Metal caught fixing cracks

For the first time, scientists have witnessed pieces of metal crack then fuse back together without any human intervention, overturning fundamental scientific theories in the process. If this newly discovered phenomenon can 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, making them safer and longer-lasting.

The research team from Sandia National Laboratories and Texas A&M University reports its findings in a paper in Nature.

“This was absolutely stunning to watch 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.”

Fatigue damage is one way that machines can wear out and eventually break. Repeated stress or motion causes microscopic cracks to form. Over time, these cracks grow and spread until – snap! The whole device breaks or fails.

The fissure Boyce and his team saw disappear was one of these tiny but consequential fractures – measured in nanometers.

“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,” Boyce said. “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.”

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. “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.”

In 2013, Michael Demkowicz – then an assistant professor in the Massachusetts Institute of Technology’s department of materials science and engineering, now a full professor at Texas A&M University – began chipping away at conventional materials theory. He published a new theory, based on findings in computer simulations, stating that under certain conditions metal should be able to weld shut cracks formed by wear and tear.

The discovery that his theory was true occurred inadvertently at the Center for Integrated Nanotechnologies, a US Department of Energy (DOE) user facility jointly operated by Sandia and Los Alamos national laboratories.

“We certainly weren’t looking for it,” Boyce said.

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 Sandia when the discovery was made. They were evaluating how cracks formed and spread through a nanoscale piece of platinum by using a specialized 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. Hattar called it an “unprecedented insight”.

Boyce, who was aware of Demkowicz’s theory, shared their findings with him. “I was very glad to hear it, of course,” Demkowicz said. The professor then recreated the experiment on a computer model, substantiating that the phenomenon witnessed at Sandia was the same one he had theorized years earlier.

A lot remains unknown about this self-healing process, including whether it could become a practical tool in a manufacturing setting.

“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.”

Yet for all the unknowns, this discovery remains a leap forward at the frontier of 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.

This story is adapted from material from Sandia National Laboratories, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.