We propose a two-dimensional (2D) model that describes toughening of nanocrystalline metallic alloys due to grain boundary (GB) segregations. Within the model, brittle GB segregations lead to the formation of satellite GB cracks near the tip of the main crack. These cracks affect the stress concentration in the vicinity of the main crack tip and lead to toughening. We performed 2D finite element simulations of crack growth in a representative volume that incorporates GB fragments without segregations and with segregations. In these simulations, GBs are modeled as interface elements, and the effect of GB segregations manifests itself in a strong reduction of the cohesive strength of these elements. We demonstrate that GB segregations in nanocrystalline alloys can increase the fracture energy and thereby toughen these solids.