Fracture Resistance of Nanocrystalline Ni

Atomic level simulations are used to study crack propagation mechanisms in nanocrystalline Ni. Digital samples with a mean grain size of 5 and 8 nm containing 125 grains were used. For both grain sizes, the mechanism of crack propagation involves the formation of nanocracks along grain boundaries in the vicinity of the main crack. Crack resistance curves for the two grain sizes indicate that the smaller grain sizes are more ductile, requiring higher stress intensities for crack propagation. This result is consistent with softer behavior for smaller grain sizes in the inverse Hall–Petch regime, where deformation is accommodated by grain boundary mechanisms. The present simulations specifically show that grain boundary sliding also plays an important role in crack blunting observed in these materials. In many cases, the crack is arrested as it encounters grain boundaries in its path, showing increased resistance to propagation. Increased ductility for smaller grain sizes in this regime indicates that there is a minimum in ductility as a function of grain size in these materials, located around the 10- to 12-nm grain size.