A tiny crack grew in a very small piece of metal when placed under repetitive stretching. The experiment, designed to study crack growth, continued as predicted for a while. But then, something unexpected happened. The crack stopped growing and instead began to get shorter, effectively "healing（愈合）"itself. This incredible observation was made by a group of researchers at Sandia National Laboratories. The findings were recently published in the journal Nature.

It would be reasonable to think, prior to this discovery, that self-healing metal was something that could only be found in science fiction. However, Dr. Michael Demkowicz, a professor in Texas A&M University's materials science and engineering department and co-author of the recent study, held no such assumptions.

Ten years ago, while an assistant professor at Massachusetts Institute of Technology's materials science and engineering department, Demkowicz and his student predicted self-healing in metals. "We didn't set out to find healing. My student, Guoxiang Xu, was doing experiments on fracture（断裂）," Demkowicz said. "We accidentally observed healing in one of his experiments and decided to follow up." Then, just like now, the results in 2013 were surprising.

Both the 2013 models and the recent experiment used nanocrystalline（纳米晶体）metals, which make studying self-healing easier because their small grain size allows for more microstructural features that even small cracks can interact with. Demkowicz said that these features are common in many metals. One condition common to both the 2013 theory and the recent experiment is that both were conducted in vacuum environments without foreign matter. Such outside matter could influence crack surfaces' ability to bond back together.

Even with this limitation, applications could still be possible for space faring technology or internal cracks that are not exposed to outside air. "The main impact of the current work is to move the original theoretical prediction off the drawing board and show that it occurs in reality," Demkowic z said. Potential applications of this work could vary widely. Demkowicz suggests that self-healing could be possible in conventional metals with larger grain sizes, but future investigations will be needed.