Variability of non-Schmid effects in grain boundary dislocation nucleation criteria

Grain boundary dislocation nucleation is simulated in a Nickel bicrystal with Sigma 21b (211)/(211) grain boundaries. Using molecular dynamics, 386 different triaxial stress states are examined for their effect on dislocation nucleation on different slip systems. The approach leads to dislocation nucleation on six of the twelve partial slip systems, enabling a study of conditions leading to nucleation. The criteria for nucleation on each of the slip systems is shown to have a linear dependence on the resolved shear, normal, and co-slip stresses, though the constants for this linear dependence are unique for each slip system. The combined nucleation criteria are used to construct a theoretical nucleation surface. The surface is reminiscent of a Mohr-Coulomb yield. surface, except that in the present case the facets of the surface correspond to nucleation on different slip systems. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.