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Thermodynamics of Solutes in Cementite Using First-Principles Calculations

Cementite precipitation from austentite in steels can be suppressed by alloying with silicon and aluminum. There are, however, no validated thermodynamic data to enable phase equilibria to be estimated when silicon and aluminum are present in cementite. The formation energies of Fe3C, (Fe11M )C4 and (Fe11M )C4, where M means an alloying element Si, Al or Mn, have therefore been estimated using first-principles calculations based on the total energy all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation to density functional theory. The ground state properties such as lattice constants and bulk moduli have also been calculated. The calculations show that (Fe11Si )C4 and (Fe11Si )C4 have about 52.1 kJ mol and 37.2 kJ mol greater formation energy, respectively, than Fe3C, while (Fe11 Al )C4, (Fe11 Al )C4, (Fe11Mn )C4 and (Fe11Mn )C4 have about 10.0 kJ mol , 13.6 kJ mol , 4.4 kJ mol and 5.0 kJ mol smaller. The formation energy for hypothetical cementite Si3C, Al3C and Mn3C have also been calculated to be about 256.4 kJ mol , 95.6 kJ mol and −52.7 kJ mol . The magnetic moments have also been estimated. The calculated electronic structures indicate that the magnetic moment reduction at the Fe(4c) site by the Si substitution at Fe(4c) site is indirect through the neighboring carbon atom, whereas at the Fe(8d) site it is direct.