Tetragonality and the distribution of carbon atoms in the Fe–C martensite: Molecular-dynamics simulation

Abstract

In the statistical theory of the ordering of carbon atoms in the z sublattice of martensite, the most important role is played by the parameter of the strain interaction of carbon atoms λ₀, which determines the critical temperature of the bcc–bct transition. The values of this parameter (6–11 eV/atom) obtained in recent years by the methods of computer simulation differ significantly from the value λ₀ = 2.73 eV/atom obtained by A. G. Khachaturyan. In this article, we calculated the value of λ₀ by two methods based on the molecular-dynamics simulation of the ordering of carbon atoms in the lattice of martensite at temperatures of 500, 750, 900, and 1000 K in a wide range of carbon concentrations, which includes c_crit. No tails of ordering below c_crit have been revealed. It has been shown analytically that there is an inaccuracy in the Khachaturyan theory of ordering for the crystal in an elastic environment. After eliminating this inaccuracy, no tails of the order parameter appear; the tetragonality changes jumpwise from η = 0 to η_crit = 0.75 at c_crit = 2.9kT/λ₀ instead of η_crit = 0.5 and c_crit= 2.77kT/λ₀ for an isolated crystal. Upon the simulation, clustering of carbon atoms was revealed in the form of platelike pileups along {102} planes separated by flat regions where no carbon atoms were present. The influence of short-range order in the arrangement of neighboring carbon atoms on the thermodynamics of ordering is discussed.