable.
The research team from Sandia National Laboratories and Texas A&M University describes their findings today in the journal Nature.
“It is absolutely amazing to see first-hand,” said Sandia materials scientist Brad 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,” Boyce said.
Fatigue damage is one way that machines wear down and eventually break down. Repeated stress or movement causes microscopic cracks to form. Over time, these cracks grow and spread until—suddenly! The whole device breaks down, or in scientific lingo, it fails.
The fissure that Boyce and his team saw disappear was one of these small but consequential fractures—measured in nanometers.
“From the solder joints of our electronic devices to the engines of our cars to the bridges we drive on, these structures often fail unpredictably due to cyclic loading leading to crack initiation and eventual fracture, ” said Boyce. “When they 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 billion dollars every year for the US”
Although scientists have developed some self-healing materials, mostly plastics, the idea of a self-healing metal is largely the domain of science fiction.
“Cracks in metals are expected to get bigger, not smaller. Even some of the basic equations we use to describe crack growth prevent the possibility of such repair processes,” said Boyce.
The unexpected discovery was confirmed by the originator of the theory
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—began chipping away at conventional materials theory. He published a new theory, based on the findings of computer simulations, that under certain conditions the metal should be able to weld the cracks formed due to the destruction.
The discovery that his theory was true came by accident at the Center for Integrated Nanotechnologies, a Department of Energy user facility jointly operated by Sandia and Los Alamos national laboratories.
“We weren’t really looking,” Boyce said.
Khalid Hattar, now an associate professor at the University of Tennessee, Knoxville, and Chris Barr, who now works for the Energy Department’s Office of Nuclear Energy, were running the Sandia experiment when the discovery was made. They simply meant to evaluate how cracks formed and propagated through a nanoscale piece of platinum using a special electron microscope technique they developed to repeatedly pull the ends of the metal 200 times per second.
Surprisingly, about 40 minutes into the experiment, the damage was back on course. One end of the crack turned again as if retracing his steps, leaving no trace of the previous damage. Over time, the crack turned in the other direction.
Hattar called it an “unprecedented insight.”
Boyce, who knew the theory, shared his discoveries with Demkowicz.
“I’m very happy to hear it, of course,” Demkowicz said. The professor recreated the experiment with a computer model, which confirmed that the phenomenon witnessed at Sandia was exactly what he had imagined years ago.
Their work was supported by the Department of Energy’s Office of Science, Basic Energy Sciences; the National Nuclear Security Administration and the National Science Foundation.
Much remains unknown about the self-healing process, including whether it can be a practical tool in a manufacturing setting.
“The extent to which these findings can be made is a subject of much research,” Boyce said. “We have shown this to happen in nanocrystalline metals in vacuum. But we don’t know if it can also be induced in conventional metals in air.”
Yet for all the uninitiated, the discovery remains a leap forward at the frontier of material science.
“My hope is that this finding will encourage materials researchers to consider that, under the right conditions, materials can do things we never expected,” said Demkowicz.
Brad Boyce, Autonomous repair of fatigue cracks by cold welding, NATURE (2023). DOI: 10.1038/s41586-023-06223-0. www.nature.com/articles/s41586-023-06223-0
Provided by Sandia National Laboratories
Citation: ‘Stunning’ discovery: Metals can heal themselves (2023, July 19) retrieved 19 July 2023 from https://phys.org/news/2023-07-stunning-discovery-metals.html
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