First-principles study on the effect of impurities at the front of cracks inα−Fe

Abstract

By using two first-principles methods, discrete variational method (DVM) and DMol, within the framework of density-functional theory, the effects of nonmetal light impurities (C, N, and O) on a fracture in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$ are studied. We calculate the curve of binding energy $({E}_{b})$ vs the displacements of the nearest-neighbor Fe atoms of the impurity, and find that C and N tend to hold onto these two Fe atoms. Calculated PDOS and deformation charge density reveal that C and N prefer to form polarized bonds vertical to the propagation direction of the crack with the nearest-neighbor Fe atoms. Furthermore, interaction of the C $(\mathrm{N})\ensuremath{-}\mathrm{Fe}$ pair which is vertical to the crack plane is much larger than that of the C $(\mathrm{N})\ensuremath{-}\mathrm{Fe}$ pair which is parallel to the plane. This anisotropic bonding character indicates that doping C or N impedes the propagation of the crack, thus can improve the mechanical property of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$. On the contrary, O tends to push its nearest-neighbor Fe atoms away, and exhibits weak interaction with its nearest-neighbor atoms. O also weakens the interaction of $\mathrm{Fe}\ensuremath{-}\mathrm{Fe}$ pair in bulk. Thus O has detrimental effects on the ductility of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$.