Nature of axial ratio anomalies of the martensite lattice and mechanism of diffusionless γ → α transformation

Abstract

The results of recent X-ray and neutron studies of axial ratio anomalies associated with carbon atom distributions in the tetragonal martensite lattice are analyzed. This analysis shows that the C-atom distribution in freshly formed martensite is usually partly disordered except for some Al and high-nickel steels in which all the C-atoms are on a single sublattice of octahedral interstitial sites. The disorder is only possible if the diffusionless γ → α a rearrangement differs from that assumed in the conventional theory. A possible mechanism due to Roitbourd and Khachaturyan, for the formation of "abnormal", partially disordered martensite with a comparatively low axial ratio involves (011)α transformation twinning. New values of the concentration coefficients of linear expansion of martensite crystal, corresponding to the case when 100% of C-atoms are in a single sublattice of octahedral interstices are deduced and used for calculation of the strain-induced energies of pairs of C-atoms. Order-disorder processes are described, and the influence of correlation effects are considered. The statistical calculation leads to the conclusion that equilibrium correlations in the arrangement of C-atoms leads to an increase of the order-disorder phase transition temperature. Ordered (tetragonal) martensite at room temperature should be observed from ~0.18 wt. %C without correlation effects and from 0.0045 wt. %C if correlation is present. © 1975.