Effect of blunt nanocracks on the splitting transformation of grain boundary dislocation piled up at triple junctions

Considering that grain boundary (GB) deformation is a structural element that hinders crack growth and promotes superplastic deformation in nanocrystalline materials (NCMs), a theoretical model is suggested to describe the effect of the blunt pre-nanocracks with surface stress on the splitting transformation of the first head grain boundary dislocation (GBD) within the pile-up at the triple junctions (TJs) of GBs in mechanically loaded NCMs. The analytic solution of the total energy change that characterizes this process of splitting transformation of GBD is derived quantitatively by the complex variable method, and then, the very beginning plastic deformation occurrence near the nanocrack tip is predicted. We theoretically evaluated the influence of the various parameters of blunt pre-nanocracks, such as nanocrack blunting and length, the characteristics of grain and GBs, such as grain size, the number of GBDs, and GB angles, and the surface stress at critical conditions for such a splitting transformation. Further analyses revealed that the positive (negative) surface stress significantly decreases (increases) the energy and obviously influences the critical conditions for splitting transformation. Our study identified the beneficial role of GB defect structure transformation, namely the splitting transformation, in enhancing the ductility and fracture toughness of NCMs. It also lays the foundation for investigating how the microstructures caused by GB deformation affect the novel mechanical properties of NCMs.