Crack propagation in gradient nano-grained metals with extremely small grain size based on molecular dynamic simulations

In this paper, the problem of crack propagation in the gradient nano-grained (GNG) metals is studied through extensive quasi-3D molecular dynamic (MD) simulations. Numerical samples of GNG copper with initial central crack are established and simulations of uniaxial deformation are carried out. The effect of the grain size gradient on the crack propagation is systematically investigated by comparing the propagation rates between the two tips of the central crack as well as by comparing the results of the GNG sample with those of uniform samples with different grain sizes. It is found that introduction of the grain size gradient can compromise the ability of the nano-grained metals to resist the crack propagation for grain size in inverse Hall–Petch regime. The crack tip profile, the stress, and the density of atoms of various defect structures were analyzed to gain insight into the synergistic interactions between the dislocation activity, the grain boundary mechanisms and the crack propagation. This work is intended to provide not only a mechanistic understanding of the fracture behavior of GNG metals, but also a guideline for ensuring the safety application of such advanced materials.