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Abstract: C and N atoms are the most frequent foreign interstitial atoms (FIAs), and often incorporated into the surface layers of steels to enhance their properties by thermochemical treatments. Al, Si, Ti, V, Cr, Mn, Co, Ni, Cu, Nb and Mo are the most common alloying elements in steels, also can be called foreign substitutional atoms (FSAs). The FIA and FSA interactions play an important role in the diffusion of C and N atoms, and the microstructures and mechanical properties of surface modified layers. Ab initio calculations based on the density functional theory are carried out to investigate FIA interactions with FSA in ferromagnetic bcc iron. The FIA–FSA interactions are analyzed systematically from five aspects, including interaction energies, density of states (DOS), bond populations, electron density difference maps and local magnetic moments. [Copyright &y& Elsevier]

Abstract: Carbon and nitrogen are the most frequent foreign interstitial atoms in α-Fe. The presence of even a little amount of these impurities in interstitial positions has a drastic influence on the properties of steels. However, the physics of the C–N interaction remains unclear. Here we perform first-principles calculations based on the density functional theory to study the interactions of C–N and those with vacancy (□) in α-Fe. When C and N atoms are in neighbor octahedral sites, a hybridization around −8eV of C 2p states with the N 2p states is observed. C and N atoms lie as far away from one another as possible. When a vacancy is introduced, it can attract two C atoms and a N atom, and a C–C–N–□ complex is formed. When the C and N atoms are the first nearest neighbors of the vacancy and stay in two neighbor octahedral sites, a covalent C–N bond is formed. And the peak of 2p states of the C atom’s density of states shifts to around −8eV, where the peak of the N atom’s 2p states lies. [Copyright &y& Elsevier]

Abstract: In this paper, the effect of N distribution on the elastic and electronic properties of hexagonal ε-Fe6N x (x=1, 2, and 3) phases is studied by first-principles calculations. The calculated formation energies of ε-Fe6N x phases decrease with the increasing distance between the N atoms, which demonstrates that the interaction between the interstitial N atoms is of repulsive nature. The theoretical elastic constants, bulk modulus (B), shear modulus (G), ratios of B to G, Young's modulus (E), Poisson's ratios (v) of ε-Fe6N x phases are obtained, which are important parameters for the crystal materials and surface coatings. The more stable interstitial conifiguration enables the ε-Fe6N x phases to possess higher B, G and E. Densities of states are given for all the configurations of ε-Fe6N x phases to discuss the origins of the variations of energetic stability and elastic properties. [Copyright &y& Elsevier]