Limitations of the structural unit model

The structural unit model (SUM) has been used to describe the structures of grain boundaries (GBs) in a range of different materials. The validity of this model for all materials has never been proven, however. The structures of Al and Ni <110> symmetric tilt grain boundaries with tilt angles between 0 degrees and 50.5 degrees are investigated. Computer simulations with the lattice statics technique and EAM potentials are used to find the lowest energy structures at a temperature of 0 K. The stability of the structures at elevated temperatures is tested by Monte Carlo annealing simulations. It is found that, while the SUM is applicable to the Al boundaries, it cannot be used to describe the Ni structures. The GB dislocation cores in the Al boundaries are more localized than they are in Ni. The effect of delocalized GB dislocations is discussed. The geometrical and materials dependent limitations of the SUM are summarized.