Mechanical amorphization drives fragmentation in a nanocrystalline Ni

We report on the peculiar fragmentation of nanocrystalline (NC) Ni under large plastic strain. X-ray diffraction and high-resolution electron microscope evidence reveal that the facture process involves a stage of deformation-induced amorphization that initiates either at grain boundaries or around the core parts of Lomer dislocations inside the nano-grains. The generation of amorphous structures provides preferred weakest-link sites for the nucleation and propagation of cleavage cracking, thereby fundamentally affecting the fracture behavior. These observations shed new insight into the fracture mechanics of NC metallic materials.