ON THE CONTINUUM VERSUS ATOMISTIC DESCRIPTIONS OF DISLOCATION NUCLEATION AND CLEAVAGE IN NICKEL

A hybrid atomistic-finite-element model is compared with the continuum-based Peierls-Nabarro model for several crack orientations in a nickel crystal. Both methods incorporate the same embedded-atom potential for Ni, in order to make the comparison as valid as possible. The agreement (expressed in terms of a stability diagram showing envelopes in loading space where fracture or dislocation nucleation are likely to occur) is excellent in the case of a crack lying on a {111} plane, with a crack front running along a [211]-type direction, subject to mixed-mode I-II loadings. That orientation involves dislocation nucleation on the prolongation of the crack plane, and hence no ledge is formed upon dislocation nucleation. In other geometries considered (involving a crack on a {100}-type plane), the agreement seems to get poorer with increasing size of the ledge that is created when a dislocation nucleates. In all geometries, the atomistic model shows that incipient dislocation-like features are present before dislocation nucleation takes place, which serves as additional validation of the continuum Peierls-Nabarro model.