Effect of interstitial carbon distribution and nickel substitution on the tetragonality of martensite: A first-principles study

By using first principles calculations, the effects of carbon distribution and of 25 at.% of nickel substitution on the tetragonality of ferrous martensite have been investigated. Different carbon concentrations have been considered (0≤xc ≤ 12.5 at.%). All calculations were based on the Projector Augmented Wave (PAW) method of the Density Functional Theory (DFT). The Special Quasirandom Structure (SQS) method has been used to model the disordered structures of the Fe16C2X and (Fe,Ni)16C2X systems. Our calculations demonstrate that the lattice parameters a and c vary linearly with carbon concentration, up to the high value of xc = 12.5 at.% (for X = 1). The nickel substitution enhances the expansion of parameter c of the Fe16C2X system up to 37%. This trend is correlated with the shear moduli calculated for bcc-iron and the disordered Fe25 at.%Ni structure. The mixing energies of the Fe16C2X and (Fe,Ni)16C2X systems indicate that the unmixing of martensite by spinodal decomposition is favoured by the nickel substitution. It is shown that carbon-carbon interactions are mostly repulsive until the distance of two bcc-iron lattice parameters.