Aim

As the first step to study the fatigue crack propagation, the molecular simulations were performed to investigate the opening, propagation and closure behaviors of intragranular and intergranular cracks in nanocrystalline Ni under single-cycle mode I loading. The atomic crack angle was proposed to characterize the crack-tip blunting and sharpening quantitatively. There are obvious different mechanisms between intragranular and intergranular crack propagation. Intragranular crack propagation could be induced by stacking faults and can change its direction very easily, but the intergranular crack perpendicular to the loading direction would propagate along the grain boundary and encounter strong resistance at the triple junction. New cracks could form in the grain boundaries ahead of the original crack. Different from the traditional understanding for intragranular and intergranular cracks, however, the dislocation density increases even in the unloading process and the crack tip is possibly sharpened temporarily during loading.

Graphical abstract