Washington: Researchers have discovered that under certain conditions, putting a cracked piece of metal under tension, exerting a force that would be expected to pull it apart, has the reverse effect, causing the crack to close and its edges to fuse together.
The surprising finding could lead to self-healing materials that repair incipient damage before it has a chance to spread.
MIT Graduate student Guoqiang Xu and professor of materials science and engineering Michael Demkowicz found that it happened because of how grain boundaries interact with cracks in the crystalline microstructure of a metal - in this case nickel, which is the basis for " superalloys " used in extreme environments, such as in deep-sea oil wells.
By creating a computer model of that microstructure and studying its response to various conditions, Demkowicz said that they found that there is a mechanism that can, in principle, close cracks under any applied stress.
Most metals are made of tiny crystalline grains whose sizes and orientations can affect strength and other characteristics. But under certain conditions, Demkowicz and Xu found, stress "causes the microstructure to change: It can make grain boundaries migrate. This grain boundary migration is the key to healing the crack," Demkowicz says.
The very idea that crystal grain boundaries could migrate within a solid metal has been extensively studied within the last decade, Demkowicz says. Self-healing, however, occurs only across a certain kind of boundary, he explains - one that extends partway into a grain, but not all the way through it. This creates a type of defect is known as a "disclination."
The results have been published in the journal Physical Review Letters.
