Structural, electronic, and magnetic properties of iron carbideFe7C3phases from first-principles theory

Abstract

The iron carbide Fe 7 C 3 exhibits two types of basic crystal structures, an orthorhombic ͑o-͒ form and a hexagonal ͑h-͒ one. First-principles calculations have been performed for the basic Fe 7 C 3 forms and for the related -Fe 3 C cementite phase. Accurate total-energy calculations show that the stability of Fe 7 C 3 is comparable to that of -Fe 3 C. The o-Fe 7 C 3 phase is more stable than the hexagonal one, in contrast to recent atomistic simulations. Furthermore, the calculations also show a rather low energy for a carbon vacancy in the o structure, which implies possible C deficiency in the lattice. Both Fe 7 C 3 phases are ferromagnetic metals. Electronic band-structure calculations show that all Fe atoms exhibit high-spin states with the majority of their 3d states being almost fully occupied. From an analysis of the structural and energetic properties, the formation of the o phase in steel treatment processes and of h form in carburization of ferrite is discussed.

Novel ultrafine Fe(C) precipitates strengthen transformation-induced-plasticity steel

“…Table 4 summarizes the size ranges, orientation relationship and crystal structures of precipitates found in this steel. Both Ti(N) and Ti 2 CS could be very effective in pinning austenitic grain boundaries [5,18] at elevated temperature during the hot rolling process, as explained above in Section 3.3. On the one hand this could result in reducing the ferritic grain…”

Stability and crystal structures of iron carbides: A comparison between the semi-empirical modified embedded atom method and quantum-mechanical DFT calculations

“…Experiments showed that graphite is about 17 meV/C more stable than the diamond phase. 18,57 Therefore, the MEAM energy for graphite is obtained from that of the diamond phase after this correction. The calculated lattice parameter for diamond is a = 3.558Å, in agreement with the experimental value.…”

“…Parameter-free first-principles approaches have been employed, but they were applied only to small systems due to the computational limits. [15][16][17][18][19][20][21][22] For large systems, atomistic simulation methods, such as the pair-potential approach (PPA), 23,24 the embedded atomic method (EAM), [25][26][27][28][29][30][31] and the recently adopted modified embedded atom method (MEAM), [32][33][34] are very valid. The main uncertainty in the large-scale simulations is the model describing interactions between atoms.…”

Role of carbon and nitrogen in Fe2C and Fe2N from first-principles calculations

“…Therefore, the C (N) 2s states do not contribute to the magnetism of these phases. The widths (about 3.1 eV) of the Fe 2 C phases are larger than those of Fe 7 C 3 (about 2.2 to 2.6 eV), of θ -Fe 3 C (∼2.2 eV), 17,37,38 and of η-Fe 3 C (∼2.5 eV). 18 The large widths of the C (N) 2s states indicate strong X-X interactions, as well as contributions to valence interactions, although they are semicore electrons.…”

“…20,21,37,38 Calculations for nitrogen were performed in a cube with the axis length a = 12.0Å, containing one N 2 molecule in order to avoid intermolecular interaction. 21 A cutoff energy for the wave functions of 1000 eV was employed to describe the strongly localized 2p bonds in the N 2 molecule.…”