INTERFACE SCIENCE 7, 45–55 (1999) c  1999 Kluwer Academic Publishers. Manufactured in The Netherlands. Atomistic Simulations of Integranular Fracture in Symmetric-Tilt Grain Boundaries FABRIZIO CLERI Divisione Materiali Avanzati, ENEA, Centro Ricerche Casaccia, C.P. 2400, 00100 Roma, Italy cleri@casaccia.enea.it SIMON R. PHILLPOT AND DIETER WOLF Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Abstract. Fracture experiments on symmetric-tilt grain boundaries in Cu are interpreted using the Peierls-Nabarro continuum model of dislocation nucleation as a starting point. Good agreement is found only when the continuum model is modified according to the results of atomistic simulations. The same experiments are also reproduced by direct Molecular Dynamics simulations of fracture propagation and dislocation emission from a microcrack placed in the interface plane of the symmetric-tilt (221)(221) grain boundary in fcc Cu. Direction-dependent fracture response is observed, namely the microcrack advancing by brittle fracture along the [11 ¯ 4] direction and being blunted by dislocation emission along the opposite [ ¯ 1 ¯ 14] direction. Moreover, the simulations allow us to establish important differences with respect to the continuum-model predictions due to the shielding of the stress field at the crack-tip and to the presence of the residual stress at the interface. Keywords: grain-boundary fracture, dislocation nucleation, Peierls-Nabarro model, molecular dynamics simulations 1. Introduction In studies of fracture behavior in real materials much has been learned by comparing the fracture response of polycrystalline materials to perfect-crystal cohesive and elastic properties [1–3]. For example, it is well known that increasing the grain-boundary (GB) density by grain-size refinement generally results in improved mechanical properties of polycrystalline metals, such as higher values of hardness and yield strength [4]; however, increase in the GB density very often also makes a polycrystal less ductile by promoting brittle intergranular fracture [5]. Insight into brittle vs. duc- tile GB fracture behavior has been obtained by recent experiments on well-characterized bicrystals: tensile- fracture experiments on Cu bicrystals provided evi- dence of direction-dependent GB fracture [6], namely the crack front advancing by brittle cleavage in one di- rection of the interface plane and by ductile yielding along the opposite direction. This suggests that the typically ductile response of pure, single-crystal Cu could change into brittle response along some integran- ular paths in the case of polycrystalline Cu, depending on the relative geometrical arrangement of the slip sys- tems in adjacent grains. Clearly, a deeper investigation of the interplay between the atomic structure of GBs and local energetics at the crack tip (cohesion, ease of dislocation nucleation) is crucial to understand the peculiar issues of GB fracture. Let us consider the idealized situation of a planar crack in the interface plane of a GB in an otherwise perfect crystal. This allows us to investigate two funda- mental issues about GB fracture, concerning: (a) the possibility that the GB can be a preferred fracture path compared to crystalline planes (intergranular vs. in- tragranular fracture), and (b) the possibility that the brittle-ductile fracture response of the bulk crystal can be modified by the presence of the GB. These two