  Nanocrystalline metals typically have high fatigue strengths, but low
resistance to crack propagation. Amorphous intergranular films are disordered
grain boundary complexions that have been shown to delay crack nucleation and
slow crack propagation during monotonic loading by diffusing grain boundary
strain concentrations, suggesting they may also be beneficial for fatigue
properties. To probe this hypothesis, in situ transmission electron microscopy
fatigue cycling is performed on Cu-1 at.% Zr thin films thermally treated to
have either only ordered grain boundaries or to contain amorphous intergranular
films. The sample with only ordered grain boundaries experienced grain
coarsening at crack initiation followed by unsteady crack propagation and
extensive nanocracking, whereas the sample containing amorphous intergranular
films had no grain coarsening at crack initiation followed by steady crack
propagation and distributed plastic activity. Microstructural design for
control of these behaviors through simple thermal treatments can allow for the
improvement of nanocrystalline metal fatigue toughness.
