Carbon ordering in martensite lattice under external stress: MD simulation

Abstract

Rapid cooling of the Fe-C fcc solution leads to the formation of a phase called martensite, which has high mechanical properties. Martensitic transformation is the basis of quench steel hardening. Experimental investigation have shown that martensite lattice parameters a and c depend linearly on the carbon content. It is well known that under stress a martensite has the ability to plastically deform within a small range, despite the large value of the macroscopic yield strength. This effect is explained in different ways, mainly on the basis of dislocation theory. Professor M.Shtremel suggested an original treatment of the effect, based on the possibility of transferring the tetragonal axis of the martensite crystal. The experimental verification of the hypothesis is hampered by the difficulty in obtaining a crystal with one orientation. We investigated the influence of external stresses on carbon ordering in dilute Fe-C solid solutions and creation of tetrahedral distortion of crystal lattice in martensite. Molecular dynamics with embedded atom (EAM) interatomic potential have been used. According to simulations, interstitial carbon atoms migrate under compressive external stress applied along tetragonality direction from z-sublattice, and tetrahedral distortion of lattice changes its orientation to other ones. The simulation confirmed the validity of Shtremel’s theory. The influence of temperature and carbon concentration on this process was studied. The discrepancy between computer simulation results and theoretical data are discussed.