The temperature dependences of the energy and structure of the symmetric tilt boundary of bcc and fcc iron were investigated by molecular dynamics simulation. A large energy cusp was observed at the bcc(112)͗110͘S3 and fcc(111)͗110͘S3 grain boundary plane, which is a twin boundary, whereas it was not observed at the bcc(111)͗110͘S3 plane in spite of it having the lowest S-value. The grain boundary energy increased at the temperature close to the melting point except for the grain boundary planes that have a large energy cusp. On the other hand, it was newly founded that the grain boundary energies at the bcc(112)͗110͘S3 and fcc(111)͗110͘S3 did not increased despite such a high temperature. The increase in grain boundary energy was due to the premelting, which is a localized disorder of the atoms near the grain boundary energy. It was confirmed that the grain boundary energy is affected by the matching at the interface rather than the periodicity described by the S-value.

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