Fracture of Ni with grain-size from nanocrystalline to ultrafine scale under cyclic loading

A two-step fatigue loading scheme was developed to study the fracture characteristics of nanocrystalline and ultrafine-grained Ni, with grain-sizes of 23 and 260 nm, respectively. Cyclic work-hardening, measured by the mid-loop width during cycling, was observed in both types of material in the first loading step. The loop width increases rapidly as the load is upgraded in the second step. Crack initiation and striation have been observed for the first time in nanocrystalline Ni.