A team of scientists from Sandia National Laboratories and Texas A&M University has recently witnessed for the first time a stunning phenomenon: pieces of metal cracking, then fusing back together without any human intervention.
If this amazing phenomenon can be harnessed, it could give rise to an engineering revolution in which self-healing bridges, engines, or airplanes could reverse damage caused by wear and tear and thus become safer and longer-lasting.
“This was absolutely stunning to watch first-hand,” said Brad Boyce, a materials scientist at Sandia. “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 of the main ways through which machine wear out and eventually break. From repeated stress or motion, microscopic cracks emerge and grow over time, ultimately leading to the complete failure of the device.
The fissure the researchers saw disappear in the current study was one of these tiny (measured in nanometers) but consequential fractures.
“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 explained.
“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 U.S.”
According to Boyce, although scientists have already created some self-healing materials (mostly plastics), a self-healing metal has been considered impossible until recently. “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,” he said.
However, the possibility of self-healing metals has already been theorized in 2013 in a study led by Michael Demkowicz, who was at that time an assistant professor of Materials Science and Engineering at the Massachusetts Institute of Technology (MIT), and is currently a full professor at Texas A&M.
Based on computer simulations, Demkowicz argued that, under certain specific conditions, metals should be able to weld shut cracks formed by wear and tear.
Now, experts from Sandia inadvertently found clear evidence for this hypothesis, while evaluating how cracks formed and spread through a nanoscale piece of platinum by using a specialized electron microscope technique that could repeatedly pull on the ends of the metal 200 times per second.
Surprisingly, after about 40 minutes, the damage in one location suddenly reversed course, with one end of the crack fusing back together as if retracing its steps, leaving no trace of the previous injury. Afterwards, the crack started to regrow along a different direction.
Further research is needed to assess whether such self-healing processes could become practical tools in manufacturing.
“The extent to which these findings are generalizable will likely become a subject of extensive research. 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,” Boyce said.
“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 concluded.
The study is published in the journal Nature.
Self-healing metals are an exciting area of research in the field of materials science. The term refers to metals that have the ability to heal themselves after being damaged or degraded.
This process typically involves the autonomous repair of damages such as cracks or corrosion that could potentially compromise the structural integrity of the metal.
To make self-healing metals, scientists generally introduce a mechanism that can respond to the initiation of damage. There are a couple of broad ways this can be done:
Small capsules filled with a healing agent are dispersed throughout the metal. When a crack or damage occurs, it breaks open these capsules, releasing the healing agent. This can then react with the metal or with another agent to “heal” the crack, preventing it from spreading further.
Certain alloys have intrinsic self-healing capabilities, as they can form oxide layers that protect the underlying metal from further corrosion. This approach can be promoted by alloying elements, heat treatment, or careful control of the material microstructure to encourage beneficial phase transformations or other self-healing processes.