Imagine a metal bridge that can repair its own cracks, or an airplane that can heal the damage caused by fatigue. Sounds like science fiction, right? Well, not anymore. Scientists have discovered a remarkable phenomenon that could revolutionize the field of materials science and engineering: metals that can heal themselves.
This discovery was made by a team of researchers from Sandia National Laboratories and Texas A&M University, who published their findings in the journal Nature. They observed for the first time that metals, such as platinum and copper, can spontaneously weld themselves back together after developing nanoscale fatigue cracks under tensile stress.
This is astonishing because metals are usually considered to be inert and brittle materials that cannot recover from damage without external intervention. In fact, most metals fail due to the accumulation of microscopic cracks that grow and spread until the whole structure collapses.
But these researchers found that metals have their own intrinsic, natural ability to heal themselves, at least in the case of fatigue damage at the nanoscale. “This was absolutely stunning to watch first-hand,” said Sandia materials scientist Brad Boyce, who led the experiment.
The experiment was initially focused on the growth of nanoscale fatigue cracks on metals, using a specialized electron microscope technique that could repeatedly pull on the ends of the metal 200 times per second. The researchers expected to see the cracks form and grow as usual, but they were stunned to see the metal autonomously welding itself back together.
“This really turns our understanding of what is possible in fracture mechanics on its head,” said Michael Demkowicz, a professor of materials science at Texas A&M University and co-author of the study. “It’s not surprising if a crack closes under compression, but if you pull on the crack and see it close, that’s very unexpected.”
The surprising results offer the first experimental validation of predictions made by Demkowicz in a 2013 study with MIT physicist Guoqiang Xu. They had proposed a mechanism that could lead to healing of metal nanocracks based on the generation of crystal defects known as disclinations by migrating grain boundaries.
Disclinations are string-like, one-dimensional defects that have a much stronger internal stress field than the more common dislocations. The geometry of disclinations can actually reverse an applied force locally, which is how the tension leads to the metal pulling itself back together.
“The stress field intensity causes the material to pull together rather than separate apart under an applied force. Depending on the kind of microstructure evolution occurring, external loads ranging from hundreds of MPa to GPa triggered the crack closure,” explained Xu and Demkowicz in their 2013 study.
The discovery of autonomous healing in platinum and copper was initially referred to as a “Bigfoot sighting” because it was so unbelievable. However, as the researchers dug deeper, they gained confidence in the observation and realized that this must already be happening in some materials.
The question for the future is whether we can harness the crack healing effect in some creative new ways. For instance, could we design materials with microstructures that use this mechanism to heal internal damage during service? Could we enhance or control the healing process by applying external stimuli such as temperature or electric fields? Could we extend this phenomenon to other metals or alloys?
These questions are not only fascinating from a scientific point of view, but also have tremendous implications for engineering applications. If we could make metals more resilient and reliable by exploiting their self-healing abilities, we could potentially reduce fatigue failures, extend service lifetimes, improve safety and performance, and save costs and resources.
Of course, there are still many challenges and limitations to overcome before we can achieve these goals. For example, how can we scale up this phenomenon from nanoscale to macroscale? How can we ensure that the healing process does not compromise other properties such as strength or ductility? How can we test and verify the effectiveness and durability of self-healing metals?
These are some of the topics that need further research and development in this emerging field of self-healing metals. Fortunately, there is already a growing interest and progress in this area, as evidenced by a recent review article by Shasha Zhang, Niels van Dijk and Sybrand van der Zwaag from Delft University of Technology.
They provide a comprehensive and updated overview of the fundamentals, design principles and performance of self-healing metals. They also identify promising strategies to achieve self-healing in metals based on different modes of potentially healable damage induced in metals and alloys, such as stress-induced damage, irradiation-induced damage in bulk materials and contact damage in corrosion protective coatings.
They conclude that “the spontaneous intrinsic healing mechanisms not requiring external assistance other than the material operating at the right temperature and an assisted healing mechanism with external intervention are reviewed. Promising strategies to achieve self-healing in metals are identified.”
Reference articles:
-Scientists Discover That Metals Heal Themselves in ‘Astonishing’ Breakthrough, VICE, July 25, 2023
-A Review of Self-healing Metals: Fundamentals, Design Principles and Performance, Springer, July 7, 2023
-Nano Focus: New mechanism heals nanocracks in metal under tensile stress, Cambridge Core, December 2023