COMPUTER-SIMULATION OF THE STRUCTURE AND DYNAMICAL PROPERTIES OF GRAIN-BOUNDARIES IN A NANOCRYSTALLINE MODEL MATERIAL

Molecular dynamics simulations have been used to synthesize and charac terize a fully dense, three-dimensional nanocrystalline material with an average grain size of 43 Angstrom by crystallization from the melt. The structures and energies of the highly constrained grain boundarie s in this material are found to be more isotropic than those of extend ed boundaries in bicrystals. Based on this observation, a simple struc tural model that combines a realistic treatment of the grain boundarie s with a finite grain size-and yet permits a comparison with the struc tures of unconstrained boundaries in bicrystals-is developed. The low temperature thermal behavior of such a model material is shown to be d ominated by low frequency phonon modes due to the grain boundaries and grain junctions.