Electronic stability effect on local strain in martensite

The lattice-strain problem around an interstitial C or N atom in martensitic iron was treated by an electronic theory. By using the DV-X alpha cluster method, the local electronic structure near an interstitial atom was calculated self-consistently. The strong bond between Fe and C (or N) atoms was attributable to the Fe 3d-C (or N) 2s, 2p covalent interactions. The bond strength increased remarkably when the first neighbour Fe atoms are displaced away from a C (or N) atom along the c axis of martensite. A series of analyses on the bond order, orbital populations of atoms, ionicities and electron density distributions show that an interstitial atom tends to exist stably in a central site of the regular octahedron, six corners of which are occupied by iron atoms. This is approximately in agreement with Jack's classical model for the structural component of martensite.