MOLECULAR ORBITAL CALCULATION ON THE INTERSTITIAL CARBON IN STEEL MARTENSITE

Abstract
Local electronic state around an intersititial carbon atom in bct iron lattice is calculated by using the discrete variational x-α cluster method in molecular orbital calculation. Hybridization between C-2s, 2p and Fe-3d,4s,4p orbitals produces bonding and antibonding levels, which control the local band structure. By taking account of the up and down spin configuration, magnetic moments of the first, second and farther neighbor iron atoms surrounding the carbon atom are calculated and compared with the result of Mössbauer spectroscopy. Although the identification of the carbon interstitial positions in the freshly quenched steel martensite by Mössbauer spectroscopy is still controversial, the result of calculation is in good agreement with one of the authors' (Fujita) former conclusion that carbon atoms occupy the octahedral and tetrahedral interstices half and half. Concerning the above molecular orbital calculation and the lattice deformation mechanism, which determines the carbon positions, experimental results by other techniques are also introduced and discussed.