Influence of external stress on martensite tetragonality in carbon steels

The theory of carbon atoms’ ordering in iron martensite based on the idea of non-uniform filling of octahedral interstices is considered. The results show that external pressure decreases the long-range order parameter and the degree of tetragonality abruptly. It is shown that an equilibrium condition needs to be considered together with free energy minimum requirement. Joint solution of these equations gives the critical stress value that turns the order parameter to zero, as a function of temperature and carbon content. Zero value of the order parameter means that the distribution of carbon atoms among sublattices is uniform, and, thus, the lattice is cubic. Molecular dynamics method with EAM interatomic potential was applied to model redistribution of carbon atoms over sublattices under external stress. It was found that migration of carbon atoms from z octahedral interstices occurs under compression along Oz axis, so that the tetragonality direction changes from Oz axis to Ox or Oy, with exact direction being determined randomly. The dependences of critical stress on temperature and carbon content were obtained. The computer simulation results are consistent with theoretical data.