Phys. Rev. Lett. 111, 145501 (2013)

Credit: © 2013 APS

Metals with nanocrystalline structure are strong — in particular if the average grain size is below 100 nm. However, because they do not tolerate plastic deformation, they can fracture at high loads. Now, using molecular dynamics simulations and finite-element-method calculations of a model bicrystal with a pre-existing nanocrack, Xu and Demkowicz show that in nanocrystalline metals stress-driven tilt-grain-boundary migration can be used to completely heal nanocracks and therefore make the metals more ductile. The researchers describe the mechanism of crack healing (which involves the creation of wedge disclinations), and designed microstructures that lead to the closing of cracks under external tensile load (which typically causes crack opening) by placing an impenetrable precipitate that blocks the motion of part of the moving tilt grain boundary. They also show that the amount of grain-boundary migration necessary for crack healing is lower than that needed for crack propagation, making the mechanism more likely to close nanocracks than advance them.