Metals Can Heal Themselves Without Human Intervention

Scientists have witnessed a first-time phenomenon in which pieces of metal crack and then fuse together again without human intervention, something that defies previously believed scientific theories and paves the way for a potential engineering revolution, they said.

A research team from the Department of Energy's Sandia National Laboratories observed the phenomenon—which already had been predicted by a fellow researcher—quite by accident.

Khalid Hattar, now an associate professor at the University of Tennessee, Knoxville, and Chris Barr, who now works for the DoE's Office of Nuclear Energy, were running an experiment at the Center for Integrated Nanotechnologies—a DoE facility—to evaluate how cracks formed and spread through a nanoscale piece of platinum. They were using a specialized electron microscope technique they'd developed to repeatedly pull on the ends of the metal 200 times per second.

About 40 minutes into the experiment, the damage to the metal reversed course, with one end of the crack fusing back to the other to show no trace of the original fissure, the researchers said. Eventually, the crack in the metal regrew along a different direction.

The researchers were surprised by what they saw. “This was absolutely stunning to watch first-hand,” shared Sandia materials scientist Brad Boyce, who also witnessed the experiment, in a press release.

However, he was aware of a theory that existed making possible the existence of self-healing metal. Boyce contacted A&M Professor Michael Demkowicz, who in 2013 as an assistant professor at MIT published a theory based on findings in computer simulations that under certain conditions, metal should be able to weld shut cracks formed by wear and tear.

“I was very glad to hear it, of course,” Demkowicz said later. He recreated the experiment that had occurred at the Sandia Lab on a computer model and found that what happened at the lab was the same phenomenon that he'd predicted in his 2013 research.

Self-Healing Revolution

If scientists can succeed in harnessing the phenomenon, it can be a game changer for manufacturing, the researchers said. It creates the potential to produce a range of self-healing metal products—including engines, bridges, and airplanes—that can reverse damage caused by wear and tear, thus becoming more safe, durable, and longer lasting.

“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.

This is a key point, as fatigue damage is a key way machines wear out and eventually break due to microscopic cracks that form and spread over time. The crack that the scientists saw heal itself was one of these tiny fractures that begin the process of eventually causing a metal part to fail; thus metal that can self-repair could potentially save billions for companies in the United States alone, Boyce said.

Self-healing materials themselves are not new—scientists already have developed self-healing polymers—but metals until now have so far not exhibited this property.

“Cracks in metals were only ever expected to get bigger, not smaller," Boyce said. "Even some of the basic equations we use to describe crack growth preclude the possibility of such healing processes."

The Work Ahead in Metals

That said, there is still a long way to go in understanding how the metal healed itself, and how scientists can reproduce this process for future manufacturing processes to reap the benefit of it.

“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.”

Still, the team is hopeful that their discovery will signify a giant step forward for materials science.

“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.