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The high-temperature structure of the Sigma = 5 bicrystalline interface of B2 NiAl with a large boundary plane is investigated by molecular dynamics simulations on parallel computers. It is observed that the atoms in the grain boundary region tend to form clusters in a thermal structural disorder transition, which is initiated at a temperature well below the thermodynamic melting point T-m (similar to0.52 T-m). The number and size of the clusters are monitored over a wide temperature range including T-m. Below T-m, the number and size of the clusters increase continuously with increasing temperature. When the temperature is up to T-m, the abrupt growth of the clusters induces melting. Once above Tm, the number and size of the clusters decrease significantly upon raising the temperature. The calculations of the potential energy also indicate that the thermal disorder transition is a continuous process, in contrast to the first-order melting transformation. Copyright (C) 2001 John Wiley & Sons, Ltd.