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Abstract

In order to understand the role of interstitial impurity atoms on intergranular fracture we need to characterize the mechanical properties of grain boundaries. Since the experimental characterization of mechanical properties of grain boundaries is extremely difficult if not impossible, we resort to precise ab initio calculations within the framework of density functional theory to calculate properties like work of separation as well as critical stress and critical strain across the interface. The benefit of using density functional theory is that on top of calculating critical values for pure grain boundaries, we can also determine the influence of segregate atoms, like carbon or hydrogen on the mechanical properties of grain boundaries and other interfaces.

These quantities calculated on the atomic scale are then used as input parameters for cohesive zone models. Within the framework of continuum modeling elastic-plastic deformation of the material around the grain boundary crack is calculated by continuum plasticity methods, whereas crack opening and advancement are described by cohesive zone models. Hence, the latter method is applied in a Griffith sense, where only the reversible work to open the crack is considered at the interfaces, whereas energy dissipation by plastic deformation around the crack tip is completely described by continuum plasticity.

Such kinds of models will be helpful to understand semi-brittle intergranular fracture, where the crack advances by breaking of atomic bonds at the crack tip rather then by damage through nucleation, growth and coalescence of plastic voids. In particular we can shed light on the role of interstitial atoms segregating to grain boundaries and characterize the influence of chemical composition on the atomic scale and use this information to study fracture of polycrystals with continuum methods on the microscale.