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Abstract

In polycrystalline materials, grain boundaries play an important role during plastic deformation and fracture. Physics based theories tell us that the fracture process begins at a scale of angstrom to nanometer. At such a small length scale, atomistic simulations can be used for the prediction of fundamental damage mechanisms, for instance the effect of impurity atoms on grain boundary strength. The interaction of a crack tip with its plastic zone, or crack propagation in a real micro-structure, are processes which can be modeled using continuum scale numerical methods, ideally by taking into account the information from atomistic scale.

In the body centered cubic metals which are the materials of interest in high-temperature applications or steel production, the reduction of strength due to grain boundary embrittlement is especially large. The presence of defects e.g. point defects at the grain boundaries affect their mechanical properties, which in turn alter the hardness or fracture toughness of poly-crystals favorably or adversely. Carbon as a point defect has been reported to increase the strength of bcc metals whereas hydrogen and oxygen are assumed to be detrimental for grain boundary strength. In order to investigate the strengthening and em-brittling nature of above mentioned impurity atoms at grain boundaries, a systematic study of a Σ 5 (310)[001] symmetrical tilt grain boundary (Σ5 STGB) in molybdenum, tungsten and iron has been carried out. Atomistic scale uni-axial mechanical tests with loads perpendicular to the grain boundary were performed for all the afore mentioned systems using ab-initio density functional theory calculations. From these results, traction separation data has been derived that is being used for the parameterization of cohesive zone model to predict the inter-granular fracture at continuum level using finite element analysis.