Special rotational deformation and grain size effect on fracture toughness of nanocrystalline materials

A grain size-dependent model is theoretically suggested to describe the combined effects of special rotational deformation and dislocations near a mode I crack tip on the fracture toughness of nanocrystalline metals and ceramics. In the model, the special rotation deformation is driven by the external stress concentration near the crack tip, and serves as a toughening mechanism by releasing part of local stresses. The lattice dislocations consist of triple junction dislocation produced by intergrain sliding and edge dislocations emitted from the crack tip. The emitted dislocations are stopped at grain boundaries. The stress fields of these dislocations suppress future dislocation emission, and the suppression depends on grain size. The results indicate that the combination of special rotational deformation and dislocations near the crack tip can lead to an increase of critical crack intensity factor (effective fracture toughness) by several times in nanocrystalline materials at finest grain size. It is also found that the fracture toughness of nanocrystalline materials is highly sensitive to grain size and there is an ideal grain size corresponding to the best toughening effects, which is qualitatively consistent with the conclusion in previous work.