Computer simulation of the structure and thermo-elastic properties of a model nanocrystalline material

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Abstract
A model of a space-filling, monodisperse nanocrystalline material, consisting of identically shaped rhombohedral grains connected by identical 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 microstructural constraints on the atomic structure and properties of the grain boundaries in the polycrystal to be elucidated. Latticestatics simulations performed on this model reveal that the relaxed zero-temperature structures of the grain boundaries differ only slightly from those of unconstrained boundaries. The vibrational densities of state of the nanocrystalline material and of the related glass, determined from lattice-dynamics simulations, exhibit low- and high-frequency modes not seen in the perfect crystal. The low-frequency modes give rise to a low-temperature peak in the excess specific heat in both types of metastable microstructures. Free-energy simulations suggest that a phase transition from the nanocrystalline state to the glass should occur below a critical grain size.