COMPUTER-SIMULATION OF THE STRUCTURE AND THERMOELASTIC PROPERTIES OF A MODEL NANOCRYSTALLINE MATERIAL

A model of a space-filling, monodisperse nanocrystalline material, con sisting of identically shaped rhombohedral grains connected by identic al grain boundaries, is presented. In the limit of infinite grain size , the model reproduces the corresponding bicrystalline grain boundary; variation of the grain size therefore permits the role of the microst ructural constraints on the atomic structure and properties of the gra in boundaries in the polycrystal to be elucidated. Lattice-statics sim ulations performed on this model reveal that the relaxed zero-temperat ure structures of the grain boundaries differ only slightly from those of unconstrained boundaries. The vibrational densities of state of th e nanocrystalline material and of the related glass, determined from l attice-dynamics simulations, exhibit low- and high-frequency modes not seen in the perfect crystal. The low-frequency modes give rise to a l ow-temperature peak in the excess specific heat in both types of metas table microstructures. Free-energy simulations suggest that a phase tr ansition from the nanocrystalline state to the glass should occur belo w a critical grain size.