In situ observation of intergranular fracture propagation in nanocrystalline freestanding ultrathin tungsten film

Crack propagation in sputter-deposited freestanding nanocrystalline tungsten films with a thickness of 38 nm was studied using an in situ transmission electron microscopy (TEM) tensile technique. Distinct from coarse-grained tungsten, a totally intergranular fracture mode and toughening mechanisms were identified in the nanocrystalline tungsten. The transition of the fracture mode indicates that the plasticity is improved, and the improvement may be attributed to the effect of nano-size grains and the freestanding surface, which facilitate a high percentage of grain boundaries (GB) and a weak binding force. A theoretical model of the energy release rate and the deflection angle of cracks was established to quantitatively characterize the preferred deflection angle. The model predictions are in good agreement with the experimental findings.