Multiparticle clusters and carbon superstructure in martensite

Abstract

On the basis of experimentally verified concentration expansion tensor values, stress induced two-particle C-C potentials have been calculated in harmonic approximation. A calculation method has been developed and expressions derived for the evaluation of multiparticle interaction potentials and cluster population. The temperature range of the applicability of the method has been estimated. On the basis of this method it has been demonstrated that in thermodynamic quasi-equilibrium, carbon atoms exist in clustered form. The clusters most frequently appearing at 300 K are of four- and five-particle type. The cluster configurations have been determined and the binding energy per atom has been estimated as about 0.5 eV. At 78 K, there exist practically only five-particle linear clusters situated along the tetragonal C axis. It has been postulated that a superstructure may exist in martensite with a binding energy per atom nearly four times higher than in the case of the above clusters. The presence of superstructure is associated with the formation of five-atom seeds in the form of pyramids having their basis in the (001) plane. The formation of seeds with different topology from the other clusters is associated with overcoming a potential barrier. The postulated form of ordering at low temperatures should exhibit high thermal stability with respect to ordering changes and order-disorder phase transitions, as well as to carbide formation.