Abstract
Previous experiments have proven that a high magnetic field can make alloy carbide M7C3 (M = Fe, Cr) precipitate ahead of time at intermediate temperatures. First-principles calculations are employed to search the source of magnetic-field-induced alloy carbides precipitation behaviors of orthorhombic M7C3. The basic building units for M7C3 crystals are polyhedrons formed by metal atoms and C atoms. The framework structure of o-M7C3 consists of metal tetrahedrons, metal octahedrons and triangular metal prisms. Magnetic calculations show that the substitution of Cr atoms causes the magnetic moments of the Fe atoms to decrease to different extents. Moreover, Cr atoms also have a magnetic moment antiparallel to the Fe atoms. Electronic structure calculations indicate that the bonds in M7C3 are a mixture of ionic, covalent and metal bonds. The substitution of Cr atoms weakens the ionic and covalent bonds between the Fe and C atoms; however, strengthen the metal bonds between the Fe and Cr atoms. The magnetic free energy change of M7C3 is larger than that of M2C and M3C at 823 K with a 12 Tesla field, which agrees well with the experimental results for a magnetic field promoting the precipitation of M7C3. This study provides a theoretical basis for the precipitation behaviors induced by magnetic fields in steels.
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