Finite temperature structure and properties of ∑ = 5 (310) tilt grain boundaries in nacl a molecular dynamics study

Molecular dynamics simulations at constant temperature and stress were employed to study structure and excess properties of [Sgrave] = 5 (310) tilt grain boundaries in NaCl from low temperatures up to bulk melting. Several metastable grain boundary core structures were found. The differences in free energies of these structures, especially at high temperatures, were found to be small. Indeed two of these different core structures related by small displacement shift complete vectors were observed to coexist at high temperatures due to their small free energy differences. The temperature at which the boundary becomes mobile can be very different for different core structures. In all cases, grain boundary migration is coupled with grain boundary sliding. Melting of the system always starts from the grain boundaries at a temperature of approximately 0·98 Tm, where Tm is the bulk melting temperature. Excess enthalpies and volumes are almost constant as a function of temperature and diverge close to the bulk melting temperature. Grain boundary stress was also calculated and its anisotropy was found to decrease as a function of temperature. Differences between the calculated structures and those in NiO deduced from high-resolution electron microscopy studies are discussed.