Scientists witness metal healing itself

Scientists from Sandia National Laboratories and Texas A&M University have witnessed pieces of metal crack and then fuse back together without any human intervention, overturning fundamental scientific theories in the process.

If this newly discovered phenomenon could be harnessed, it could usher in an engineering revolution — one in which self-healing engines, bridges and airplanes could reverse damage caused by wear and tear.

Fatigue damage is one way machines wear out and eventually break. Repeated stress or motion causes microscopic cracks to form; over time, these cracks grow and spread until the whole device fails.

“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.”

Although scientists have created some self-healing materials — mostly plastics — the notion of a self-healing metal has largely been the domain of science fiction. As noted by Boyce, “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.”

From theory to reality

In 2013, Michael Demkowicz — then an assistant professor at Massachusetts Institute of Technology, now a full professor at Texas A&M — published a theory in Physical Review Letters, based on findings in computer simulations, that under certain conditions metal should be able to weld shut cracks formed by wear and tear. The discovery that his theory was true came inadvertently at the Center for Integrated Nanotechnologies, a Department of Energy user facility jointly operated by Sandia and Los Alamos national laboratories.

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 running an experiment at Sandia when the discovery was made. They only meant to evaluate how cracks formed and spread through a nanoscale piece of platinum using a specialised electron microscope technique they had developed to repeatedly pull on the ends of the metal 200 times per second.

Surprisingly, about 40 minutes into the experiment, the damage reversed course. One end of the crack fused back together as if it was retracing its steps, leaving no trace of the former injury. Over time, the crack regrew along a different direction.

“This was absolutely stunning to watch firsthand,” Boyce said.

“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, who was aware of Demkowicz’s theory, contacted the professor to share the team’s findings. Demkowicz then recreated the experiment on a computer model, substantiating that the phenomenon witnessed at Sandia was the same one he had theorised years earlier.

A lot remains unknown about the self-healing process, including whether it will become a practical tool in a manufacturing setting. As noted by Boyce, “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.”

Yet for all the unknowns, the discovery — which has been published in the journal Nature — provides a leap forward for the field of materials science. Demkowicz concluded, “My hope is that this finding will encourage materials researchers to consider that, under the right circumstances, materials can do things we never expected.”

Image caption: Green marks the spot where a fissure formed, then fused back together in this artistic rendering of nanoscale self-healing in metal, discovered at Sandia National Laboratories. Red arrows indicate the direction of the pulling force that unexpectedly triggered the phenomenon. Image credit: Dan Thompson.