Scientists Discover That Metals Heal Themselves in 'Astonishing' Breakthrough

Scientists have observed metal “healing” itself for the first time, an unexpected discovery that challenges basic tenets of materials science and could pave the way toward more durable metal structures, reports a new study.

The breakthrough was made at Sandia National Laboratories during an experiment that was initially focused on the growth of nanoscale fatigue cracks on metals, such as platinum. Researchers including Brad Boyce, a staff scientist at Sandia, watched the tiny cracks form and grow in the metal as expected, but were stunned to see the metal autonomously welding itself back together.  

“Our goal was to observe the mechanistic processes associated with fatigue loading, in particular the phenomena of grain boundary migration,” said Boyce in an email to Motherboard. “So we weren’t exactly looking for the healing process in our first experiments. But after we saw it the first time, we did intentionally repeat the experiment to observe the healing process again.”    

The surprising results offer the first experimental validation of predictions made by Michael Demkowicz, a professor of materials science at Texas A&M University. Boyce and Demkowicz, along with several other researchers, said this “evidence that pure metals may occasionally heal themselves at the nanoscale is astonishing” and “challenges the most fundamental theories on how engineers design and evaluate fatigue life in structural materials” in a study published on Wednesday in Nature.

“I think skepticism is a scientist's knee-jerk reaction to these kinds of findings,” Demkowicz told Motherboard in an email. “In fact, the [Sandia] team initially referred to it as a ‘Bigfoot sighting.’ However, as we dug deeper, all the pieces were fitting together and we gained confidence in the observation.”

“In particular, since more than half of the experiments ended up showing some form of crack healing, it became apparent to us that this must actually already be happening in some materials,” he continued. “We just didn’t know it until now. The question for the future is whether we can harness the crack healing effect in some creative new ways.”

Human beings, along with countless other lifeforms, have evolved ways to heal ourselves from injuries and illnesses, but reproducing this restorative ability in artificial materials can be a tall order. Some substances, such as plastics and ceramics, demonstrate remarkable self-healing abilities, but few experts had considered that metals might be able to autonomously mend their wounds.

Demkowicz, in contrast, had proposed a mechanism that could lead to healing of metal nanocracks in a 2013 study with MIT physicist Guoqiang Xu. However, he told Motherboard that at that time, he wasn’t sure how to follow up on the hypothesis in the laboratory. 

“I was at a bit of a loss as to how one might validate it experimentally—the required experiments are extremely challenging,” Demkowicz said. “Thus, I think the [Sandia] team really deserves a lot of kudos.”

Indeed, nearly a decade later, Boyce and his colleagues at Sandia were using an electron microscope to produce wear-and-tear on extremely small and thin metal samples in a vacuum environment. During the experiment, the researchers saw the minute cracks merging back together, a process caused when compressive forces activate a “cold-welding” effect inside the metal—just as predicted by Demkowicz and Xu.

“Those observations, along with a supporting computational model that predicted the behavior, helped us gain confidence that this was real and reproducible,” Boyce said. “Moreover, the model gave us insight into the mechanistic process by which the healing occurs. Now, I think we are curious—curious to explore the detailed conditions associated with the healing process in greater detail.” 

“We want to explore the possibility that this can happen in atmospheres other than vacuum, where species like oxygen form a thin oxide on the crack flanks; and explore the possibility that the healing process might also occur in conventional metal alloys,” he added.

While the discovery may evoke visions of T-1000 robots or indestructible metalworks, the long-term implications of this self-healing process remain unclear. That said, the researchers noted in the study that “self-healing has the potential to impact numerous structural applications of metals, in particular fatigue failures under cyclic loading where delayed catastrophic failure is difficult to anticipate, even given extensive empirical data.”  

“While it's easy to get excited about making metals that can heal damage caused by fatigue, I think we should temper that expectation with some reality: our observations were on tiny cracks, about a thousand times smaller than a human hair, and the crack didn’t heal in its entirety—only the leading segment of the crack healed, Boyce said. “Nonetheless, I think this result can help engineers develop a more thorough understanding of the fundamental process of fatigue failure, and thereby develop principles that help us mitigate fatigue failures in our engineered structures.” 

“I am always an optimist about these things,” concluded Demkowicz. “The question is: how soon? I’m guessing that we’re looking at another decade before we can work self-healing into a technological application. Since our work was done in vacuum, I think a strong early contender might be in space technology (vehicles, infrastructure, machinery).”