Martensite crystal lattice,mechanism of austenite-martensite transformation and behavior of carbon atoms in martensite

Results concerning the crystal lattice of freshly formed martensite with abnormally low and abnormally high axial ratioc/a as well as the results of neutron diffraction studies of the positions of carbon atoms are reviewed. Mechanisms of the austenite to martensite transformation are considered, which can explain the formation of martensite with differentc/a for the same carbon content in the initial austenite. Occurrence of (011)m transformation twins explains both a lowering ofc/a and an orthorhombic distortion of the martensite lattice. It follows from analysis of the experimental results that the well known dependence of martensite lattice parameters and axial ratio on the carbon content relate to a partly disordered distribution of carbon atoms between three sublattices of the octahedral interstitial sites (ois). Using new values of the concentration coefficients of linear expansion of the martensite lattice, calculated for the case of carbon atoms distribution only in a single sublattice of the ois, leads to some corrections of the previous temperature and concentration dependences of order-disorder processes in martensite. Phenomena such as the reversible change of axial ratio due to the redistribution of carbon atoms between their normal positions and “traps” in irradiated martensite are described.