Place: Fall Meeting of the Materials Research Society (MRS), Boston (MA), USA
The mechanical strength of grain boundaries determines the deformation behavior of ultra-fine-grained and nanocrystalline metals. Sliding of grain boundaries has been suggested contribute significantly to the plastic deformation, on top of conventional dislocation plasticity. Furthermore, grain boundaries and triple lines are potential sites of crack initiation and crack advance. A multiscale model is presented in which the mechanical properties of grain boundaries under shear and tension are calculated by electronic structure methods within the density functional theory. From the resulting tensile force-displacement curves physical parameters like the work of separation, maximum stress and displacement across the interface are derived. For the case of shearing and grain boundary sliding generalized gamma-surfaces are calculated that can be used to classify the different types of grain boundaries into three categories. The ab initio results can be used to parameterize cohesive zone models that describe the mechanical behavior of interfaces directly on the continuum scale. Preliminary results obtained with a simplified model indicate that grain boundary sliding itself is not a significant deformation mechanism during plastic deformation of nanocrystalline and ulta-fine grained metals, but rather facilitates recovery processes close to grain boundaries and thus weakens the material.